CONTENTS

INTRODUCTION

ABOUT CANCER

1. Types of Cancer

2. Causes of Cancer

3. Treatment of Cancer

HISTORY

LINEAR ACCELERATOR

1. Principle

2. Major Components

3. Description of Components

ADVANTAGES OF LINAC

TROUBLESHOOTING IN LINAC

DISADVANTAGES OF LINAC

1. INTRODUCTUON

A linear accelerator (LINAC) is the device most commonly used for external beam radiation treatments for patients with cancer. The linear accelerator is used to treat all parts/organs of the body. It delivers high-energy x-rays to the region of the patient's tumor. These x-ray treatments can be designed in such a way that they destroy the cancer cells while sparing the surrounding normal tissue. The LINAC is used to treat all body sites, using conventional techniques, Intensity-Modulated Radiation Therapy (IMRT), Image Guided Radiation Therapy (IGRT), Stereotactic Radiosurgery (SRS) and Stereotactic Body Radio Therapy (SBRT)

2. ABOUT CANCER

Cancer is a general term for a group of disease caused by the uncontrolled growth of abnormal cells. Cancer may occur in any region or any organ in the body.

The DNA in each cell is responsible for “programming” that cell’s characteristics and growth. When this program derailed, cells lose the ability to grow and reproduce normally. As a rule, abnormal cells are detected by the defence system and eliminated. But when the body is no longer able to do this abnormal cell continue to multiply and can eventually form a tumor.

2.1 Types of tumor

Benign Tumors are not cancer. They often can be removed and in most cases, they do not come back. Cells in benign tumor do not spread to other parts of the body. More importantly, benign tumor are rarely life threatening.

Malignant tumors are cancer. Cells in malignant tumors are abnormal and divide without control and order. These cancer cells can invade and destroy the tissue around them. In a process called metastatis, cancerous break away from the organs on which they are growing and travel to other parts of the body, where they continue to grow.

2.2 Types of Cancer

Cancer is classified on the basis of tissue. They are of four types:

CARCINOMAS: This type is mainly derived from epithelial cells. They include cervical, breast, skin, brain, lung, stomach cancer etc.

MELANOMAS: Cancerous growth of melanocyte (A type of skin cells) is called melanomas.

SARCOMAS: These cancers are located in muscular tissue derived from mesoderm. Thus they include the cancer of bones, cartilage, tendons adipose tissue and muscles.

i. Cancer of bones is called Ostermas.ii. Cancer of adipose tissue is known as lipomas.

iii. They are rare in human about 1% of all tumors are sarcomas.

LEUKEMIAS and LYMPHOMAS: These are cancers of haematopoctic cells. Leukemia’s are commonly called blood cancer.

The most common cancer in India is Mouth-Throat Cancer in men and uterine cervical cancer in women.

2.3 Causes of Cancer

Chemical and physical agents that can cause cancer are called carcinogens and belong to these categories:

ONCOGENIC TRANSFORMATION: They are agents or factors, which bring about changes in genetic materials.

TUMORS PROMOTORS: They promote proliferation of cells, which has undergone oncogenic transformation. E.g. some growth factors are hormones.

TUMORS VIRUSES: Some viruses are known to be connected with oncogenic transformations.

RADITION: The X-rays, cosmic rays, UV-rays can cause cancer.

PHYSICAL AGENTS:

i. Betel and tobacco chewing causes Oral cancer.ii. Heavy smoking causes lung cancer and may also cause cancer of oral

cavity, pharynx, and larynx.iii. Jagged teeth may cause tongue cancer.iv. Excessive exposure to sunlight may cause skin cancer.

CHEMICAL AGENTS: Several chemicals are known to cause cancer. These are caffeine, nicotine, products of combustion of coal and oil for pesticides, constant use of these substances can cause cancer. Some sex hormones, steroid if are given in large amount may cause cancer.

2.4 Detection and Diagnosis of Cancer

i. Bone marrow lecopsy and abnormal count of WBC In leukaemia.ii. Biopsy of tissue, direct or through endoscopy.

iii. Pap tests (cytological staining) used for detecting cancer of genetic tracts.iv. X-rays (using dyes), CT scan, MRI scans, ultrasound, SPECT, PET, PET/CT of internal

organs e.g. Kidney Pancreas. Mammography: it is a radiographic examination of breasts fir possible cancer.

