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MAHADEVAPPA MAHESH, MS, PhD,JAMES M. HEVEZI, PhDTHE MEDICAL PHYSICS CONSULT

536

linical Management of Abutting Electronields: An Overview of a New Paradigm

ohn R. Gentry, MS, Lawrence Klonowski, MS

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adiotherapeutic technique isriven by the goal of achieving theost conformal dose distribution

ossible and sparing surroundingissue wherever possible. Image-uided and intensity modulated ra-iotherapy with photons is an em-odiment of this goal. However,arriers of convention stemming inart from the nature of electronose deposition prevent the gainshat have been achieved with pho-ons from being translated to super-cial radiotherapy using megavolt-ge electrons. In addition to theose heterogeneity created on theentral axis of an electron field,he current convention used toompute the number of monitornits for electron treatment in-reases the problems of dose heter-geneity at the junction of abuttedlectron fields. As a result, the clin-cal management of patients withbutting electron fields is greatly in-reased over that of abutting pho-on fields because of the necessity tohoose two or more new field junc-ions, possibly requiring the cre-tion of additional electron cut-uts. This column touches on thesessues and lays the foundation for aew principle of conformal abut-ent with examples.

URRENT ELECTRONRESCRIPTIONARADIGM INCREASESOSE HETEROGENEITYREATED BY ELECTRONIELD ABUTMENT

uperficial therapy with electrons

s the boost or primary treatment t

odality enables clinicians tohoose a megavoltage electroneam that provides adequate dos-ng near the surface while sparingissue that is distal to the tumor vol-me. Megavoltage electron depthose curves in tissue are character-

zed by an entrance dose that in-reases until the depth of maximumose is reached, falling off rapidlyhereafter. Therefore, the most re-ent guidance from both the Inter-ational Commission on Radiationnits and Measurements [1] andmerican Association of Physicists

n Medicine Task Group 70 [2]ontinues to suggest that the 90%sodose line be used to cover thelanning target volume. This gui-ance, along with ensuring uni-ormity in the way that electronrescriptions are written, also in-roduces 10% dose heterogeneity.or a single field, the impact of thisractice results only in the creationf a single region of heightenedose on the central axis of that field.However, there is a situation in

hich the impact of an inherentose heterogeneity (IDH) hasamifications that extend beyondhe dose delivered on the centralxis. When two electron fieldsust be abutted, it has been shown

y Harms [3] that the dose at theunction of the two fields canxceed 120% of the central-axisose despite the use of a gap de-ending on the energy and source-o-surface distance (SSD) used.ecause more monitor units areecessary to prescribe to the 90%

sodose line than the depth of peakose, the current electron prescrip-

ion paradigm causes the dose het- a

0091

rogeneity on the central axis toct as a multiplier to the alreadyeightened dose found to exist athe junction. When it is necessaryo treat to two different depths withifferent electron energies, the cur-ent electron prescription paradigmoosts the magnitude of the doserom the penumbra of the lower en-rgy electron field that is receivednto higher energy electron field.lso, when treatment requires thatlectron fields must abut at an angleith respect to each other, such as

he curvature of the chest wall for acar boost, the penumbra of theelds overlap at the higher isodose

ine levels.Khan [4] discussed the chal-

enges presented by the abutmentf electron fields. If the abuttinglectron fields are kept too far apart,cold spot at the junction of the

elds at the surface is created, in-reasing the potential for under-ose and loss of local control. If theelds are brought close together sohat the surface dose at the junctions adequate, large dose heterogene-ties are created, especially at theepth of peak dose. If the fields areept apart and the treatment islanned for extended distance, theurface dose may be made accept-ble, but volumes of heightenedose heterogeneities are enlargedeyond the surface. Therefore, aundamental step that can be takeno reduce or eliminate the high doseolumes created by the conven-ional abutment of two electronelds is to reduce or remove IDH.n photon radiotherapy, it is axi-matic that we desire only that

mount of dose heterogeneity that

© 2010 American College of Radiology-2182/10/$36.00 ● DOI 10.1016/j.jacr.2010.04.009

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The Medical Physics Consult 537

s consistent with complete cover-ge of the planning target volume.lthough the physics of electronepth dose has necessitated pastnd current practice, a superficialreatment paradigm for abuttedlectron treatment that reducedose heterogeneity would be ofenefit.A variety of techniques to im-

rove the abutment of electronelds have been devised [5-9], rang-

ng from the use of geometry toimple devices that expand the pen-mbra and increase the surfaceose. Some methods require a fullommissioning procedure beyondhat traditionally made for mega-oltage electron treatment. Theimplest technique, based on theork of Harms [3], results in a gap-ing procedure that is both energynd SSD dependent. However, thisechnique results in low entranceose in many cases unless extendedistance is used with its attendantonsequences. And this techniquetill causes the dose at the junctionf two fields to be increased by theDH factor from both fields.

