[c. j. karzmark, robert j. morton] a primer on the(bookos.org)

A Primeron Theory and Operation

of Linear Acceleratorsin Radiation Therapy

C.J. Karzmark, Ph.D.Professor of Radiology (Radiological Physics Section)

Department of RadiologyStanford University School of Medicine

Stanford, California

Robert J. Morton, M.S.National Cancer Institute

Bethesda, Maryland

First published December 1981 by theBureau of Radiological Health

Revised January 1989 byMedical Physics Publishing Corporation

(A non-profit organization)

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Copyright © 1989 by Robert Mortonand C.J. Karzmark

All rights reserved. No part of thi1 publication may be reproduced ortransmitted in any form or by aIy means , without permission inwriting from the publisher. Perm~sion is granted to quote excerptsfrom articles in this book in sciertific or technical works with acknowledgement of the source, inch:ding the editors' names, the booktitle, and year of publication.

Originally published by the Bureauof Radiological Health, 1981.

Reprinted and published by 1\在edid Physics Publishing Corporation,27B, 1300 University Avenue , MadEon, Wisconsin, 53706.

ISBN: 0-944838-07-3

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Preface

Electron linear accelerators evolved from the microwave radar developments of World War II. The klystron tube, invented at Stanford,provided a vital source of microwave power for radar then as it doesnow. In the late 1940s, the high-power klystron and the microwaveprinciples incorporated in its design were used to construct and powera linac for use in physics research and later for industrial radiography. By the mid 1950s, a linac suitable for treating deep-seated tumorswas built in the Stanford Microwave Laboratory and installed at Stanford Hospital, which was located in San Francisco at that time. Itserved as a prototype for commercial units that were built later.

Since that time medical linear accelerators have gained in popularity as major radiation therapy devices , but few basic training materials on their operation have been produced for use by medical professionals. Dr. C. J. Karzmark , a radiological physicist from StanfordUniversity, has been involved with medical linacs since their development and he agreed to collaborate with Robert Morton ofthe National Cancer Institute, in writing this primer on the operation of medicallinear accelerators. This publication provides an overview of the

components of the linear accelerator and how they function and interrelate. The auxiliary systems necessary to maintain the operation ofthe linear accelerator are also described. The primer will promote anunderstanding of the safe and effective use of these devices. It hasbeen produced in cooperation with the Division of Resources, Centers ,and Community Activities of the National Cancer Institute, and is intended for students of radiation therapy technology, radiologicalphysics, radiation oncology, and radiation control.

For ease of understanding, much of the text describes the components as they appear in a specific electron linear accelerator treatment unit, the Varian Corporation's Clinac 18. This choice in no wayconstitutes an endorsement of this particular equipment. Variationsin design do occur and several are described in Appendix A. Table 1in Appendix B lists pertinent specifications of all radiotherapy linacsknown to be commercially available at this time. A three-part videotape, titled "The Theory and Operation of the Linear Accelerator inRadiation Therapy," has been produced in conjunction with thisprimer and can be ordered from Pam Gorman at the National Audiovisual Center, 8700 Edgeworth Drive, Capitol Heights, Maryland ,20743. Order numbers are A07313 (VHS videotape format) and A01822

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(3/4" tape format).The current revision takes cognizance of signific~nt advances oc

curring in radiotherapy linacs since the original publication. Again ,the level of treating these advances is simplified so that the audienceof technologists, as well as physicians, engineers and physicists canbenefit. A new Section 9, Dual X-ray Energy Linacs, describes theseversatile new units which provide two X-ray and several electronbeams for a variety of clinical situations. Providing these varioustreatment modalities requires changes in how the standing wave andtravelling wave accelerator structures are energized with microwavepower. Section 10, Bending Magnet, has been revised to describe morefully the properties of complex (doubly achromatic) magnets used incontemporary treatment units in contrast to the simplified (singlyachromatic) magnet shown in Fig. 36. Additional technical information on advances in accelerator design may be found in the addedreference, Karzmark, 1984 and the projected publication of a text onMedical Electron Accelerators, by C. J. Karzmark, Craig Nunan andEiji Tanabe. Appendix A has been revised to include descriptions ofcontemporary 1inac treatment units，缸ld Appendix B provides an updated Table I of performance specifications of radiotherapy linacs.

Acknowledgments

The need to simplify complex microwave and physics phenomenawhile retaining rigor in the treatment of these phenomena presented asignificant dilemma in writing this primer. We are deeply indebtedto our many colleagues who gave generously of their time in criticallyreviewing the manuscript, suggesting changes, simplifying analogies, and identi马ring areas that were unclear. Their incisive comments enabled us to have a better perception of how the primer shouldbe written. We wish also to acknowledge the assistance, critical review, and encouragement of BRH staff members Frank Kearly andMarcia Shane.

This work has been supported in part by Research Grant CA-05838from the National Cancer Institute, NIH, and in part by an Interagency Agreement with the National Cancer Institute; NCI 2Y01-10606.

Contents

ForewordPrefaceAcknowledgment1. Introduction 12. Energy Designation in Accelerators 23. An Elementary Linear Accelerator 54. Similarities and Differences Between Linacs and Diagnostic

X-Ray Generators 85. Major Linac Modules and Components 96. Introduction to Microwave Power Sources 10

6a. Microwave Cavities 106b. The Klystron 126c. The Magnetron 13

7. The Waveguide and Circulator 148. Introduction to Accelerator Structures 15

8a. Traveling-Wave Accelerator Structures 168b. Standing-Wave Accelerator Structures 19

9. Dual X-Ray Energy Mode Linacs 219a. StandingWave 219b. Traveling Wave 23

10. Bending Magnet 2411. TreatmentHeads 2512. Retractable Beam Stopper 2613. Functional Block Diagram and Auxiliary Systems 2714. Operational Review 29Bibliography 30Appendix A - Representative Linac Treatment Units 31Appendix B - Specifications of Radiotherapy Linacs 41

Abstract

Karzmark, C. J. and R. J. Morton: A Primer on Theory and Operation of Linear Accelerators in Radiation Therapy. HHS Publi(:ation(FDA) 82-8181 (December 1981); reprinted 1988.

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IntroductionCancer patients are treated by radiation , surgery or chemotherapy.

A treatment method proving increasingly effective is radiation , usedby itself or in combination with other modalities. The principal radiation modality for the treatment of deep-seated tumors is x-rays ofvery high energy and penetrating power. Such x-rays are createdwhen high energy electrons are stopped in a target material such astungsten. Alternatively, the electrons themselves may be used directly to treat more superficial cancers. The electron linear acceleratoraccelerates charged particles in a straight line, in contrast to the circular orbits that characterize the betatron and cyclotron. The purposeof this primer is to explain the principles of operation and use of theelectron linear accelerator and to acquaint the reader with pertinentfeatures and terminology.

The medical linear accelerator will be introduced by first examin-

ing the treatment room. Fig. 1 shows a patient being readied for treatment with a linac. The thick concrete walls of the treatment roomshield the technologist and other staff from the penetrating radiation.The linac is mounted in a gantry which rotates on a stand containingelectronic and other systems (Fig. 2). The linac can be rotated into position about the horizontal gantry axis for use in treatment. The radiation beam emerging from the collimator is always directed throughand centered on the gantηaxis. The beam central axis intersects thegantIγaxis at a point in space called the isocenter. In the majority ofcases, the couch is positioned so that the patient's tumor is centered atthe isocenter. Usually , the patient lies supine or prone on the treatment couch (sometimes called patient support assembly). The couchincorporates three linear motions and a rotation motion about the isocenter to facilitate positioning the patient for treatment. Side and ceiling lasers project small dots or lines that intersect at the isocenter.These facilitate positioning the patient in conjunction with referencemarks , often tattoos, placed on the patient's skin. The digital positionindicators display the treatment field size together with collimatorand gantry rotation angles. The isocentric system facilitates comfortable , precise reproducible treatment when using multiple fieldsdirected at the tumor from different gantry angles (Fig. 3). In thisunit, a constant radiation Source-gantry Axis-Distance (SAD), usual-

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Fi职lre 1. A contemporary high-energy radiotherapy linear accelerator. This andmany ofthe illustrations which follow pertain to the Varian Clinac 18

ly 100 centimeters (em), is employed. Alternatively, some treatmenttechniques use a constant radiation Source-Skin (of patient)-Distance(SSD), usually for large fields at distances of 100 em or greater.

The technologist operator views the patient, and presets and monitors the treatment from a control console outside the treatment room(Fig. 4). Much of the auxiliary electronics (as well as control andmonitoring devices) is housed in the electronic card rack cabinetmounted at the console. A nearby modulator cabinet houses auxiliaryelectronics for the larger linacs.

The discussions and illustrations which follow this brief description of the linac will introduce the necessaηr concepts behind its operation and extend them to the building of an elementary electron linear accelerator. Later, the major modules of a medical linac will beidentified. Their principles of operation and how they function collectively to produce x-ray and electron treatment beams will be described. First, however, there will be an important digression on designating the energy of radiotherapy beams.

Energy Designationin Accelerators

Fig. 5 shows a simple device that will accelerate electrons. It consists of a one-volt battery connected to two conducting plates spaced 1em apart in an evacuated glass tube. The glass tube is an electricalinsulator. The negative plate is termed the cathode and the positiveplate the anode. In order to set up these charges, the battery causeselectrons to flow from the anode to the cathode via the external circuit.This results in a deficiency of electrons at the anode (positive charge)and an excess of electrons at the cathode (negative charge) as shown.This charge distribution creates an Electric Field "E" (denoted by anarrow) in the region between the plates in the direction shown. Theelectric field is the force that a unit positive charge would feel if placedbetween the two plates. Its strength or magnitude in this example isone volt per em (1 V/cm). That is, the difference in the electrical potential between the plates, divided by the distance between them, is one

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FigJ山 re 3. The "isocentric" treatment technique. The tumor center, shown within apatient's cross-section, is positioned at the isocenter with the aid of skin marks and thelasers shown. The tumor is now positioned for easy and accurate irradiation from anydesir巳d gantrγangle. The dashed circle depicts all possible x-ray source locations at100 em radius (Source-Axis Distance = 100 em).

Fi♂He 4. The control console and modulator cabinet of a Clinac 18 installation. Thetechnologist initiates, monitors, controls and obs巳rves the treatment from th 巳 control

console on a closed-circuit television monitor. The modu lator cabin 巳 t contains primarγpower controls and auxiliary electronic units. The card rack cabin 巳t also contains auxiliary electronics.

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volt per cm. By definition , the arrows, which identify the direction of"E," are in the direction in which a positively charged particle wouldmove; an electron with its negative charge would move in the oppositedirection. It is not possible to see "E" fields but they are known to existbecause of the force they exert on charged particles such as electrons. If electrons, denoted by "e" in Fig. 6, are released from thenegative plate (the cathode) they 耳IVill be accelerated by the force of theliE" 自eld to the positive plate (the anode). An electron volt (eV) is theenergy gained by an electron accelerated across a potential differenceof one volt.

Exerting a force through a distance is a basic measure of work andenergy. On the atomic scale, the electron volt, or multiples of it, is theadopted unit of energy. In Fig. 6 we are dealing with a force of onevolt per cm exerted on an electron through a distance of onecentimeter.

Imagine now that a thousand one-volt batteries are connected in series to provide 1,000 volts, or one kilovolt potential differential, acrossthe plates of this device as in Fig. 7. The accelerated electron wouldarrive at the anode with an energy of 1,000 electron volts or one kiloelectron volt (1,000 eV = 1keV). Note, also, that the strength ofthe associated "E" field now is 1,000 volts per cm (l kV/cm).