2.5 Different Sites of Cancer

Some of the important sites of cancer are skin, mouth, oesophagus, stomach, colon, rectum, liver, gallbladder, blood, lymph, adipose tissue, lung, cervix, breast, brain, penis, prostate, muscle, thyroid, kidney and bone.

2.6 Treatment of Cancer

All cancer treatments focus on destroying malignant cells. The most commonly used treatment, regimens consist of surgery, radiation therapy, chemotherapy, hormonal therapy, immunotherapy, either alone or in some combination. Because certain tumors respond best to specific treatments, there is no best treatment for all tumor types.

Surgery

Surgery is the oldest form of treatment for cancer. It also has an important role in diagnosis and staging (finding the extent) of cancer. Here the surgeon removes the cancerous tissue along with a sufficient margin of healthy tissue. Today more limited (less invasive) operations are often done to remove tumors while preserving as much normal function as possible.

Surgery offers the greatest chance for cure for many types of cancer, especially those that have not yet spread to other parts of the body.

Chemotherapy

Chemotherapy is the term used to describe the treatment of cancer with drugs that can destroy cancer cells by stopping them from growing or multiplying healthy cells can also be harmed, especially those that multiply quickly. Normal body cells usually repair themselves after chemotherapy is completed. There are more than 100 different chemotherapeutic agents available today and can be administered as infusion, injections, pills or as ointment, depending upon the drugs chosen and the type of tumor to be treated, the chemotherapy drugs may be fixed together or given individually.

Immune Therapy

Immune therapies try to force the patient’s own immune systems to stop the growth of cancer cells. This can be accomplished by stimulating the immune systems to work harder, or by given the patient synthetic immune system proteins. Immune therapy is the most effective when used to treat small tumors, or when the patient’s cancer is not very advanced.

Radiation Therapy

Radiation therapy (sometimes called radiotherapy, x-ray therapy, or irradiation) is the treatment of the disease using penetrating beams of high energy waves or streams of particle called radiation. Radiation is used to treat cancer and other illness.

The radiation used for cancer treatment comes from special machine or radioactive source. Radiation therapy equipment aims specific amount of the radiation at tumor or area of the body where there is a disease.

Radiation in high doses kills cells or keeps them from growing and dividing. Because cancer cells grow and divide more rapidly than most of the normal cells around them, radiation therapy can successfully treat many kinds of cancer.

2.7 Treatment Objective

The goals of any forms of cancer treatment is the destruction of the tumor and its spread into the neighbouring lymph system or the palliation (relieve) of symptoms when a cure is not attainable. These goals are realized by the delivering a precisely measured uniform lethal dose of radiation to the tumor while minimizing the potentially harmful irradiation of normal tissue.

2.7.1 Procedures

External beam therapy (EBT): External beam therapy is a method for delivering a beam of high energy x-rays to the location of the patient’s tumor. The beam is generated outside the patient (usually by a linear accelerator) and is targeted at the tumor site. This x-ray can destroy the cancer cells and careful treatment planning allows the surrounding normal tissue to be spared. No radioactive source is placed inside the patient’s body.

Intensity -Modulated Radiation Therapy (IMRT):IMRT is an advance mode of high precision radiotherapy that utilizes computer controlled x-ray accelerator to deliver precise radiation doses to a malignant tumor or specific areas within the tumor. The radiation dose is designed to conform to the three dimensional(3-D) shape of the tumor by modulating or controlling the intensity of the radiation beam to focus a higher radiation dose to the tumor while minimizing radiation exposure of surrounding normal tissue. Treatment is carefully planned using 3-D CT images of the patient in conjunction with computerized dose calculations to determine the dose intensity pattern that will best conform the tumor shape.

Because the ratio of normal tissue dose to tumor dose is reduced to a minimum with the IMRT approach, higher and more effective radiation doses can safely be delivered to tumor with fewer side effects as compared with conventional radiotherapy techniques. IMRT also has the potential to reduce treatment toxicity, even when doses are not increased.