OUNDATIONS OF AEW APPROACH TOLECTRON FIELDBUTMENT

ecause of the nature of the prob-ems faced by treatment plannersho must abut electron fields, theroblems listed above must all beolved simultaneously. To mitigatehe problems of conventional elec-ron abutment, the penumbra muste expanded in such a way that theurface dose is improved, even for00-cm SSD treatment, while re-ucing the size of dose heterogene-

ties, if any. We propose an electronnergy and electron intensity mod-lation approach that builds on theork of McKenzie [10] as well as

hat of Richert et al [11]. McKenzie’s s

poiler technique showed that thelectron penumbra could be spreadithout resorting to the use of ex-

ended distance and that acceptableurface dose at the junction of abut-ed fields is achievable even with00-cm SSDs. There may be occa-ions when abutting fields mustreat drastically different depths. Inhat case, differences in electroneam pernumbra associated withach field being used might createatching problems. This can be

olved by expanding the penumbraf one electron beam by treating atxtended distance, as the variable-SD technique of Richert et al hashown.

A melding of the premises thathe dose on the central axis can beiven with reduced IDH and aechnique that expands the penum-ra without the need for extendedistance would accrue the advan-ages of (1) reducing hotspots onhe central axis and on the border ofbutted electron fields and (2) im-roving the surface dose distribu-ion for abutted fields that have amall gap at 100-cm SSD. This ishe essence of the conformal iso-etric abutment operation

CIAO).

MPROVING ELECTRONIELD ABUTMENTHROUGH SYNERGY

adiotherapy optimization enablesosing goals to be met through stra-egic manipulation of various as-ects of dose delivery. Althoughuch tools exist for the delivery ofhotonic radiotherapy, they are notsed for megavoltage electron treat-ent planning. Current practice

equires a quality assurance checkf the treatment planning systemy independent planning software.his software only verifies whether

he primary treatment planning

ystem in the clinic is computing t

he dose correctly. Such software isn “after-the-fact” tool and doesot have a role before the dose cal-ulation in improving the quality ofosimetry planning that a patienteceives. And its primary use is withhotons. Using an inverse planningechnique discussed by Gentry et al12], Trumpet e-IMRT Pre-Plan-ing Software (Standard Imaging,nc, Middleton, Wisconsin), offers“before planning” means of im-

roving patient dosimetry tailoredpecifically for megavoltage elec-ron therapy. Trumpet e-IMRTre-Planning Software is a hybrid-

zation of two important conceptsn electron radiotherapy: boluslectron conformal therapy and in-ensity-modulated electron therapyIMET). The dose prescription en-ered into the Pre-Planning Soft-are is to a depth, not an isodose

ine. The Pre-Planning Softwarean customize the dose distributionn a patient using simple bolus suchs that found in the clinic. The Pre-lanning Software’s inverse plan-ing engine is used to computehe weight of two differentially bo-used beams having the same or dif-erent energies to meet specifiedose constraints on the central axis.his capability makes Trumpet

-IMRT Pre-Planning Software arue IMET tool that is availablelinically (US FDA approval in004). Because the focus of this hy-rid methodology began with con-rolling the dose on the central axis,he gains on the central axis areultiplied at the abutment of elec-

ron fields. And moving beyondraditional notions of IMET withnly 5 electron energies, the Pre-lanning Software enables customepth dose creation to meet user-pecified dose goals through an in-erse planning engine.