Suppose the plates are spaced 10 cm apart and that a thousand one-

kilovolt batteries are connected in series to provide a one-mil1ion-voltpower supply (Fig. 8). The plate spacing and glass tube have beenlengthened to withstand this higher voltage without electrical breakdown. An electron released from the cathode now gains one millionelectron volts of energy during its transit and arrives with an energyof one-million electron volts (1,000,000 eV = 1,000 keY = 1 MeV). Notethat the energy gained by the electron· depends only on the potentialdifference between the anode and the cathode, and not the distancetraveled. The corresponding electric field strength "E" is one million volts divided by 10 cm, or 100,000 volts per cm (100 kVlcm). To establish the higher electric field strengths of Figs. 7 and 8, the + and charge distributions at the anode and cathode are proportionatelylarger, as compared to Figs. 5 and 6. To simplify the Fig.s which follow, the "E" lines and associated charge distributions at the anodeand cathode will sometimes be omitted.

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F,igure 8 , !"- s.i~'p}e_~le~~ron linear accelerator of energy designation one millionelectron volts (l MeV). Note the increased electric charg~ 仆， -) distributions at theanode and cathode compared to those for the lower voltages ofFigs. 5, 6 and 7.

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the window and emerge with only a small loss of energy. In this elementary linac, the electron beam emerges with an energy only slightly less than 4 MeV.

To adapt this for x-ray therapy, the positive anode placed outside thewindow is a tungsten target which stops the electrons abruptly, therebyproducing penetrating x-rays (Fig. 11). These x-rays will have energies from a fraction of an MeV up to 4 MeV, all initiated by electronsof 4 MeV energy, since the electrons can give up their energy all atonce in a single collision or in parts due to several collisions. The resuIting spectrum of x-ray energies is designated by "4MV." The notation convention of dropping the "e" from "MeV" indicates that the xray beam will be made up of x-rays of different energies produced asthe 4 MeV electrons are slowed and stopped in the target.Instθad of energizing this simple linear accelerator with a battery,

substitute an alternating voltage, as shown in Fig. 12. The magni·tude and polarity of such a voltage changes regularly and repeats itself periodically with time in this cyclic pattern which is called a sinewave. For the single cycle shown in Fig. 12, the horizontal axis denotes time; the vertical axis denotes the magnitude and polarity of theanode voltage, V, relative to the cathode, that establishes the "E" field.Many electrical and mechanical phenomena change smoothly in thisregular pattern of a sine wave. The number of complete sine wave

An Elementary LinearAccelerator

It is possible to convert the simple "linac" just described to a moresophisticated, yet still elementary, electron linear accelerator. First,a heated cathode is substituted for the negative plate (Fig. 9). (In the linac this cathode becomes the electron gun.) The cathode shown here isa simple filamen t. The small battery, B, heats the filament causing itto literally "boil off' electrons just as in a light bulb filament. Next, atheoretical 4 MV batteη， is connected between the cathode and anode.This battery voltage corresponds to the electron energy desired, i.e. , 4million volts for 4 MeV electrons. Now, electrons are boiled off thefilament and accelerated to an energy of 4 MeV just as they strike theanode.

To adapt this electron therapy, a thin metal "window" becomes thepositive plate or anode (Fig. 10). Such a thin, solid, metal sheet maintains the necessary vacuum and yet permits the electrons to penetrate

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Figure 9. An elementary four million volt (4 MeV) electron Hnac. The filament-typecathode for the electron source is heated by a small battery, B.

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Figure 11. The elementary four million volt linac of Fig. 10 modified to provideexternal 4 MY x-rays

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cycles per second (+ and - or up and down excursions) is called the frequency and is expressed in hertz (Hz), kilohertz (kHz) or megahertz(MHz). One hertz equals one cycle per second. Typically, the frequency of home electric power is 60 Hz , a standard broadcast radiowave can be 1000 kHz, and the energizing power for medical linacs is3000 MHz. The latter high frequency is referred to as a microwavefrequency. The time for completing a single cycle is called the periodand , for the above examples , coincides with 1I60th second , 111,000,000th second (one microsecond) and 1/3,000th of a microsecond,respectively.

Now, investigate the performance of the elementary linac whenpowered by an alternating voltage , as depicted in Fig. 12. With thetarget positive and trr-e filament negative , as shown in Fig. 13, electrons emitted from the cathode during interval a-b-c of Fig. 12 are accelerated to the target. At point "c" in Fig. 12, the voltage reverses polarity and 咀" field direction. With the target negative and filamentpositive, during interval c-d-e as shown in Fig. 14, electrons are stillemitted but are not accelerated to the targe t.

Electrons are accelerated only during the first half of the cycleshown in Fig. 12. At a time one-fourth through the cycle , point "b," thevoltage, V, reaches a positive maximum , and "E" is directed as inFig. 13. An electron released from the cathode at this time would gain

a maximum of energy. Conversely, at three-fourth's time through thecycle, point "d ," V reaches a negative maximum. Then, "E" is directed as in Fig. 14, and maximum energy would have to be expendedin "pushing" the electron from the cathode if it were to reach the anodeagainst the opposing "E" field. Therefore, no electrons are accelerated at this time. At other times, intermediate amounts of energy wouldbe gained or expended, including zero at points a , c and e. In this illustration , we assume that the electron travels between cathode andanode instantly; that is, the electron's travel time is zero. Note thatnow the elementary linac accelerates electrons and emits radiationonly half of the time , and the electrons vary in energy sinusoidallyduring this time.

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Figure 13. The elementary electron linac modilled to utilize an alternating voltagepower source, whose polarity reverses everγhalf cycle (see Fig. 12). For the polarityshown, electrons will be accelerated to the anode. Note that "E" and the electron velocity"V" are in opposite directions.

Figur它 14. The elementary linac powered by an alternating voltage. For the polarityshown (opposite ofFigure 13), electrons remain near the fJlament and will not beaccelerated to the anode.

Similarities and DifferencesBetween Linacs and DiagnosticX-Ray Generators

There are many similarities between the 1in~c and conventionaldiagnostic x-ray generators. Both provide a source of electrons from ahot filament or cathode in an evacuated tube. Both require an accelerating voltage between the cathode and target anode. This voltage isadjustable in a diagnostic generator, depending on the procedure ,from about 30 kV to 150kV. In contrast, 1inac accelerating voltagesare fixed in a particular unit and range from about 4MV to 35 MV. Diagnostic x rays often involve a single 0.01 to 10 second pulse with a 60Hz to 720 Hz frequency, while linac radiation consists of short burstsof about five-millionths of a second duration repeated several hundred times per second, each burst having a 3000 MHz frequency. Bothemploy collimators to shape the x-ray beam, but these must be thickerin the case of 1inacs. Because of their high energy, x-rays from 1inacsare much more penetrating than diagnostic x-rays. This is a distinct

8

advantage for treating a deep-lying cancer since the cancer can be destroyed by the linac beam with less damage to healthy, overlyingtissues.

Linacs require heavily shielded rooms to protect the persons outside.Such rooms are constructed with thick concrete walls. In contrast, di

agnostic rooms are usually shielded by a sheet of lead a few millimeters thick hidden in the walls. Diagnostic x-rays reveal anatomicalstructures based on differences in atomic number as well as physicaldensity, e.g. , bone versus soft tissue or air; megavoltage x-ray attenuation is primarily based on density differences. A film produced withmegavoltage x-rays would show little difference between bone andsoft tissue. The importance of x-ray diagnostic beams is in the information contained in the transmitted beam which produces an imageon a receptor. The importance of x-ray therapy beams, such as provided by linacs, is in the energy absorbed in the tumor.

Orthovoltage (about 250 kV) radiation equipment, which dominatedtreatment energies of the 1930s, has properties closer to diagnostic xrays than megavoltage energy therapy beams, and continues to be appropriate for some specific treatments.

Now return to examining a 1inac , such as seen earlier being read- r

ied for use. It consists of a number of major modules and componentsthat will be identified, and includes operating principles which will

be explained. Then , this information will be combined and interrelated to explain an operational linac.

Major Linac Modulesand Components

The major modules in the linac are the gantry, the stand, the con饷1

console and the treatment couch. Some Hnacs also have a modulatorcabinet, as this sample linac does. Fig. 15 identifies the componentshoused in the stand and gantIγof a medium-energy linac and will bereferred to frequently. The stand is anchored firmly to the floor andthe gantry rotates on bearings in the stand. The operational accelerator structure, housed in the gantry, rotates about a horizontal axisfixed by the stand. For ease of understanding, most of the text will describe the components as they appear in the Varian Clinac 18. Variations in linac design do occur and are described in Appendix A.

The major components in the stand are the:

1. Klystron - which sits atop an insulating oil tank and provides asource of microwave power to accelerate electrons;2. Waveguide - which conveys this power to the accelerator in thegantry;3. Circulator - a device inserted in the waveguide to isolate the klystron from microwaves reflected back from the accelerator; and4. Cooling water system - which cools various components that dissipate energy as heat and establishes a stable operating temperature suf二

ficiently above room temperature to prevent condensation of moisturefrom the air.

The major components found in the gantry are the:1. Accelerator structure - which is energized by the microwave powersupplied from the klystron via the waveguide;2. Electron gun (or cathode) - which provides the source of electronsinjected into the structurβ;

3. Bending magnet - which deflects the electrons emerging from theaccelerator structure around a loop in order to strike the target to produce x-rays or to be used directly for electron treatments;4. Treatment head - which contains beam shaping and monitoringdevices;5. Beam stopper - which reduces room shielding requirements for thetreatment beam emerging from the patient, and extends from the bot-

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Figure 15. Schematic diagram of a high-energy radiotherapy linac idcnti(ying majorcomponents housed in stand and gantry.

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tom of the gantrγas shown in Fig. 40a.The modulator cabinet (Fig. 4) contains components that distribute

and control primary electrical power to all areas of the machine fromthe utility connection and also supplies high voltage pulses.

The treatment couch motions are controlled by a hand pendant control operated by the therapy technologist. The three-dimensional positioning of the patient on the couch is motor-driven. Fast and slowspeeds or variable speed motor control are provided for the couch , together with control of gantry rotation and secondary collimator positioning (Fig. 2). Most couches also provide couch rotation around avertical axis passing through the isocenter, and some permit attachment of a treatment chair.

The control console (Fig. 4) is the operations center for a linac. Itsupplies the timing pulses that initiate each pulse of radiation. It provides visual and electronic monitors for a host of linac operating parameters including the individual patient's dose prescription. Treatment cannot proceed when the value of pertinent parameters exceedslimits which have been previously established.

In addition to these major modules and components, there are anumber of auxiliary systems including: vacuum, pressure, cooling,automatic 仕equency control (AFe), and monitor and control (seeSection 12).

10

Introduction to MicrowavePower Sources

The klystron and magnetron are two special types of electron tubesthat are used to provide microwave po~er to accelerate electrons. Microwaves are similar to ordinary radio waves , but have frequenciesthousands of times higher. The microwave frequency needed for linac operation is three billion cycles per second (3000 MHz). The voltage and "E" fields associated with microwaves change sinusoidallyin direction and magnitude in a regular manner, producing an alternating voltage as shown in Fig. 12. Microwave cavities, which arecentral to the construction and operation of klystrons and magnetronsas well as to accelerator structures, will be described next.