A medical linear accelerator generates the photon or x-rays used in IMRT. The machine is the size of a small room-approximately 10 feet high and 15 feet long. The intensity of each beam’s radiation dose is dynamically varied according to the treatment plan shrinking or eliminating tumors.

Stereotactic Radiosurgery:Stereotactic radio-surgery is a highly precise form of radiation therapy used primarily to treat tumors and other abnormalities of the brain. Stereotactic radio-surgery is a non-surgical procedure that uses highly focused x-rays to treat certain types of tumors, inoperable lesions and as a post-operative treatment to eliminate any leftover tumor issue.

The treatment involves the delivery of a single high dose or sometimes smaller, multiple doses of radiation beams that converge on the specific area of the brain where the tumor or other abnormalities resides. Using a helmet-like device that keeps the head completely still and three dimensional computers aided planning software; stereotactic radio-surgery minimizes the amount of radiation to healthy brain.

These are the basic forms of stereotactic surgery, each of which uses different instruments and sources of radiation. Linear accelerator (LINAC) machines deliver high energy x-ray photons or

electrons in the curving paths around patient’s head the linear accelerator can perform radio-surgery on larger tumors in a single session or during multiple sessions, which is called fractionated stereotactic radiotherapy.

RADIOSURGERY USING LINEAR ACCELERATOR

Linear accelerator (LINAC) radio-surgery is similar to the gamma knife procedure and its four phases: head frame placement, imaging, computerized dose planning and radiation delivery. Unlike the gamma knife which is motion less during the procedure, part of LINAC machine called a gantry rotates around the patient delivering radiation beam from different angles. Compared to the gamma knife, the LINAC is able to use a larger x-ray beam, which enables it to treat larger tumors more uniformly and with less reposition.

3. HISTORY

The Early Years

Initially developed in the early 1900s, radiation therapy was used primarily for relieving pain by shrinking tumors, but not often for cure. The earliest radiotherapy devices used primitive X-ray tubes to generate very weak radiation — not enough to effect cures or to penetrate the body very deeply. Next came cobalt machines that offered higher energy but which delivered relatively slow treatments that lengthened in time as the radioactive source within the machine weakened. The weakened radioactive cobalt source also presented hospitals with a problem of how to dispose of potentially dangerous radioactive waste.

A Technology Is Born

Modern radiation therapy traces its origins back to the invention of the "klystron" by brothers Russell and Sigurd Varian in 1937. The Varian brothers first used their invention in radar systems. However, after World War II, either the klystron or the magnetron, another invention of the time, was used to propel charged particles through a vacuum tunnel, resulting in a device called a linear accelerator or linac. The linac was initially used for research in high energy physics.

Kaplan proposed that a linac be specifically designed to generate high energy X-rays for the treatment of cancer. The idea was that klystrons would accelerate electrons to near the speed of light. The electrons would then be made to strike a tungsten target causing an emission of X-rays of comparable energies. These high-energy X-ray beams would then be used to bombard a cancerous tumor.

The linac 6 could generate sharply defined beams of 6 MV X-rays in a gantry that could be rotated 360 degrees around a patient. Though limited in production, the linac 6 established that linacs could be used to treat cancer, with intrinsic medical advantages over the cobalt irradiators that had been used to treat cancer throughout the 1950s.

The Informative Years

In 1968, Varian introduced the linac 4, a machine that deployed "standing wave-guide" technology which, along with other advances, helped to reduce the size, cost, and complexity of a medical linac. For the first time, the linear accelerator technology became economically competitive with cobalt irradiators and was ready to seize centre stage.

Another innovation — an achromatic bending magnet — made it possible to achieve higher energy beams without increasing the machine size. The Clinac 18 was a "user friendly" high-energy machine that could be used by hospital radiation therapists without specialized physics training.

In 1981, Siemens introduced the linac 2500, a machine that could operate at, and was easily switched between, two widely separated X-ray energy levels, depending upon the depth of the tumour being treated using a patented energy switch. Subsequently, Siemens introduced the "C" series linac, a computer-controlled linac model. The three distinct uses for computers in radiation treatment are to promote efficient management of information about patients and their

treatment, to perform the complex calculations that plan the best way to administer radiation therapy and to control the linear accelerator’s movement and operating functions.

4. LINEAR ACCELERATOR

4.1 Principle of LINAC

LINAC is short form of linear accelerator. Linear accelerator machines produce radiation that is referred to as high energy x-ray.

A linear accelerator machine is designed to be a general purpose radiation delivery machine and in general requires modifications to enable it to be used for radio surgery or IMRT. Linac emits a well-defined beam of uniformly intense x-ray photon radiation of different energies, depending on the accelerator. Some also produce electron beams. Cobalt radiotherapy units use cobalt-60, a man made isotope. Radiation is used to treat at least 50% of all cancer cases. Radiation therapy can be either curative or palliative, depending on the stage and prognosis of the disease. For treatment to be successful, the radiation field should of a uniform intensity and predictable energy level and must be well defined to avoid irradiating healthy tissue.

4.2 Working

The linear accelerator uses microwave technology (similar to that used for radar) to accelerate electrons in a part of the accelerator called the "wave guide," then allows these electrons to collide with a heavy metal target. As a result of the collisions, high-energy x-rays are produced from the target. These high energy x-rays are shaped as they exit the machine to conform to the shape of the patient's tumour and the customized beam is directed to the patient's tumour. The beam may be shaped either by blocks that are placed in the head of the machine or by a multileaf collimator that is incorporated into the head of the machine. The beam comes out of a part of the accelerator called a gantry, which can be rotated around the patient. Radiation can be delivered to the tumor from any angle by rotating the gantry and moving the treatment couch.

4.3 Major Components

LINAC consists of five major components:

a) A modulatorb) An electron gun c) A radio frequency (RF) sourced) A klystron or magnetron

An accelerator guide

The electron beam produced by a Linac can itself be used for treatment or can be directed toward a metallic target to produce x-rays.

The modulator amplifies the three phase AC power supply i.e primary power, rectifies it the DC power, and produces high voltage DC pulses that are used to power the electron gun and RF power source. The high voltage cables electrically connect the electron gun and RF power source to the modulator, which can be located in the gantry, the gantry supporting stand, or a separate cabinet.

The electron gun injects electrons into the accelerator guide in pulses of the appropriate duration, velocity, and position to maximize acceleration. The electron gun can be attached to the accelerator guide by a removable vacuum flange which allows easy replacement of the gun.

The RF power source either a magnetron or a klystron, supplies high frequency electromagnetic waves(2856MHz+_200KHz), which accelerate the electrons injected from the electron gun down the accelerator guide.

Linacs are classified according to their energy levels.

a) Low energy units produce 4 or 6 million volt (MV) photons.b) High energy Linac produces between 15 and 25 MV.c) Electron energies ranging from 4 to 20 MeVs.

3-D VIEW OF A HIGH ENERGY LINAC

MAGNETRON:

Generally, a magnetron is used in low energy Linac and a klystron (an amplifying electron tube) with RF cavities arranged in a straight line, is used in high energy accelerators. In a klystron, the electron beam interacts with the microwaves, which modulate the beam’s velocity to concentrate the electrons into bunches. Klystron requires low RF power and is more expensive than magnetron because they have specialized circuitry, provides higher output, and last longer. However, for low energy applications, frequency instabilities are small, and magnetrons are most cost effective than klystrons. The microwaves are transported to the accelerator guide by a waveguide, a hollow metallic tube closed at both ends by a ceramic window that are transparent to microwaves. The waveguide is filled with a pressurized gas (SF6) to prevent arcing.

ACCELERATOR GUIDE:

It consists of several copper, resonant cavities soldered into a single structure, accelerates the electrons to the desired energy.

Two types of guide are used – the standing wave and the traveling wave. Although very different, both require the use of ion pumps, which maintain an internal pressure of 10-7 to 10-10 torr to remove any gas molecules that could interact with the electron gun and cause gun failure. Mostly standing wave guide is used in high energy Linac because the accelerating electric field oscillates in place within the tube, which is sealed at each end to reflect the microwave energy, thereby multiplexing the intensity of the incoming wave. In the traveling wave accelerator, the length of the accelerator is directly proportional to the acceleration energy produced. High energy traveling wave units, which require lengths of 2.5 meters or more, increase the overall length of the accelerator and may require large treatment rooms. After the electrons are accelerated, they are aimed by a bending magnet to produce radiation for treatment. Most systems use a 270° achromatic magnet to position the beam.