6a. Microwave CavitiesMicrowave devices , including klystrons , magnetrons and acceler

ator structures, make extensive use of resonant microwave cavities.

that exist in a cavity have a complex dependence on time and havebeen separated arbitrarily in Figs. 16, 17, and 18a, band c, for clarity.The polarity of the electric charge and current, and the "E" and "H"field directions reverse twice each microwave cycle; that is, six billion times a second! The patterns of Figs. 16 and 17 are one-half cycleapart in time. In order to take advantage of these intense "E" fields tobuild a klystron or an accelerating structure, circular openings onaxis at the cavity ends are cut as shown in Fig. 18 so that electronbeams can be introduced to interact with these fields. The electronbeam current passes through these openings along the cylindricalaxis Z. The large cavity wall currents I should not be confused withthe electron beam current which originates from an electron gun in aklystron or in an accelerator structure. The arrows denoting I inFigs. 16a, 17a, and 18a point in the direction that a positive charge current would flow. The electrons, which in actuality are the charge carriers, flow in the opposite direction.

Earlier, the energy transfer from a static, and then an alternating,electric "E" field to an electron transported between two conductingplates was studied. Recall that in one direction of the "E" field energyis transferred to the electron. An electron traveling at high speed inthe reverse direction of the "E" field can transfer energy from theelectron to the "E" field. This latter phenomenon will be examined in

A simple microwave cavity similar to that used in medical linacs, butwith closed ends , is shown in Figs. 16, 17, and 18. It is an accuratelymachined cylinder, about 10 ern in diameter and several ern inlength. Such a cavity has the approximate size and shape of a 7 oz; tunafish can. In Fig. 18, thecavity is shown modified by cutting openings in its two ends along the axis , for use in a klystron or an accelerator structure. A microwave cavity is an enormously efficient devicein the sense that the intense "E" fields needed for these applicationsare established by a small amount of electrical power. This is a resonance phenomenon that occurs at one frequency , in this case 3000MHz , which is determined by the dimensions of the cavity much as amusical organ pipe of a particular length resonates to a particularpitch. Such cavities are formed of copper walls for high electrical andthermal conductivity. An electric current I flows on these innerwalls, moving electric charge from one cavity end to the other, asshown in Figs. 16, 17, and 18. These end regions of dense electriccharge are central to both klystron and accelerator structure operationbecause they give rise to the intense "E" fields along the axis of thecavity as in Figs. 16b, 17b, and 18b. The magnetic "H川自eld pattern ofFigs. 16c, 17c, and 18c that exists in the cavity will be omitted in the illustrations which follow , since they are unimportant for our purposes.

The electric and magnetic fields , currents and charge distributions

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more detail. It provides the basis for the operation of both the klystronand magnetron.

6b. The KlystronThe elementary klystron, depicted in Fig. 19, is a microwave am

plifier tube that makes use of two cavities of the type illustrated in Fig.18. The cross-section drawing shown in Fig. 19 is a view that contains the cylindrical Z axis of the cavities similar to the view shownin Fig. 18b. On the left is the cathode, the source of electrons for theklystron , which is given a negative pulse of voltage. This accelerateselectrons into the first, or buncher cavity, as it is called. The bunchercavity is energized by very low-power microwaves which set up alternating "E" fields across the gap between left and right cavity walls.The "E" fields vary in time , as shown in Fig. 19b. Recall that it is thenegative "E" field that accelerates the electrons. Those electronswhich arrive early in the microwave cycle , at times between points "aand b ," encounter a retarding "E" field and are slowed. The velocityof those electrons arriving at time 飞，" when the tiE" field is zero, isnot affected. Electrons arriving at later times, between points "b andc," are speeded up by the negative "E" field. This process is called velocity modulation , since it alters the velocity but not the average number of electrons in the beam and causes the electron stream to be

formed into bunches. The drift tube connecting the two cavities provides the distance along which the electrons moving with different velocities merge into discrete bunches as shown.

The second, or catcher cavity, is resonant at the arrival frequency ofthe bunches. As the electron bunches leave the drift tube and traversethe catcher cavity gap, they generate a retarding "E" field by inducing charges on the ends of the cavity and thereby initiate an energyconversion process. By this process, much of the electron's kinetic energy of motion is converted to intense "E" fields in the second cavitycreating microwave power which is used to energize the acceleratorstructure. The residual beam energy that is not converted to the microwave power is dissipated as heat in the electron beam collector onthe far right, and the heat is removed by the water cooling system. Thebeam collector of high-powered klystrons is shielded with lead to attenuate hazardous x-rays created by these stopped electrons. Suchklystrons have three to five cavities and are used with high energy linacs, e.g. , 18 MeV and above. The additional cavities improve highcurrent bunching and increase amplification. They can provide atremendous (e.g. , 100,000:1) amplification of microwave power. Theklystron is located in the stand as shown in Fig. 15.

Fig. 20 illustrates a 3-dimensional cu仁away high power klystronwhich produces about 5 MW of peak power and is similar to that used

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Fi肌He 19. (a) Cross-sectional drawing of an elementary two-cavity klystron tube usedas a microwave power amplifier. The two cavities are shown in cutaway sections similar to the section shown in Fig. 18b. The anode is not a single distinct component oftheklystron but constitutes the entire structure exclusive of the cathode. (b) The timing diagram is for the "E" field of the first or buncher cavity. The "E" fields valγsinusoidal

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with a cathode and anode. The magnetron is usually a less costly (buta less stable) microwave power source than the klystron. The magnetron shown in Fig. 21 has cylindrical geometry (circular in crosssection). As shown in the circular cross-section of Fig. 22 , the centralcylindrical cathode is surrounded by the evacuated drift space andthen by an outer anode having twelve cavities. The cylindr:i cal cathode is heated by an inner filament connected to each end of the cylinder, one end of which can be seen in Figs. 21a and b. Circular geometηis characteristic of the magnetron; linear geometry is characteristic ofthe klystron. (Contrast Figs. 19 and 20 with Figs. 21 and 22.)

Fig. 22 is a cross-section made by cutting a slice at mid-depth, paralleI to the surface shown in Fig. 21a. A static magnetic field , H , is applied perpendicular to the plane of the cross-section shown. In addition , a pulsed electric field , Ep, directed radially inward all around,is applied between the central cathode and the segmented anode thatincludes twelve cavities arranged peripherally on the outer circularwal l.

The electrons emitted from the cathode are accelerated by the pulsedelectric field , Ep, toward the anode across the evacuated drift space between cathode and anode. The accelerated electrons induce an additional (+, -) charge distribution shown on the anode poles and an electric field , Em, of microwave frequency between adjacent segments of

Figure 21. Cut-away magnetron of a type widely used in medicallinacs. (a) The cylindrical cathode is surrounded by 12 peripheral cavities of the segmented anode (the cavity on the top is obscured by the filament lead used for heating the cathode). Two smallcoupling loops, just visible in the bottom cavity, connect the microwave power to the output waveguide just below it. The cooling water connections are on the right. (b) The twofilament connections for heating the cathode are on the top; the output waveguide is onthe bottom. A fine-tuning knob is on the right.

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6e. The MagnetronThe magnetron is the microwave source usually employed to power

lower energy 1inacs, typically 12 MeV or less, but occasionally ashigh as 20 Mev. Like the klystron , it is a two-element tube or diode

for the Clinac 18. The electron gun (cathode) of the tube is at the bottom. The center section contains four amplifying cavities separatedby drift tubes; the upper section consists of the water-cooled collectorand output waveguide. This klystron is about one meter in length andsits atop an oil-filled tank with its cathode-electron gun portion submerged to provide the requisite electrical insulation (see Fig. 15). Thecathode is pulsed with a negative voltage of about 120 kV. The fourcavities each have tuning adjustments (Fig. 20b) that provide smallchanges in cavity dimensions , bringing them to the correct resonancefrequency of operation (some klystrons are pretuned at the factory).The buncher cavity nearest the cathode (Figs. 20a and b) is energizedfrom a low power microwave source. Cylindrical current carryingcoils, not shown here, surround the cavities and drift tubes and provide a magnetic field to confine and focus the electron beam traversing the klystron along the axis. The rectangular waveguide conductsthe microwave power pulses out ofthe tube from the output cavity to theaccelerating structure.

Figure 20. (a) Cut-away four-cavity klystron, similar to that employed in the Clinac 18.Views (b) and (c) are cutaway individual cavity sections. (b) Enlarged view of the bot-tom cavity, the input power coupling loop is on the right and a fme tuning device is onthe left. (c) Enlarged view of cavity number three; the fme tuning device has been cutaway in this view

the anode (see Fig. 22) in a manner similar to that in the catcher cavity of the klystron. In ~ddition， the magnetic field, H, imparts a circular arc component to the electrons' motion. Thus, they move in complex spirals , S, under the combined influence of Ep; the magneticfield , H; and the induced microwave electric field, Ern. In the process , approximately 60 percent of the kinetic energy of the electronbeam is converted into microwave energy. Magnetrons almost invariably function as high-power oscillators; that is, originators of microwave power, but klystrons usually operate as amplifiers driven bya low-power oscillator. However, if one feeds back a small portion ofthe output of a klystron to its first cavity, it can function as anoscillator.

The power output of magnetron and klystrons is measured in thousands of watts (kW) or millions of watts (MW). The watt is the unit ofelectrical power; that is, the rate at which electrical energy is expended. A household electric iron or toaster consumes about 1 kW of electrical power. Typically, magnetrons that operate at a frequency of3000 MHz (corresponding to a 10 cm wavelength) provide 2 MW peakpower output during a burst of radiation , although 4 MW to 5 MW versions are available at increased cost. The magnetron need only be energized for one one-thousandth of the time to provide the usual shortbursts of radiation. Thus, the magnetron shown in Fig. 21 operates at

14

2 MW peak power and 2 kW average power output and is widely usedin medical linacs.

The Waveguide and CirculatorMicrowave power is conveyed from the klystron (or magnetron) to

the accelerator structure by a system of hollow pipes called waveguides (Fig. 15). These are either rectangular or circular in crosssection, as shown in Fig. 23. For example, the waveguide between thestand and gantry (Fig. 15) facilitates rotation and involves a shortcircular section between rectangular waveguides. Waveguides replace the traditional electrical wires and cables which are inefficientin transmitting power at microwave frequencies. Waveguides confine microwaves by reflecting them forward off the walls like a hoseor pipe confines water flowing through it. They are pressurized withFreon or sulphur hexafluoride gas which reduces the possibility of

Fi伊re 22. Cross-sectional drawing of the magnetron of fig. 21 showing representativeelectric fields Ep (pulsed) and Em (microwave) , with associated electric charge 仆， -)distributions. At a particular time and place in the drift space, electrons move in typical'paths, S, under the infiuence of the magnetic field which is perpendi∞lar to thecross-section and the sum of electric fields Ep, and Em, which are shown separately.

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electrical breakdown and thus increases their power-handling capacity. Two ceramic windows separate the pressurized waveguide fromthe evacuated klystron at one end and from the evacuated acceleratorstructure at the other end. The windows are transparent to themIcrowaves.

The circulator (Fig. 15), placed in the waveguide between the klystron and the accelerating structure, acts like a one-way sign, permitting traffic to move forward through a street intersection but allowingtwo-way traffic just beyond the intersection. Reflected microwavepower is diverted aside in the circulator and absorbed , similar to theway traffic approaching the intersection from just beyond it can be diverted to a side street. Microwave power is allowed to proceed forwardfrom the klystron and through the circulator to the accelerator; but microwave power that is reflected back from the accelerator structure isprevented from reaching the klystron (or magnetron) where it couldlead to instabilities and damage.