ELECTRON BEAM:

The high energy electron beam is either directed at a tungsten target to produce photons. Because the photon beam produced from the tungsten target is most intense at its center, a flattening filter, usually made of lead, is provided to modify the beam’s intensity distribution for clinical use. For electrons scattering foils (0-5mm) are used as no target is involved.

All Linac have a Dosimetry system in the treatment head that terminates the radiation at the preset dose. This system incorporates a compartmented, dual-system ionization chamber, which should be sealed against temperature and pressure fluctuations. Most Dosimetry systems detect asymmetries in the treatment beam and then terminate irradiation if the asymmetry exceeds a preset value. Some systems also have beam-steering circuitry to automatically compensate for changes in the angle or position of the beam caused by gantry or collimator rotation. The radiation beam is shaped by the collimators, which are motor driven, movable blocks of material that define the treatment field. A light field projected onto the patient outlines the area to be irradiated. Field sizes of up to 40 cm on a side are available, as are digital readouts of collimators positions. Adjustable collimator jaws are available on both units. Shaping wedges can be placed on the interface mount (part of collimator) to further customize the beam shape.

For electron treatment electron applicators are used.The entire collimator assembly rotates about an axis that passes through the centre of the treatment field and the isocenter.

Major manufacturers of Linac offer multileaf collimators (MLC’s). Multileaf collimators use multiple (up to 120 leaves), which are individually motorized, to define the treatment field. This computer-controlled collimation facilitates modification of the treatment field and replaces custom-made lead blocks for many treatments.

HIGH ENERGY CLINAC BLOCK DIAGRAM

HIGH ENERGY BEAM GENERATION SYSTEM

4.4 Description of Components

RF POWER

The electrons (e-) need to stay in motion. In order for this to happen RF power must be pumped into the accelerator. The RF power is what causes the electrons to a high energy level.

There are two different systems that generate RF power

i. Low energy accelerators magnetrons acts as RF source.ii. High energy accelerators RF source is used.

iii. Klystron acts as an amplifier.

KLYSTRON

The main difference between the magnetron and the klystron RF system is that the klystron produces a great (higher) RF energy level than the magnetron. It has the ability to produce 6 -9 million watts of power.

High energy machines utilize RF power system. It uses a pulse tank to help generate RF power.

The electron gun successfully injected the electrons (e-) into the accelerator. RF energy was added and the electrons (e-) were accelerated to almost the speed of light.

KLYSTRON

BENDING MAGNETS

The electrons have now passed through the accelerator and headed for the beam bending envelope. The beam bending envelope is surrounded by an electromagnet. The magnetic field causes the electrons (e-) to turn 270 degrees.

The bending magnet does more than just turn the electrons (e-) 270 degrees. It serves as a band pass filter to remove that part of the beam spectrum that falls outside of the useful range.

First, determine the dose of radiation deliver to the patient. This will determine the output energy of the beam. The machine controls the energy output, when the electrons enter the envelope. The bending magnet controls the electrons through its current and adjusts them to the proper level we are trying to achieve.

CROSS-SECTION VIEW IN THE RADIAL PLANE

TRANSVERSE SECTION AND VIEW OF THE MAGNET SYSTEM

Collimation (The Defining Head)

Once the beam has bean bent by the bending magnet 270 degrees, it will travel downward in the direction of the patient. The beam now is ready to enter the collimator, also called defining head, is responsible for shaping and defining the radiation beam.

Upper defining head

The upper defining head defines the beam profile and determines the type of output (either x-ray or electron mode).

Electron mode

In electron mode the beam first reaches the primary foils. The first foil is used to scatter electrons. The beam travels down the electron dose chamber for monitoring beam and finally reaches the secondary scattering foils.

There are two sets of jaws: inner jaws (A) are on the top and the outer jaws (B) are located just below. The jaws are used to determine the size of the exiting beam. The jaws play an important role in achieving a square, uniform beam that will enter the patient. Below the jaws are the multi leaf collimators (MLC’s). There are total 80 leaves [40 leaves on carriage (A) and 40 on carriage (B)].