Introduction toAccelerator Structures

A linac accelerator structure (sometimes called the acceleratorwaveguide) consists of a long series of adjacent, cylindrical, evacuated microwave cavities and is located in the gantry as shown in Fig.15. It makes use of the cavity principles for power generation thathave been discussed and applied to klystrons. Here, however, the objective is to transfer energy from the cavity "E" fields to an accelerating electron beam. Typically, medical accelerator structures vary inlength from 30cm for a 4 MeV unit to one or more meters for the highenergy units.

The first few cavities vary in size. They both accelerate and bunchthe electrons in a manner like that of our klystron buncher cavity described earlier. Typically, only about one-third of the injected electrons are captured and accelerated by the microwave "E" field. Asthey gain energy, they travel faster and faster until they reach almost

15

the speed oflight. Therefore, these first cavities are designed to propagate an "E" field with an increasing velocity in order to stay in stepwith the electrons and to further bunch and accelerate them. Latercavities are uniform in size and provide a constant velocity travelingwave, just less than the velocity of light. Initially, electrons gain energy predominantly by increasing their velocity; later, by an increase in their mass since their velocity cannot attain the speed oflight. For example, a 2 MeV electron moves at 98 percent the speed oflight. Its mass in motion is almost five times its mass at rest. Here,we are invoking Einstein's famous mass-energy equivalence concept; that is, increased energy of rapidly moving particles appears asincreased mass.

Accelerator structures are of two types: traveling-wave and standing-wave. The "E" field patterns behave differently in these structures, and are central to understanding the linacs. First, travelingwave linacs will be looked at from this "E" field viewpoint.

8a. Traveling-Wave Accelerator StructuresA hollow, cylindrical pipe, such as the waveguide used for micro

wave power transmission in Fig. 23b, has an "E" field pattern asshown in the cross-section in Fig. 24a. This pattern travels one waydown the pipe from the klystron (or magnetron) faster than the elec-

16

trons can keep up. Hence, a hollow pipe would not be useful for accelerating electrons.

These traveling-wave fields are slowed by "loading" the pipe withwasher-like inserts called disks as shown in Fig. 24b. Now the waveguide pipe has been transformed into a long series of resonant cavities. (Compare Fig. 24 with Fig. 18.) When energized, very high "E"fields which are suitable for electron acceleration (Fig. 24) are developed along the axis.

The microwave cavities of the accelerator structures are constructedfrom copper. Copper is used because of its high heat conductivitywhich improves temperature control and because of its high electricalconductivity which reduces power losses. The accelerator structureshown in Fig. 25 consists of a series of precisely-machined parts:washer-like disks sandwiched between short cylindrical sections.This sequence of disks and short cylinders is assembled on a longspindle for a particular length (energy) structure and soldered together in a furnace. The soldering material is in the form of very thinsilver washers, shown at the bottom of Fig. 25. These are placed between each disk and cylinder junction surface and, when melted,fuse the components together. Once fused , the sections become a rigid,vacuum-tight accelerator structure. Higher energies require morecavities and longer structures.

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也土罗〉Figure 25. Components of an accelerator structure (right and bottom). Short section ofan assembled acceJerator structure (left). (Courtesy of Stanford Linear AcceleratorSection, Stanford, California.)

The process is still not finished , however. The structure must betuned to a single, precise, resonant frequency. Machining of the cavity components is the first step in establishing the correct dimensionfor each cavity. This amounts to a "rough tuning" and results in acrude resonance with most cavities "off-tune." Next, each cavity is"fine-tuned" by mechanically squeezing it to create very small dimensional changes , perhaps a few thousandths of a cm. Then, like afinely-tuned symphony orchestra, they all play the same note. Thecavities now resonate to the same frequency and provide optimal energy gain for the accelerating electrons.

As noted earlier, the electrons are captured and bunched on a moving "E" field , gaining energy by traveling in-step on the advancingelectric wave. At the far end , the residual microwave power is absorbed by resistive material fused to the wall of the last cavity, andnone is reflected. Further detail of how this wave progresses appearsin Fig. 26. The "E" field along the axis varies smoothly in a sinewave pattern , as shown for three sequential instants of time, and thepattern moves smoothly from left to right as time progresses. The solid arrows along the axis denote the instantaneous positions of themaximum positive (to the right) and maximum negative (to the left)values of the traveling electric wave ("E" field). Electrons are accelerated on the negative portions ofthe "E" wave; that is, just to the right

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of the sine wave crest identified by arrows directed to the left. In anyone cavity, the "E" field maximum reverses direction from time t 1 tot 3 (a half-cycle of time) but a given wave crest (direction arrow)travels forward by one cavity from time t 1 to t 2 and again from time t2

to t3 . From this traveling wave , an electron at a corresponding speedwill gain energy in each successive cavity. An early prototype traveling-wave accelerator structure , cut in half along its cylindrical axis ,is shown in Fig. 27. Note that the buncher section on the left incorporates larger and variable aperture sizes and more closely spaceddisks than the uniform section on the right. The input waveguide attaches on the left (buncher) end. Bunchers in contemporary linacsare significantly shorter in length.

A boy surfing on a water wave provides a useful traveling-waveanalogy in Fig. 28a. Here, he is shown riding the forward edge of thecrest, moving in step with the wave traveling to the right. If he slipsbackward over the crest of the wave, he will just bob up and down as thewaves pass under him and he will move slowly, if at all, towards theright. Similarly, electrons move forward on the front of the advancing negative "E" wave or are lost from it (Figs. 28b and c). The waterparticles themselves just go up and down and do not move forward , yetthe wave of ma立imum water particle height travels forward. Similarly, the conduction electrons in the cavity walls are confined to

17

Figure 27. Cut-away traveling-wave accelerator structure; the buncher section is on theleft and the uniform section is on the right.

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Figure 29. Standing命wave principle illustrating forward (f moving to the right) andbackward (b moving to the left) "E" wave maxima at three sequential instants of time.They exist simultaneously in a standing-wave accelerator structure, but are shownhere in two separate structures, for clarity.

moving back and forth between walls of a single cavity, yet the position of the "E" field maximum travels forward as a result of thatmovement as shown in Fig. 26.

8b. Standing-Wave Accelerator StructuresMost present-day medical linacs are of the standing-wave type be

cause the accelerator structure can be much shorter and, therefore, thetreatment unit is less bulky than traveling-wave linacs of comparable energy. Standing wave linacs operate somewhat like the traveling-wave unit just described but with one significant difference. The"E" wave varies in magnitude with time in a sinusoidal manner butthe pattern remains stationary along the axis and does not advancelike the traveling "E" wave or water wave just studied. A good standing-wave analogy is the pattern of a violin string fixed at both endsand vibrating up and down to produce a musical note.

In the case of traveling-wave accelerators, microwave power is fedto the structure via the input waveguide at the proximal (electron gun)end. The residual power is absorbed at the distal (target) end of thestructure. In the standing-wave accelerator the microwave power canbe fed anywhere along the length of the structure, because the powerproceeds in both forward and backward directions from the inputwaveguide and is reflected at both ends. The incident forward wave

is reflected backward from the distal end, and the backward wave isreflected forward from the proximal end. There are now two waves:an advancing incident wave and a reflected wave. These two wavesare reflected back and forth from one end to the other end of the accelerator structure about one hundred times during a five microsecondpulse. The circulator, described earlier, stops reflected power fromreaching and detuning or damaging the klystron or magnetron.

Fig. 29 shows the "E" field maximum values , denoted by arrows forthese two waves at three sequential instants in time t1, t2 , and t3. Theforward wave crests (instantaneous positions denoted by arrows)moving to the right advance one cavity length during the time interval from tl to t2 , t2 to t3 , etc. Similarly, the backward wave crestsmove at the same speed to the left. These sequential movements can beseen by examining each of the two patterns of arrows at the threetimes. Here, the sine wave "E" 丑eld patterns have been omitted andattention is confined to the wave crests denoted by arrows.

The effective "E" field , in accelerating the electron beam , is the sumof the forward and backward waves, as shown in Fig. 30. Its magnitude, assuming 100 percent reflection and no losses, is double that ofeither the forward or backward wave when the fields are in the samedirection. But it is zero when the fields added are in opposite directions. The effective "E" field exhibits a sinusoidal variation with

19

distance along the accelerator structure as shown in Fig. 30. Thecrests of the sine wave pattern oscillate up and down with the progression of time.

Note that eveηr other cavity of this standing-wave structure in Fig.30 has a zero "E" field at its center at all times; at times t1 and t3 because both the forward and backward "E" fields are zero, and at timet2 because the forward and backward "E" field are equal in magnitude but opposite in direction , and cancel completely (see Fig. 29).These zero "E" field cavities are essential in transporting microwavepower but do not contribute to electron acceleration. Their role is totransfer or couple power between accelerating cavities. Because theyplay no role in acceleration , they can be moved off-axis and the lengthof the structure can be shortened.

Fig. 31 illustrates how the shortened , side-coupled standing-wavestructure evolves from the standing-wave structure of Fig. 30. First,evelγother cavity of Fig. 31a, which couples power between accelerating cavities, is shortened in length as in Fig. 31b. Next, they aremoved off-axis as in Fig. 31c. and finally , in Fig. 31d, placed on alternating sides of the axial accelerating cavities. The spatial "E"field pattern shown below each sequential accelerator structure is forthe same time in the microwave cycles. In Figs. 26 and 28b and c, the"E" wave repeats every four cavities and there are four cavities per

wavelength λ. At any given instant, only one of four cavities is accelerating the electron bunch and the other three cavities are "coasting."In Figs. 31c and d, the "E" wave repeats every two axial cavities sothat, at any instant, half of the axial cavities are accelerating the electron bunch , and the relatively lossless off-axis coupling cavities replace half of the cavities of the traveling-wave accelerator; hence, theshorter length and greater efficiency for the standing响wave design.Fig. 32 illustrates in detail how the axial "E" spatial pattern changesin time over a complete microwave cycle for a standing-wave linac.Contrast the time variation of this pattern to that for a traveling-wavelinac in Fig. 26. Note that the "E" field pattern does not advance, butchanges in magnitude and direction with time. We can now optimizethe cavities along the beam axis for acceleration , and the off-axiscoupling cavities for microwave power transport. Fig. 33 is a cutaway view of such an optimized standing-wave accelerator structure.This is called a bimodal or side-coupled accelerator structure. Twostanding-wave accelerator structures constructed in this way areshown in Figs. 34 and 35. They are shorter in length than a traveling-wave structure for a given energy gain and a given klystron ormagnetron power.

Electrons injected into standing-wave structures, such as those illustrated in Figs. 34 and 35, are captured, bunched and accelerated in

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=二==匠 "ElO /OIl...别.. 9U \JFigure 31. Evolution of side-coupled standing-wave accelerator structure from a linearstanding-wave structure and related "E" wave patterns along the axis. The"patt怡ern below each s创tru飞咀u冗ctωUl陀吧 shows the spatial field along the axis at the same tim丑le in them丑II刊crowave cycle.

Fi♂lTe 32. Asequential look at the axial standing-wave "E" field pattern for one fullmicrowave cycle oftime for the bimodal structure depicted in Fig. 31d. Note that the "E"field pattern does not advance, but changes in magnitude and direction with time.