As the tumor is in irregular shape, it is not possible that the tumor can be rectangular or square, therefore to define that irregular shape of the tumor MLC’s are used so that the beam attack on the desired area otherwise if the beam falls on the living tissues it can result in serious hazards to the patient.

MULTILEAF COLLIMATORS (MLC’s)

ELECTRON GUN

In order to produce radiation you need electrons (e-). This drawing demonstrates how the electron guns release electrons into the system by thermionic action.

The High Energy Clinac Gun employs a dispenser cathode, as well as a grid to control electron emission. Finely-ground particles of barium are distributed evenly within the tungsten cathode. Most tungsten cathodes operate at about 1750°C and the embedded electron emitter is thorium. However, in Clinac guns, 1750°C would be hot enough to melt the grid (and warm-uptime would be too long). Therefore barium is used allowing the cathode to operate at a lower temperature. When the temperature reaches 714°C, the melting point of barium, the particles migrate to the surface, where they form a thin film.

As the surface barium is used up, more barium is “dispensed” from within the cathode to the surface, hence the name “dispenser cathode”.

CONSTRUCTION OF TYPIVSL DISPENSER-TYPE CATHODE

THE ACCELERATOR

Once the electron (e-) leaves the electron gun they are immediately injected into a system of accelerating cavities known as the accelerator (or waveguide). The purpose of the accelerator is to accelerate electrons to almost the speed of light. Simply said, this occurs has to do with the fact that electrons (e-) are negatively charged, so as they travel down the accelerator they are chasing a positive charge. As the electrons enter the resonant cavities on the accelerator they are reflected. This causes them to sufficiently gain both energy and mass. Below is an illustration what an accelerator would look like if it was cut in half. This view allows you to see resonant cavities and the path of the electrons (e-).

ACCELERATOR

GUN MOUNTING

4.5 Construction and Operation

4.6 Schema of LINAC

A linear particle accelerator consists of the following elements:

i. The particle source: The design of the source depends on the particle that is being moved. Electrons are generated by a cold cathode, a hot cathode, a photocathode, or radio frequency (RF) ion sources. Protons are generated in an ion source, which can have many different designs. If heavier particles are to be accelerated, (e.g. uranium ions) a specialized ion source is needed.

ii. A high voltage source for the initial injection of particles.iii. A hollow pipe vacuum chamber: If the device is used for the production of X-

rays for inspection or therapy the pipe may be only 0.5 to 1.5 meters long. If the device is to be an injector for a synchrotron it may be about ten meters long. If the device is used as the primary accelerator for nuclear particle investigations, it may be several thousand meters long.

iv. Within the chamber, electrically isolated cylindrical electrodes are placed, whose length varies with the distance along the pipe. The length of each electrode is determined by the frequency and power of the driving power source and the nature of the particle to be accelerated, with shorter segments near the source and longer segments near the target. The mass of the particle has a large effect on the length of the cylindrical electrodes; for example an electron is considerably lighter than a proton and so will generally require a much smaller section of cylindrical electrodes as it accelerates very quickly. Likewise, because of its small mass, electrons have much less kinetic energy than protons at the same speed. Because of the possibility of electron emissions from highly charged surfaces, the voltages used in the accelerator have an upper limit, so this can't be as simple as just increasing voltage to match increased mass.

v. One or more sources of radio frequency energy used to energize the cylindrical electrodes. A very high power accelerator will use one source for each electrode. The sources must operate at precise power, frequency and phase appropriate to the particle type to be accelerated to obtain maximum device power.

vi. Target: If electrons are accelerated to produce X-rays then water cooled tungsten target is used. Various target materials are used when protons or other nuclei are accelerated, depending upon the specific investigation. For particle-to-particle collision investigations the beam may be directed to a pair of storage rings, with the particles kept within the ring by magnetic fields. The beams may then be extracted from the storage rings to create head on particle collisions. As the particle bunch passes through the tube it is unaffected (the tube acts as a Faraday cage), while the frequency of the driving signal and the spacing of the gaps between electrodes are designed so that the maximum voltage differential appears as the particle crosses the gap. This accelerates the particle, imparting energy to it in the form of increased velocity. At speeds near the speed of light, the incremental velocity increase will be small, with the energy appearing as an increase in the mass of the particles. In portions of the accelerator where this occurs, the tubular electrode lengths will be almost constant.

vii. Additional magnetic or electrostatic lens elements may be included to ensure that the beam remains in the center of the pipe and its electrodes.