21

provide a 6 MV x-ray mode , an 18 MV x-ray mode , and an electronmode with energies of 6, 9, 12, 15 and 18 MeV. For patients benefitingfrom treatment in more than one mode, greater precision of patientpositioning can be obtained because the patient does not have to bemoved from one treatment couch and treatment room to another whenreceiving radiation. There are also cost savings in having the fullrange of radiation modes within only one treatment room and machine instead of two , such as for radiotherapy departments havingmodest volume. Dual x-ray energy mode linacs have become available using either a standing wave or traveling wave acceleratorstructure.

ga. Standing WaveIf, in order to reduce beam energy (e.g. , from 18 to 6 MeV), the mag

nitude of the accelerating "E" field is reduced in the second portion(output end) of an unmodified standing-wave accelerator structure,the magnitude of the "E" field drops correspondingly in the first portion (凯III end). Similarly, if the phase of the sine wave "E" field isshifted in the second portion, it shifts equally in the first portion. Because the RF power is reflected back and forth in a standing-wavestructure, the first portion senses and adjusts to the field in the secondportion , and vice versa. If the amplitude of the "E" field was correct

Dual X-Ray Energy ModeLinacs

BEAMCHI‘NNEL

the first few cavities, just as in the traveling-wave accelerator. Theypass through the following cavities during the "E" wave's negativeexcursion and are accelerated (recall that electrons are accelerated inthe opposite direction of E). During that time, the "E" wave of the nextadjacent cavity is positive and electrons are not accelerated in it.However, as the electron bunch crosses the boundaηbetweenadjacentcavities, the "E" wave in the next cavity starts its negative excursionand the electron bunch is again accelerated. Each cavity accelerateselectrons only when its "E" field is negative. This process continuesuntil the electrons acquire their final energy.

In recent years there has been a move toward use of dual x-ray energy mode linacs in radiotherapy. For example, such a machine might

COUPl iNGCAVITY

Figure 33. Cut-away of a bimodal or side-coupled standing-wave accelerator structure.The accelerating cavities are shaped for optimum efficiency. The coupling cavities arestaggered to reduce asymmetries introduced by the coupling slots. (Courtesy of Los Alamos Scientific LaboratoJγ ， Los Alamos, New Mexico.)

Figure 34. Cut-away of a standing-wave linac structure. Electrons attain an energy of4 MeV in this 30 em long structure having five accelerating cavities. The input waveguide is on the bottom, the electron gun attaches on the left and the x-ray target is permanently sealed into the structure on the right end. (Clinac 4 , courtesy ofVarian Associates.)

in the first portion (for optimal capturing, bunching and positioningof electrons injected from the electron gun on the "E" wave crest foracceleration through the remainder of the structure) e.g. 18 MeV, thenthe amplitude of the "E" field is reduced for acceleration to e.g. 6MeV. In present dual x-ray energy linacs there are two fundamentally different ways to modify standing-wave accelerator structures toeliminate or reduce this problem of otherwise incorrect electron capture and bunching and positioning in low x-ray energy mode:1). Change the ratio of RF power fed to the first and second portions ofthe standing wave accelerator structure. This can be done by use of acompact energy switch in a side cavity located between the first andsecond portions. In one position of the energy switch, the side cavityprovides high coupling between the first and second portions, creatinghigh amplitude of "E" field in both portions of the accelerator structure for high x-ray energy mode. In the second position of the energyswitch, the side cavity provides low coupling, creating low amplitudeof "E" field in the second portion of the accelerator structure for low xray energy mode. In either position of the energy switch, the sameamplitude of "E" field is maintained in the first portion of the accelerator structure in order to maintain optimal capture and bunching ofthe injected electrons from the gun, positioning the resulting bunch onthe crest of the "E" wave in the second portion of the accelerator struc-

~

Figure 35. Linac standing-wave structure. The input waveguide is on the top just rightof center. Water cooling tubes, which are soldered to the structure, can also be seen.(Clinac 18, courtesy of Varian Associates.)

tur飞 This results in an electron beam with a narrow energy spreadand stable energy, hence high transmission through the bendingmagnet and minimal leakage radiation from electrons lost beforereaching the x-ray target. This provides high dose rate and a stable xray beam in both low and high energy modes. The energy switch employs a moving part, a plunger which was unreliable in some earlymachines. Instead of an energy switch, a high power microwave circuit can be used, employing a power divider and a phase shifter. Suchsystems are quite bulky and employ many moving parts.2). Use of a broad band buncher in a standing wave accelerator structure. In the first portion of such an accelerator structure, (the buncher), the cavities are made very short. Also, the coupling slots to theside cavities may be small to reduce the amplitude of the "E" field inthis first portion. There is no energy switch, so the amplitude of the"E" field is one value for high energy x-ray mode and a much lowervalue for low energy x-ray mode, throughout the accelerator structure.

Because the initial cavities are so foreshortened, the electrons injected from the gun are captured and bunched around a position veryfar forward of the crest of the accelerating "E" field sine wave. Following this bunching section there is one especially long cavity. Inpassing through this cavity the electron bunch slips backward relativeto the "E" field sine wave to near (but not on) its crest, for acceleration

through the rest of the accelerator structure; ahead of the crest in a high"E" field for high x-ray energy mode; behind the crest in a low "E"field for low x-ray energy mode. This technique avoids use of a mechanically moving part, namely the plunger in the energy switch.However, it is wasteful of RF power, requiring a higher power klystron or magnetron , and the off-crest acceleration produces an outputbeam with larger energy spread and greater energy instabilities.This makes it more difficult to obtain high transmission of the electron beam through the bending magnet to ensure a high dose rate flattened fully to the corners of large fields in low energy x-ray mode ,with stable dose distribution over all gantry angles.

9b. Traveling WaveIn traveling-wave accelerator structures without RF feedback

through an external circuit, the first portion does not sense the field inthe second portion because the wave travels only forward. The amplitude of the accelerating "E" field can be changed in the second portionof the accelerator structure without significant effect on the captureand bunching properties of the first portion. One way of producing adownward taper of this "E" field from first to second portions is bybeam loading, simply increasing injected beam current from the gunand ke-eping the klystron or magnetron power constant_ Because the

RF power is being transferred to the high current electron beam, a progressively decreasing fraction of the RF power flowing through theaccelerator structure is left to produce the "E" field in the cavities ofthe second portion. Also, the phase of the "E" field sine wave can be tapered from first portion to second portion , simply by changing the frequency of the klystron or magnetron; the electron bunch then slips inphase over the "E" field sine wave, receiving less than maximal acceleration. As pointed out in Section 8b, traveling-wave structuresare much longer than standing-wave structures for the same input RFpower, beam energy and beam current. Such long accelerator structures for dual x-ray mode accelerators can be accommodated moreeasily in a drum-type gantry because the accelerator structure canproject through the drum bearings which support the rotating gantryfrom the stand. External RF feedback from second portion to first portion is used for some traveling-wave structures in order to improvefrequency stability (so that a magnetron can be used in a high energyaccelerator) and in this respect the first portion senses the field in thesecond portion. Variable coupling is used in such feedback circuits tomaintain the same accelerating "E" field in the first portion both inhigh x-ray energy mode with light beam loading and in low x-ray energy mode with heavy beam loading.

Because the frequency and beam stability of traveling wave struc-

2'3

higher energy component is deflected through a loop of larger radius.The important property of the achromatic magnet is that these components of energy are brought back together to the same position , angleand beam cross-section at the target, as they were when they left the accelerator structure. The achromatic focusing property is analogous toan achromatic camera lens wherein the different colors(wavelengths) of light from the object are focused to the image on thefilm. Thus, a 3 mm diameter beam out of the accelerator is reproduced as a 3 mm diameter beam at the target. In the singly achromaticmagnet of Fig. 36, a variation in beam energy will result in a changein the angle of the beam at the target, producing angular x-ray fieldasymmetry, even though the focal point position stays fixed. To correct for this angular dependence on energy, modern magnets aredoubly achromatic. The mean energy of the beam can vary withoutchanging the mean position or angle of the beam at the x-ray target,hence, maintaining symmetry of the treatment field. This small xray focal spot will help ensure that the x-ray treatment fields will havesharply defined edges (i.e. , a small penumbra), a feature which is ofassistance in treatment. This feature improves uniformity of radiation of the tumor, and spares nearby critical organs. Medium andhigh-energy accelerators employ bending magnets. However, manylow-energy units have straight-through beams without bending mag-

tures and magnetrons is inherently less stable than standing wavestructures and klystrons , such linac designs rely more heavily onelectronic feedback , such as with computer look-up tables, to maintaintreatment beam stability.

Bending Magnet

The electron beam leaving the accelerator structure continuesthrough an evacuated bending magnet system. It is deflected magnetical1y so as to either strike a target for x-ray therapy or to exit throughthe treatment head, via a thin metallic window, for electron therapy.Note the location of the bending magnet in Fig. 15.

The bending magnet deflects the beam in a loop of approximately2700 (Figs. 36 and 37). This magnet configuration provides desirableachromatic focusing properties. As shown in Fig. 36, the lower energy component is deflected through a loop of smaller radius and the

24

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Figure 36. A simplified achromatic 2700 beam-bending magnet with focusing properties as shown. The magnetic, H, is perpendicular ωthe plane of the electron orbits. Afull achromatic magnet includes additional angular and spatial focusing propertiesnot shown here but described in the text. For example, it also provides transverse focus ,that is, focusing in a plane at right angles to that shown. (Cou 此esy of Physics and Medicine in Biology, Vol. 18, pp. 321-354, 1973 and C.J. Karzmark, Ph.D.)

nets. This is because these accelerator structures can be made shortenough to be vertically mounted and still allow isocentric rotation.

Treatment Heads

The treatment head (Fig. 37) contains a number of beam-shaping,localizing, and monitoring devices. The high-energy x-rays emerging from the target are forward-peaked in intensity, being of higherintensity along the beam central axis and of progressively less intensity away from it (see Fig. 38). The forward-peaked x-ray lobe is flattened to provide uniform treatment fields. This is accomplished bythe flattening filter, a conical metal absorber, placed on the axis asshown.

The dual ionization chamber system samples the radiation beam (xrays or electrons) passing through the treatment head and produces

electrical signals that terminate the treatment when the prescribeddose is given. Two independent ionization chamber channels ensurethat the prescribed dose is delivered accurately and safely; one serving as a check on the other.

The field defining light simulates the x-ray field and facilitatespositioning the patient for x-ray treatment. It provides an intenselight field , duplicating in size and shape the x-ray field incident onthe patient as defined by the collimators or other beam-limitingdevices.

A range finder light projects a numerical scale on the patient's skinto define the source-skin distance (SSD) from 80 em to 130 em.

The x-ray target is retractable and is moved off-axis for electrontherapy.

Additional details of the treatment head beam subsystem for x-raytherapy are shown in Fig. 38. A primary collimator limits the maximum field size for x-ray therapy. The effect of the flattening filter onbeam uniformity is shown. The forward peaked x-ray beam has beenflattened. Treatment field size is defined by the secondary collimatorconsisting of four thick metal blocks, often made of tungsten. To helpprovide sharp edges for treatment fields , the movement of the blocks isconfined to arcs so that the block faces present a flat edge to the beamdiverging from the target. They are adjustable in pairs and, in some

25

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linacs, provide rectangular treatment fields as large as 40 cm x 40 cmat one meter from the target. The secondary collimator rotates aboutthe beam axis, allowing angulation of fields. Accessories to modifythe emergent x-ray field externally, such as wedges , tissue compensators , individually shaped apertures and shadow blocks , may bemounted on trays that slide into slots of an accessory mount attachedto the treatment head.