QUADRAPOLE MAGNETS SURROUNDING LINAC TO FOCUS ELECTRON BEAM

viii. Very long accelerators may maintain a precise alignment of their components through the use of servo systems guided by a laser beam.

Generating an Electron Beam

Early radiation therapy machines used a radioactive source (cobalt) to produce the ionizing radiation needed to treat cancerous tissue. But most radiation therapy today is done with a linear accelerator. In this, the electrons are accelerated by the gun in the back of the monitor and directed at the inside of the screen, where phosphors absorb the electrons and produce light. A medical linear accelerator produces a beam of electrons about 1,000 times more powerful than the standard computer monitor. The longer a linear accelerator is, the higher the energy of the beam it can produce. The innovation of Therac-25 was that the designers found a way to fold the beam back and forth so a very long accelerator could be fit into a smaller space. Thus powerful beams could be produced within a reasonable amount of space.

Getting the Beam into the Body

Patients can be treated directly with the resulting electron beam, as long as the beam is spread out by scanning magnets to produce a safe level of radiation. The medical linear accelerator spreads and directs the beam at the appropriate place for treatment. The picture below shows a typical medical linear accelerator in operation.

But a difficulty with the electron beam is that it diffuses rapidly in tissue and cannot reach deeper tissue for treatment. The picture below is a simulation (produced by the Stanford Linear Accelerator Center) of an electron beam traveling

through air and entering human tissue. You can see the beam quickly diffuses and therefore does not penetrate deeply.

To solve this problem, Therac-25 and many other machines can switch to a mode in which X-ray photons are used for treatment. These penetrate much more deeply without harming intervening tissue. For this, the electron beam is greatly increased in intensity and a metal foil followed by a beam "flattener" is placed in the path of the electron beam. This transforms the electron beam into an X-ray (called photons in some literature). This process is inefficient and requires a high intensity electron beam to produce enough X-ray intensity for treatment. Therac-25 used a 25 MeV electron beam to produce an X-ray for treatment. 25 MeV is 25 million electron volts (eV) (eV is the energy needed to move one electron through a potential of one volt).

Therac-25 was called a dual-mode machine. It could produce the low energy electron beams for surface treatment and it could also produce a very high intensity electron beam that would be transformed into an X- ray by placing the metal foil in the path of the beam. The serious danger in a dual mode machine is that the high-energy beam might directly strike the patient if the foil and flattener were not placed in its way.

Radiation Absorbed Dose

Although MeVs are used to measure the strength of the electron beam, the measure used for therapeutic uses is the radiation absorbed dose (rad). This is a measure of the radiation that is absorbed by tissue in a treatment. Standard single radiation treatments are in the range of 200 rads. 500 rads is the accepted level of radiation that, if the entire body is exposed to it, will result in the death of 50% of the cases. The unprotected electron beam in the Therac-25 is capable of producing between 15,000 and 20,000 rads in a single treatment. The unprotected beam is never aimed directly at a patient. It is either spread to a safe concentration by scanning magnets or turned into X-rays and reduced by a beam flattener.

Siemens Linear Accelerator Comparison Chart

Models Artiste Oncor (Impression,Expression,&

Avante Garde)

Primus K Primus M KD2 MD2

Year(s)Manufactured*

2009 & newer

2004 & newer 1998-2005 1998-2005 1990-1999 1990-1988

Powersource Klystron Klystron Klystron Magnetron Klystron Magnetron

Photon EnergyConfiguration

6&10/15/18 6&10/15/18 6&10/15/18 6MV 6&15/18 6&10/15

Electron Energies Yes Yes Yes Yes Yes Yes

Multi-Leaf Collimator

(MLC)**

160 MLC 58; 82; 160 MLC

(optional)

58 MLC 58 MLC 58 MLC (optional)

58 MLC (optional)

Portal Imager(EPID)**

Optivue(Amorphous

Silicon)