Additional details of the treatment head subsystem for electronbeam therapy are shown in Fig. 39. The x-ray target is moved out ofthe beam and a thin scattering foil replaces the flattening filter onaxis. A rotating carousel facilitates the latter exchange. The scattering foil spreads out the small, pencil-like beam of electrons and provides a flat uniform electron treatment field. For electron therapy adetachable electron applicator is attached ωthe accessory mount of thetreatment head. Field definition is provided by a removable aperturelocated at the end of the applicator close to or in contact with the patient's skin. In addition, the secondary x-ray collimator is set to afield size somewhat larger than that defined by the applicator. Insome linacs the small, pencil-like beam of electrons emerging fromthe accelerating structure is scanned in a television-like raster pattern to achieve uniformity over the electron treatment field.

26ELECTRON BEAM:

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RetractableBeam Stopper

In most cases the major portion of the treatment beam is absorbedin the patient; the remainder continues on through. This exit beam,which emerges from the patient, spreads out (to widths of a few meters)on the walls, ceiling and floor of the treatment room. Concrete barriers of approximately two meters thickness are needed to reduce thex-ray intensity and protect personnel outside the room from these direct beams. The extra barrier thickness requirement in the regionexposed directly by the beam can be_reduced significantly by use of abeam stopper (Fig. 40a). The beam stopper, constructed of steel andconcrete, absorbs 99.9 percent of the incident radiation. As a result,only the leakage and scatter radiation need be shielded and a concretebarrier of more uniform thickness for all walls will then be sufficient, thus sir呻lifying room construction and also saving space. AIthough use of a beam stopper reduces barrier thickness requirements,

Figure 40. Retractable beam stopper (Clinac 18). (a) Fully extended. (b) Retracted.(Courtesy of University of Arizona, Tucson, Arizona, and Varian Associates.)

access to the patient being readied for treatment is more restricted unless it can be retracted as in Fig. 40b. The beam stopper is fully extended by motor control prior to treatment and is interlocked to prevent treatment when it is not in position. The treatment unit illustrated in Figs. 1, 2, and 15 incorporates a counterweight instead of a beamstopper and would be installed in a room of sufficient wall thicknessto protect personn~l.

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A number of auxiliary systems are essential for operation, control ,and monitoring of the linac treatment unit. These systems, togetherwith the major components described earlier, are shown in Fig. 41; afunctional block diagram. The modulator cabinet and control console, shown on the left, are located outside the treatment room; thestand, gantry, and treatment couch, shown on the right, are inside.

The modulator cabinet may be placed inside the treatment room insome installations.

The modulator cabinet contains a pulsed power supply, as shown inFig. 41 , which energizes the klystron and the electron gun when triggered by a timing pulse (Fig. 42a) from the control console. Thepulsed power supply provides a 120 kV pulse of approximately 5 microseconds duration to the klystron which generates the microwave power, and a similar 18 kV pulse which speeds electrons from the electrongun into the accelerator structure (Figs. 42b, c, and d). The timingpulse rate, which is set by the technologist, provides a convenientmethod of varying the linac output dose rate.

Electrons are injected into the structure on axis from the electrongun as shown in the upper left of the gantry in Figs. 15 and 41. Thegun is pulsed with a negative 18 kV pulse. As a result, electrons enterthe cavities with about 18 keY of energy and a velocity approachingone-fourth the speed of light.

The vacuum system provides the extremely low pressures needed foroperation of the electron gun, accelerator structure, and bending magnet system. Without a vacuum , the electron gun would rapidly "burnout," like a light bulb filament exposed to air. In addition , the accelerated electrons would collide with air molecules, deflecting them andreducing their energy, and the small, pencil-like beam of electrons

27

STAND

TR EATMENT CDUCH

Figure 41. Block diagram of a high energy bent-beam medicallinac. Major components, auxiliaη， systems and interconnections are identified. (Clinac 18, courtesy ofVarian Associates.)

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Figure 42. Timing diagram showing major time/event relationships for two sequential bursts of radiation. Note the two different abscissa time scales, one in microseconds (μsec)， the other milliseconds (msec)

would be diffused and broken up. The vacuum is maintained by anelectronic ion pump. It was this latter development, more than anyother, that transformed the linac from a laboratory instrument into apractical clinical tool. Earlier accelerator vacuum systems involvedoil-based rotary and diffusion pumps which required significantmaintenance.

The pressure system pressurizes the waveguide with Freon or sulphur hexafluoride gas.· This is needed to prevent electrical breakdown from the high power microwave electric HE" fields.

A cooling system, providing temperature-controlled water, establishes the operating temperature of sensitive components and operatesprimarily to remove residual heat dissipated in other components.Temperature control is particularly critical for the accelerator structure itself. Without it, the series of cavities comprising the acceleratorstructure will change dimensions slightly. The effect of this is t。

"detuneH them in the same way a musical instrument changes itspitch; they are then "off-frequency," and their acceleration capabilityis seriously impaired.

An automatic frequency control (AFe) system continuously sensesthe optimum operating frequency of the accelerator structure to maximize radiation output. It uses this information to "tune" the klystronor magnetron to this microwave frequency.

28

An elaborate monitor and control system maintains control of linacoperation and patient treatment. It monitors operation to assure proper linac performance and to ensure that the prescribed treatment isfaithfully delivered in a safe manner. Deviations, depending ontheir nature and magnitude, will give rise to fault warning signals ortermination of the treatment, when appropriate. The center of thismonitor and control function is at the control console with connections to all other units. The control console provides status information on treatment modality accessories in use, prescribed dose anddose delivered, interlock status, emergency off, as well as other datapertinent to linac operation and patient treatment. Frequently, themonitor function is directly linked to the control function and currentstatus information is used in a feedback manner to maintain optimalperformance.

A multitude of quantitative and procedural checks are incorporatedin the console to assure correct, safe operation. The digital logic circuits used in modern computers are the basis for these checking procedures. They can be carried out in a few seconds and are assessed automatically, prior to each treatmen t.

A counting system, tied to the dose monitor, terminates the treatmentwhen the preset dose monitor prescription is delivered. An intervaltimer is set to terminate treatment in the event of dose-monitor fail唰

ure. The technologist monitors the treatment both visually and aurally. A closed-circuit TV system provides visual contact, and a twoway audio system facilitates instant communication with the patient.

Operational ReviewThe patient is positioned on the treatment couch; the gantry angle ,

collimator angle , field size, and treatment distance are set. Accessory beam-modifying devices such as blocks or wedges are attached andpositioned. The technologist then proceeds to the control console andpresets the controls and dose monitor to deliver the dose prescriptionfor that treatment field. The technologist must select the treatmentmodality: electrons or x-rays. If electrons are selected, the energymust also be selected. In typical medical linacs, electrons used diredly for treatment have energies from about 3 MeV to 35 MeV. However, in the case of x-ray selection , a particular medical linac has

only one energy, although it may be anywhere from 4 MeV to 35 MeV.For example, the Clinac 18 allows electron selection of 6 MeV, 9 MeV,12 MeV, 15 MeV, or 18 MeV, and produces beams of 10 MV x-rays.

Before the treatment begins, however, an internal check system isautomatically activated which sequentially verifies linac operatingparameters for correct values. In many units a method for testing thedosimetry system is used to ensure that the prescribed dose will be delivered. The treatment may also include a computer-based recordand veri句r program, which compares the treatment that has been set upwith a record of the intended treatment (Fig. 4). This treatment prescription assessment may include field size and collimator angulation , gantry angle , couch position, the daily dose for that field, and thepreset monitor readings to provide the dose. Such record and verifyprograms identi命 setup errors prior to treatment so that they can becorrected when they exceed a preset magnitude; for example, morethan one degree of arc. Such programs veri马r and record each treatedfield on a continuing, daily basis throughout the course of treatment.

Typically, the linac is pulsed several hundred times per second,with the exposure for each treatment field lasting a few minutes.When the 飞earn on" button is pushed, an elaborate sequence isinitiated, in part, as described by the timing diagram of Fig. 42. First,the modulator accumulates energy for the first pulse of radiation. It

m

sends out two high voltage pulses in unison: one to give the electronsleaving the electron gun their first boost of energy as they enter the accelerating structure (Fig. 42d), the other to energize the klystron (Fig.42b). The klystron then delivers the microwave power to the accelerating structure (Fig. 42c) and, in turn, to the electron beam emergingfrom the electron gun. Here, the intense "E" fields come into play:bunching the electrons and accelerating them to their final energy.

The electron beam next traverses the bending magnet and is directed on the x-ray target, or scattering foil , in the case of electron therapy. The emerging cone of radiation traverses the two monitor ionization chambers and is further shaped by the collimator and other beamshaping devices.

Fig. 42 summarizes pertinent time relationships for two sequentialbursts of radiation. In this diagram of idealized timing, the linac ispulsed every 5 milliseconds, that is, 200 times per second. The timingpulse that initiates each sequence is very short, and all other pulsesare of about 5 microseconds duration. During this 5 microsecond interval , 15,000 complete microwave cycles occur (3 ， 000/μsec x 5). Thismicrostructure is also present in the radiation burst (Fig. 42e) but thetiming details have been omitted for simplicity.

m

BibliographyKarzmark, C.J. and N.C. Pering. "Electron Linear Accelerators for

Radiation Therapy: History , Principles and ContemporaryDevelopments." Physics in Medicine αnd Biology 18:321-354(1973).

Karzmark , C.J. "Advances in Linear Accelerator Design forRadiotherapy." MedicαI Physics 11(2):105-128, 1984.

Kramer，丘， N. Suntharalingam, and G.F. Zinninger, Eds. HighEnergy Photons αnd Electrons: Proceedings of αn' Internαtionαl

symposium on the Clinical Usefulness of High-Energy Photons αnd

Electrons (6-45 Me叨 in Cαncer Mαnαgement. Thomas JeffersonUniversity, Philadelphia, Pa. , May 22-24, 1976. John Wiley andSons, N.Y. (1976).

Tapley, Norah duV, Ed. Clinicαl Applicαtions of the Electron Beαm.

John Wiley & Sons , N.Y. (1976).The Use of Electron Lineαr Accelerαtors in Medical Rαdiation

Therαpy: Physical Chαrαcteristics. HEW Publication (FDA) 768027 (1976).

Varian's CLINAC 18 features a standing-wave accelerator structUI飞It provides a beam of 10 MV x-rays and electrons from 6 MeV to

31

18 MeV in five discrete steps. It delivers a flattened x-ray beam at adose rate up to 500 rads/minute at 1 meter target-skin distance, forfield sizes continuously variable from 0 cm x 0 cm ω35 cm x 35 cm.Arc therapy is available whereby the treatment is delivered as thegantry rotates slowly through a defined arc. For this modality, precisely controlled dose rates are programmable from 0.5 rads to 5.0rads per degree.

The electron treatment beam covers field sizes up to 25 cm x 25 cm at1 meter at similar dose rates for energies of 6 Mev, 9 MeV, 12 MeV, 15MeV and 18 MeV. A number of fixed field size electron applicatorsare provided. The accelerator is a 1.4-meter standing-wave structure,powered by a 5 megawatt klystron tube. The isocenter is 130 cm abovethe floor. The treatment couch is equipped with motorized, continuously variable speed control of vertical , longitudinal and transversemotions. All gantry and couch motions are controlled from a singlehand-held pendant to facilitate fast, precise setup by one technologist.An electronic check of the dosimetry system is made prior to each irradiation. A retractable beam stopper is available as an option. Manyhuman engineering features , which increase its versatility and easeof operation by trained technologists, are incorporated in the design.

Varian's newest product development is the C-Series family of accelerators. It is a complete line of computer-controlled accelerators

Varian's CLINAC 2100C Linear Accelerator.

AppendixRepresentative Linac Treatment Units

Descriptions' and photographs of a variety of medical linear accelerators have been solicited from various manufacturers. The manufacturer is solely responsible for the information provided. This sectionis provided for the education of the student and is not an endorsementof any product by the editors or publisher.

Varian Associates, Inc.611 Hansen WayPalo Alto , CA 94303

For X-ray energies a number of options are available as alternatives 臼 those stated.

Essential features are: drum gantry construction, low isocenter,dual foil electron scattering systems, automatic prescription entry,computer control system and ease of service. Electronic componentsare mounted on the gantry to give extra space in the treatment room.

notes the energy.The SL75/5 is a single X-ray energy accelerator which may be set to

4, 5 or 6 MV. The gantry is of the C-arm type with a 900 bending spectrometer bending system. The system is microprocessor controlled.

The SL series is a range of computer controlled accelerators. Theseries is based on a common design configuration and comprises thefollowing models all with dual X-ray energies and multiple electronenergies. To suit different use requirements the energies are set asfollows:

and simulators which provide the radiotherapy community with themost advanced tools available for treating cancer. The C-Series Clinacs are:τ'he Clinac 600C combines advanced micro-processor control with thereliable , straight-through accelerator design used in Varian lowenergy machines.The Clinac 2100C is literally two accelerators in one. Its dual x-rayenergy standing-wave accelerator provides multi-modality treatments with a choice of x-ray combinations from 6 to 18 MV and fiveelectron beams energies ranging 企om 4 to 16 MeV or 6 to 20 MeV.四le Clinac 2500C offers two widely separated x-ray energies of 6 MVand 24 MV, as well as six electron energies ranging from 6 MeV to 22MeV, giving you the most powerful and penetrating medical accelerator available today.

Philips Medical Systems Linear Accelerators710 Bridgeport Ave.

Shelton, CT 06484

Philips Medical Systems manufactures two types of linear accelerators. They are prefixed with SL and a subsequent number which de-

ModelSL15SL18SL20SL25

X-ray energies (MV)6+106+156+186+25

Electron energies (MeV)4, 6, 8, 10, 12 + 154, 6, 8, 10, 12, 15 + 184, 6, 8, 10, 12, 15, 18 + 204, 6, 8, 10, 12, 15, 18,20 + 22

32

Philips Medical System's SL75/5 Linear Accelerator. A Philips Medical System's SL Series Linear Accelerator.

There are several design differences between Philips and othercompanies' accelerators; the following are of particular interest.Traveling waveguide structure and spectrometer bending system.

All Philips accelerators have a high efficiency traveling waveguidestructure for high dose rates and maximum versatility. The low vacuum requirement leads to easy replacement of vacuum componentsand rapid return to clinical use. The spectrometer type bending systerns , 90° for the SL75/5 , and "3-in-line" Slalom , for the SL series,give very accurate control of energy.Demountable Electron Guns.

All Philips accelerators are fitted with easily removable electronguns and have quickly replaceable filaments. This is a major costand time saving advantage when compared with accelerators whichrequire an entire sealed accelerating waveguide assembly to be replaced following a failure in any of its components parts.1∞ cm Source Axis Distance· 1251118 cm Is倒到自由icHeight.

Philips accelerators are designed for ease of use. The low isocentricheight of 125 cm above floor level (118 cm in the case ofthe SL75-5) enabIes operators to.work at a convenient level without the need for stoolsor other objects in order to see patient alignment aids. This is a critical factor when trying to improve throughput and accuracy.Applicaω，rAccessories.

Philips provides a complete range of electron applicators which haveunique hook and latch mechanisms to facilitate mounting and removal. On the SL series, individual endframes may be encoded foreach patient and can be included for verification in the patient prescription. This encoding and verification also applies to the use of individual shadow trays.Automatic Wedge Filter System (SL7515 and SL series).

The microprocessor control console or computer control systemsused with all Philips accelerators enable operators to select the precisewedge angle required by automatically combining a 60° wedged fieldwith a normal field. By varying the dose with and without the wedge,any wedge angle from 0 to 60° can be obtained without the operator having to insert individual wedges manually.Con位。，1 Systems (SL series).

The control system of the SL series is computer based and forms anintegral part of the linear accelerator, controlling both patient treatment management and machine performance.

The patient prescription, which is stored within the system, includesnot only gantry angle, field size , radiation modality, energy and dosebut also details of the planned series of treatments which can be customized to match existing department practice.

With the prescription stored in this manner, the operator has only to

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identify the patient to call up the complete prescription. This is automatically set in the system and greatly assists patient throughput byreducing data entry.

Verification is a standard feature and only when the patient set-upis in accordance with the prescription, is treatment commenced.

The "Vericord" option on the SL series incorpora:tes a dose recording system which provides a hard copy record of treatment delivery together with positive patient identity which can be read by bar codereader.Slalom Beam Bending System

The slalom system consists of 3 in-line beam bending magnets.These are positioned along the evacuated flight tube carrying the electron beam as it leaves the accelerating waveguide.

The first electro-magnet acts as an energy analyser and turns thepencil beam of electrons through an angle of about 45°. It has shapedpole pieces to produce a dispersed beam having a spectrum of energieswith high energy electrons on the one side and low energy electronson the other.

The strength of the magnetic field is adjusted to transmit the desiredmean energy electrons. Complete control of the energy is achieved byenergy sensors which continually monitor the energy dispersed beamin the high and low energy positions.

34

The second converging electro-magnet reverses the 45° deflectionand its pole pieces are shaped to start the focusing action in two orthogonal directions as the electrons enter the third magnet which directthem into the target or window.

The third electro-magnet turns the electron beam through an angleof about 112° and also has shaped pole pieces to complete the two dimensional focusing action started by the second magnet. The electronsare focused on a small area of the target approximately 2 mm indiameter.

The Philips system thus provides a very small diameter beam ofelectrons which is positionally fixed and inherently stable.

Asea Brown Boveri, Ltd.CH-5401 BadenSwitzerland

ABB's DYNARAY linear accelerators all have the same mechanical design with a drum-type gantry. The only difference betweenthem seen from the treatment area is isocentric height, the multimodality models being at 127 cm and the single photon energy modelat 122 cm. The accelerating structures are of the traveling wave typewith replaceable cathode. There are three basic models with the desig-

Asea Brown Boveri's DYNARAY Linear Accelerator.

nations LA6, LA16 and LA20. The LA6 is a magnetron driven , photononly linac with an energy between 4 and 6 MV. Both the LA16 andLA20 are equipped with a klystron RF source, the LA16 having a single photon energy and five electron energies and the LA20 dual photonenergies and seven electron energies.

The beam bending system in all cases is 270°. The features of thetreatment head are an advancement on anything previously available. It is fully motorized and continuously rotatable in either direction. The distance from the focus to the face of the head is only 52 em,leaving adequate working space between the head and the isocenter.The maximum field size of·40 x 40 cm is achieved without any rounding of the corners. Asymmetrical fields are optionally available onthe LA6 , standard on one pair of jaws to 10 cm beyond central beam onthe LA20. The basic capability of asymmetry on both pairs of jaws andwith a further option of 15 em beyond central beam is available on allmodels.

The head movements , as is the case with all relevant movements onthe accelerators, are prepared for a second verification channel for theimplementation of dynamic therapy techniques. An automatic wedgesystem is standard equipment, permitting any wedge angle between0° and 60° to be set. The wedge does not obstruct the light field. Theanti-collision system operates capacitively in the manner of a prox-

imity switch and any accessories attached to the head are automatically included in the system.

ABB Dynaray linear accelerators are controlled by a rugged microprocessor, which has proved itself in many arduous industrial applications and environments. The settings for treatment are entered atthe console in twO' logically arranged tables , one for dose , dose rateand other beam parameters and the second one for the geometricalparameters. The linac status and the operation of interlocks are alsosignalled on the screens. Assisted set-up is a standard facility. Geometrical parameters are shown in large characters on the screens inthe treatment room to 0-I° or 0-I em.

The Dynaray micro-processor also provides many aids for the physicist and service engineer. By operating a key-switch a series of service screens can be viewed, which provide information on all aspectsof linac operation.

The linacs are supplied with a fully motorized treatment table, having extremely wide ranges of movement and yet requiring a floor pitof only 30 em.

-11i1lll

35

Siemens Medical Systems, Inc.70 Jackson DriveCranford, NJ 07016

Siemens produces a product family , the MEVATRON, unique for itsmodularity, high-level of structural and component commonalityand compactness. Both magnetron and klystron driven models feature dual photon and electron beam capability. The table below exhibits the variety of models available as a result of the basic buildingblock approach.

The 6740, with its electron beams, is a clinical workhorse for head,neck and breast treatments and up to 80% or more of the typical clinical cases. The MD models provide, in a single unit at modest price,the capability to cover 90% to 95% of all clinical cases. The KD!KDSmodels are used in a variety of clinical environments and can beusedωtreatthe entire spectrum of cases.

The product line also features two types of control technology, thatprovide the users further choices and flexibility to achieve the best fit totheir particular clinical emphasis. The choice is available of hardwired, IC-chip control technology or microprocessor-based, multitasking operating system that employs instructional code embeddedin firmware. The latter is designed to provide for more sophisticated

m

Nominal Beam EnergiesSlnagl|e and Electron

Dual Photon Unit Low High Energys)Basic Groups 丁ypes Photon Photon (6 step

6300 4MVMCLASS 6700 6MV

6740 6MV 5-12 MeV

63-6700 4MV 6 MVMD CLASS 67-7445 6 MV 10 MV 5-14 MeV

67-7745 6 MV 15 MV 5-14 MeV

67-7460 6 MV 10 MV 6-18 MeVKD S CLASS 67-7760 6MV 15 MV 6-18 MeV

67-7860 6MV 18 MV 6-18 MeV

67-7467 6MV 10 MV 6-21 MeVKD CLASS 67-7767 6 MV 15 MV 6-21 MeV

67-8067 6MV 23 MV 6-21 MeV

Available models of Siemen's MEVATRON series of Linear Accelerators.

control functions and the possibility to add new functions , or makechanges and upgrades as therapy practice evolves. For either type,beam performance characteristics are the same , as well as the modular and commonality features across the product line.

Commonality of system logic, operator interface and accessory systern give the clinical advantage to the Mevatron system to reduce delays and errors. The well-designed and optimized control logic forthe accelerator and couch, as well as for the control console make possible the handling of unusually high daily patient loads. Other features provided include independently adjustable photon collimators tobe used as beam splitters, independently programmable radiographicsetup, low dose rate photon output (for total body irradiation) and massdose selection for electron beam output.

All Mevatrons use a 2700 triple focusing bending magnet to place asmall beam spot on target or scattering foil , and to assure that its position , size and angle remain stable. Photon beams are producedthrough bremsstrahling production from a thin target. The use of athin target reduces the photon yield, but provides (with the use of additional beam conditioning elements) a beam with higher than typicalaverage photon energy, or stated otherwise, with fewer low energy("soft") photons. The result is a build-up profile in the depth dosecurve that is consistent on- or off-axis, with or without wedges or other

Siemen's MEVATRON Linear Accelerator.

beam modifying devices. The same properties also limit the shift ofDmax under the same conditions and also limit the shift from large tosmall fields. The profile of the attenuated depth dose curve is scarcelyaltered.