Optivue(Amorphous

Silicon)

BeamView,Optivue

(optional)

BeamView,Optivue

(optional)

BeamView(camera based)

BeamView(camera based)

Treatment Delivery

3D, IMRT, SRS

3D, IMRT, Rapid Arc, SRS

3D, IMRT, SRS

(optional)

3D, IMRT, SRS

(optional)

3D, IMRT, SRS

(optional)

3D, IMRT, SRS

(optional)

KV Imaging for IGRT

K-Vision N/A N/A N/A N/A N/A

CBCT In room CT M-Vision N/A N/A N/A N/A

StereotacticRadiosurgery**

Cones orMLC Based

Cones orMLC Based

Cones orMLC Based

Cones orMLC Based

Cones orMLC Based

Cones orMLC Based

4.7 ADVANTAGES OF LINAC

The linac is the most widely selected medical accelerator and is used to treat thousands of patients around the world every day. In the standard-bearer of the

industry, the linac is an industry leader in high uptime and can be easily upgraded to match the needs of your linac as it grows.

Administrator

A flexible and efficient treatment delivery system helps improve patient care and increase the number of patients you can treat. This translates to an enhanced reputation for the facility, increased revenues, and an accelerated return on investment.

In addition, a Varian linac leads all other linear accelerators in reliability, and comes with the best customer support service in the industry. With decades of experience as a world leader, Varian is committed to ensuring your machines are always up and running and delivering the highest quality treatment care.

Physician/oncologist

The versatility of the linac treatment machine enables any area of the body to be treated using the latest and most effective treatments available. With exceptional under couch access and multi-treatment modalities, there is no limit to the type of treatment clinicians can offer. In the case of IMRT, it includes:

a) Step and shootb) Sliding windowc) Small to large fieldsd) Coplanar or non-coplanar fieldse) Radical or palliative plans

The linac is designed to deliver a high dose rate for fast treatment times and to help ensure effective hypo fractionation. As a result, more patients can be treated quickly and accurately.

Physicist

The machine and dose stability of the linac enables you to deliver IMRT, IGRT and IMIGRT easily and effectively. The linac is designed so the beam remains consistent and can be quickly turned on and off. This high degree of accuracy and reliability enables gating and other advanced treatment techniques and ensures that the dose output is the same according to treatment.

The linac also features streamlined matching of machine dosimetry. This means that machines can be beam matched across the department, so patients can be easily and quickly transferred from machine to machine if required.

a) Medical Linac and cobalt radiotherapy units are used in external beam radiation therapy to treat cancer.

b) Low energy Linac is used primarily to treat bone cancer and tumors of the head, neck & breast.

c) High energy Linac is used to treat deep seated neoplasm and tumors of the pelvis and thorax.

d) The linear accelerator is used to treat all parts/organs of the body.

4.8 TROUBLESHOOTING IN LINAC

1. Pentode, Klystron, Magnetron, Thyratron & Field Light bulbs :

Replacement2. MLC (motors, Potentiometers, Encoders):

Calibration/ Replacement3. Wave Guide (RF problem):

Calibration/ Tuning4. Gantry (Electronics & Electricals):

Repairs/Replacement5. Console/Control unit (Software):

Update/Rectification6. Table/ Couch(Electronics & Electrical Motors):

Repair/Adjustments7. Dosimeter Board:

Calibration/Replacement8. External Water cooling system9. SF6 (Sulphur hexafluoride) gas (Used as Dielectric in Wave Guide)10. Electronic Portal Imaging Device(EPID)-Camera System(used to take images before

treatment):Repair Camera/ Imaging System

4.9 DISADVANTAGES OF LINAC

a) The device length limits the locations where one may be placed.b) A great number of driver devices and their associated power supplies are required,

increasing the construction and maintenance expense of this portion.c) If the walls of the accelerating cavities are made of normally conducting material and

the accelerating fields are large, the wall resistivity converts electric energy into heat quickly. On the other hand superconductors have various limits and are too expensive for very large accelerators. Therefore, high energy accelerators such as SLAC, still the longest in the world (in its various generations), are run in short pulses, limiting the average current output and forcing the experimental detectors to handle data coming in short bursts.