Electron beams are produced by scattering twice. A primary foil ,different for each energy, provides the first scattering. A fixed ,shaped secondary foil placed further downstream scatters the beamagain. The result of this technique is to yield a beam at isocenter thatis flat over a large diameter. Inserting the applicator merely blocksout the beam without affecting the depth dose curve characteristics,and further blocking of the beam (even off-axis) has little or no further impact. Other benefits of this approach are to reduce energy lossof the primary beam, reduce energy straggling (which gives a steepfall-off in depth dose profile) and to reduce x-ray contamination.

For both electron and photon beams, as well as for dual photon models, the energy of the accelerator beam is changed by varying the highvoltage applied to either magnetron or klystron. RF power output varies approximately linea盯r甘 with this change. Injected beam current iscontrolled through a grid on the tungsten matrix gun. The correctcombination of RF power, injected beam and bending magnet current, all controlled through independent pre-set values for each mode,with the correct pulse repetition frequency provide a controlled dose

rate for treatment.Dose rate is controlled through a servo system that varies the pulse

repetition frequency in a way to maintain constant output. Each beampulse amplitude on target is held constant. In this way dose chamberresponse remains constant over a wide range of dose rate , so that linearity and saturation effects do not occur.

An array of safety interlocks protects against an improper setup ofbeam producing elements for any mode selected, and beam sensinginterlocks protect against excessive beam even if an improper setupoccurred. The dose per pulse and dose rate must remain within narrow limits or an interlock will be activated. In addition to beam symmetry interlock (primarily a protection against positioning errors ofbeam elements or beam wander), a beam flatness interlock protectsagainst a variety of effects, such as incorrect energy, incorrect flattening filter, incorrect scattering foil , or incorrect mode.

During operation, the real-time operating parameters of the accelerators can be remotely monitored through a phone line modem conn肘，

tion. Service personnel can observe performance from service offices. In addition, extensive interlock, time-average and shape-averageinformation are stored and accessed by phone. This service tool isavailable on all Mevatrons (Mevanet).

Each Mevatron can be equipped with one of several types of

37 川

verification and recording systems which have different levels ofcapability ranging from essential functions to sophisticated computerbased systems. The digital verification system is a microchip basedsystem that can store and be used to verify approximately 300 patientsetups. Its operation is simple and rapid , yet provides safeguardsagainst treatment setup errors. Optional equipment extends its performanceωrecordingand other useful features. The Mevamatic 3 isa sophisticated minicomputer based patient data base managementsystem capable to store up to 2000 patient files , provide data linksamong several Mevatrons and external data systems (through DECNET) in addition 切 basic verification and recording functions.

The Siemens treatment couches, ZII and ZIV, are standard clinicworkhorses that are designed to assist the technologist in the manycomplex setups used in clinical practice. Their well-conceived design speeds the work and reduces the stress placed on the technologistin a busy clinic.

Finally, Siemens has produced an accelerator, the ME , designedspecifically for intraoperative applications. It is a wall-mounted ,magnetron driven accelerator with an electron beam energy to 8MeV. It is designed to be mounted on upper floors in operating rooms,requiring little space and minimal room shielding. A unique designis used to place the electron cone in the patient in a desired position

m

and angle , secure it to the table and then move the patient, cone andtable under the accelerator and position and align the acceleratorbeam with a system of laser lights. This speeds the process and because there is not a rigid connection from patient to accelerator, therisk of injury during the "docking" procedure is eliminated.

GE Medical SystemsP.O. Box 414Milwaukee, WI 53201

With the acquisition of CGR in 1987, GE Medical Systems expandedto meet the needs ofthe radiation therapy community. The full rangeof products currently available has been developed from the years ofexperience gained by CGR-MeV in the field of particle accelerators.CGR-MeV pioneered their application of high energy x-ray and electron medical linear accelerators in the treatment of cancer with theSagittaire™ in 1967. Sagittaires with 12 and 25 MV photons and 7 to 40MeV electrons are still in clinical use today; however, they are nolonger in production.

Today, GE-CGR continues to manufacture a full range of radiotherapy systems, including the Saturne 4™ and Orion™ series of linacs.

The streamlined magnetron powered Saturne 41 provides the user with two photonbeams between 6 and 15 MV and 8 electron beams from 6 to 16 MeV

High Energy CapabilitiesWall-mounted, Saturne family of linacs are useful for a broad spec

trum of treatment challenges. Optimized use of a double accelerating(standing wave) section enables the working energy points. Thestreamlined magnetron powered Saturne 41 provides the user with twophoton beams from 6 to 15 MY and 8 electron beams from 6 to 16 MeV.*The Saturne 42, a medium range klystron powered accelerator, provides users with 2 photon beams from 6 to 18 MV; and 12 electronbeams between 3 and 20 MeV. The high energy, klystron powered Saturne 43 provides 3 photon beams 仕om 6 to 25 MY and 16 electron beamsfrom 3 to 15 MeV.

Each Saturne system offers a number of capabilities designed to further extend its clinical utility; for eJtample, clockwise and counterclockwise photon and electron arc therapy, multiple microprocessorbased control consoles and automatic pre-setup are standard.

The Saturne series provides users with electron and photon beamsmeeting exceptionally high standards of homogeneity and purity.

*The Saturne 41 product described here will become commercially available only following GE's receipt of clearance from the Food and Drug Administration (FDA) to sellthe device. Such FDA clearance in no way constitutes FDA approval ofthe device.

The 2700 achromatic beam path and the use of an automatically adjusted energy slit provide excellent electron energy definition. Anoptional high dose rate of 1,000 ~min. is available.

Other key features of the Saturne 42 and 43 include:Irradiation Fields. Saturne series linacs offer exceptionally large,unclipped treatment fields; the fixed pre-collimator defines a 56 emdiameter circle at 1 m from the target.

In photon mode , the Saturne 42 and 43 provide continuously variable,unclipped fields of up to 40 x 40 em; in electron mode, up 切 30 x 30 em,via two pairs of trimmers on the photon jaws.Photon Mode. A single, internal wedge filter, integrated with thebeam limiting device, permits automatic wedge angles of up to 600 formaximum fields of 40 x 20 em.Electron Mode. The combination of the double foil principle for lowenergies and the scanning procedure for medium and high energiesmaximizes the advantage of both techniques; excellent homogeneityand minimum photon contamination over the available range of electron energies are the results.Dosimetry. To promote symmetry, homogeneity, high and low doseoutput and offset of cumulated dose rate , Saturne linacs employ twoseparate, sealed chambers associated with two independent dosimetrychannels.

m

Beam Limiting Device (Collimator). The beam limiting systemworks in both symmetrical and asymmetrical mode. Asymmetry ispossible on one pair of jaws up to the central axis of the field in bothphoton and electron modes.Light Simulation. To permit coincidence between the light field andthe radiation field, a lamp housed in the target bar takes the place ofthe target during light simulation.Commands in Treatment Room. A newly designed control pendantallows all geometrical parameters to be set manually or automatically in the treatment room. A monitor within the treatment room provides the necessary display. All commands for the couch are also onthe control pendant.Con缸'01 Console. The control console has as its principal function thedialogue between operator and unit, as well as maintaining controlof:- geometric and dosimetric treatment parameters- beam quality control- safety device management

Two other computer functions are also included, one for physics, research and testing, and one for technical adjustments and maintenance. In addition, the Sincer™ treatment management system canbe connected to the control console.

40

Low Energy Alternatives. GE-CGR also offers a series of singlephoton linear accelerators for low-energy applications in spacelimited environments. The Orion and Orion 6 are housed in one ofthe smallest linac structures available, and their surface-mountablegantries require only a shallow couch pit.

Both systems are magnetron powered. The Orion linac supplies asingle photon beam, with 64% (±2%) ofthe dose at 10 cm; the Orion 6, asingle photon beam delivering 67.7% (±2%) of the dose at 10 cm.

In addition, both systems offer many features of far larger linacs,including the industry's largest unclipped treatment fields; continuously variable fields of up to 40 x 40 cm; symmetrical and completelyasymmetrical collimation; variable penumbra; arc therapy; andcomputerized controls.

The Orion and Orion 6, with microprocessor-based control consoles,offer pre-select and free-select modes to streamline operation and accommodate new techniques as they evolve.

AppendixSpecifications of Radiotherapy Linacs

The table on the following page illustrates a partial list of specifications for contemporary radiotherapy linacs. All linacs listed operateat S-band frequenCies of approximately 3000 MHz , corresponding to afree space wavelength of about 10 cm. The accelerator structure notation TW stands for traveling-wave and SW stands for standingwave design. The specifications of energy for electron beams varyand , hence, may not be comparable. The items in the last column ofthe table , Comments or Special Features, are illustrative but not exhaustive. Manufacturers provide a wide variety of options, accessorydevices, and special features for their units.

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yes40x40

Motorized wedge, assist. -ups, 1pair asymm. fields standard, fullymotorized movements. R&Vavailable

40x40270。TW±185 。- /1271:8-12MV5: 5-16 MeV

LA 16ASS yes

Motorized wedge, assist. -ups, 2pairs asymm. fields standard ,fully motorized movements.R&Vavailable.

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Sincer R&V system available.Asymmetric collimators and arctherapy for both photons andelectrons. Pre-setup andparameter storage ofprescription. Single internalwedge. Variable collimation withuse of trimmer bars. Three modemultiple microprocessor controlconsole. Choice of non- orisocentric couch

yes40x40unclipped

270。6MWKlystron

2.2m SW370。100/1312: from 6-25 MV16: from 3-25MeV (dualscattering at lowenergies,scanning at highenergies.

Saturne 43GE

Sincer R&V system available.Computerized system with hardwire. Asymm. collimators. Presetup of prescription parameters.Variable penumbra. Choice ofnon- or isocentric couch.

yes40x40unclipped

no in-linesection

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.23m SW370。- /1005 MV photonsOrionGE

Sincer R&V system available.Computerized system with hardwire. Asymmetric collimators.Pre-setup of prescriptionparameters. Variable penumbra.Choice of non- or isocentric∞uch.

yes40x40unclipped

no in-linesection

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yes40x40unclipped

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370。100/1312: 6-20 MV12: 3-20 MeV

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40x35270 05.5MWKlystron

- /1286/200, 10/500EXL-17DPMitsubishi

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yes40x4090。2MWMagnetron

1.25m TW4200100/1184/5/6 MV x-raysSL75-5PhilipsMedicalSystems

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'The Saturne 41 product described here will become commercially available only following GE's receipt of clearance from the Food and Drug Administration (FDA) to sell the deviceSuch FDA clearance in no way constitute.s FDA approval of the device

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yes40x40270°2.5MWMagnetron

104m SW370。100/1306 MV x-rays5-12 MeVelectrons

Mevatron M6740

Siemens

50 MU low dose rate photonsand 90 MU high dose rateelectrons for total body. Alsooptional electron rotation , indojaws, digital verification systemand remote service monitoringsystem.

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Siemens

50 MU low dose rate photonsand 900 MU high dose rateelectrons for total body. Alsooptional rotation electrons, indojaws, digital verification systemand remote service monitoringsystem.

yes40x40270。8MWKlystron

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50 MU low dose rate photonsand 900 MU high dose rateelectrons for total body. Includesupperllower indo jaws, rotationelectrons, remote servicemonitoring system and digital∞ntrol console with colormonitor and single patientverification.

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Clinac2500C

Varian

[c. j. karzmark, robert j. morton] a primer on the(bookos.org)

Documents

radiation oncology

radiation control

physics research

radiological physicist

stanford microwave laboratory

appendix b

bureauof radiological

effective use