The CIAO concept uses the elec-ron energy modulation ability of

he Pre-Planning Software to syner-

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538 The Medical Physics Consult

ize the spoiler and variable-SSDenumbral techniques. By itself,he spoiler technique spreads theenumbra several centimeters in allirections, giving unwanted dose tother nearby normal tissue. Alone,he variable-SSD method allowshe penumbra to be matched atnly one depth, leaving the poten-ial for cold spots above that pointnd hot spots beyond the depth ofhe match point. Compared with

cKenzie’s [10] technique, theIAO framework reduces the dose

o normal tissue outside of the fieldecause the computational solu-ions from the Pre-Planning Soft-are require the bolus to be directlyn the skin. Because the CIAOramework harnesses the variable-

ig 1. Film results from the abutm-mm gap. Even with no gap, thealue but �120%. For the 2-mm-he open-field value. The relative

butment of two 12-MeV electron fi

SD technique to an energy modu-ation tool, the treatment planners able to develop complementaryenumbral solutions on both sidesf the junction that are matched athe surface and beyond.

A treatment solution for a sin-le field generated by the Trum-et e-IMRT Pre-Planning Soft-are is delivered in two parts witharying thicknesses of bolus. Theenumbra at any depth is there-ore a composite of the two seg-ental penumbras. If two pre-

lanned fields are abutted, it isossible to find global solutionshat solve the dose heterogeneityroblem on the border betweenhe fields. By using solutions thatave penumbras that are comple-

nt of two 12-MeV fields, the uppese on the junction between the tw

scenario, the dose in the gap bese does not increase with depth,

elds.

entary to each other on eachide of the junction, adequateosing can be ensured at the sur-ace while reducing or eliminatingarge dose heterogeneities withinhe planning target volume andeyond. If the penumbras onoth sides of the junction cannote matched at 100 cm, one of theelds can be treated at extendedistance to facilitate penumbralatching.After a solution has been chosen

y the user, the monitor units arentered into the treatment plan-ing system for computation of theose in 3-D and display. Eclipseersion 8.6 (Varian Medical Sys-ems, Palo Alto, California) has autton that allows the user to “cal-

ith no gap and the lower with afields is �110% of the open-fieldeen the fields is comparable withis expected in the conventional

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The Medical Physics Consult 539

ulate with pre-set monitor units,”utility that allows the direct entryf the Pre-Planning Software solu-ion into Eclipse. The Pre-Planningoftware solution can then be com-ared with a conventional solutionia the dose-volume histogram.

ONFORMAL ISOMETRICBUTMENT OPERATIONXAMPLES

he usefulness of an abutmentechnique can be measured in sev-ral ways. Dosimetric accuracy, theafety and robustness of the solu-ion, as well as ease of planning andreatment delivery are realisticenchmarks. Two cases in point arerovided.Figure 1 shows two examples

hat illustrate the robustness of theIAO solution to electron field

butment. The upper portion ofigure 1 with “no gap” betweenwo electron fields shows the filmesult of delivering 2 Gy to a depthf 2 cm on the right side of theunction and 2 Gy to a depth of 2.5m on the left side of the junctionsing 12 MeV and the CIAOethod. Because the film gives rel-

tive results, we see that the maxi-um dose at the junction at each

epth shown differs by �20% ofhe dose in the open-field portionf the treatment, even with no gapetween them. When a 2-mm gaps introduced, the dose at the junc-ion differs by �10% from the dosen the open field. This is in con-rast to the result obtained byarms and Purdy [3], who found

hat the dose heterogeneity cre-ted by the abutment of two 12-

eV electron fields was �130%or fields that had no gap betweenhem. Harms and Purdy foundhat the introduction of a 2-mmap reduced the dose heterogene-ty to 20% over the depth from 1

o 3 cm for 100-cm SSD treat- s

ents. The introduction of ex-ended distance to solve the prob-em of surface underdose at00-cm SSD increases the sizend scope of the heterogeneitieslready found to exist at 100-cmSD. It is worth noting that theontributions to the dose hetero-eneity from both sides of theunction are clinically larger be-ause the prescription is writteno the 90% isodose line for anctual patient treatment.

A second example (Figure 2)hows how the CIAO techniqueolves the problem of surface un-erdose without the creation ofotspots through the abutment ofwo 10 � 10 cm 12-MeV electronelds at 100-cm SSD with a gap ofmm on a phantom composed in

art of a 2-D ion chamber arrayMatrixx; IBA Dosimetry, Mem-his, Tennessee) with a 3-mm in-erent buildup. The small buildupn the device permitted dose mea-

Fig 2. Comparison of measured ated fields at 100-cm source-to-sugap between the fields. The wrectangle correspond to the comptively, using the conformal isommethod. The large black rectangsingle 12-MeV fields that are abcomputed and measured ratio rethat achieved by the delivery of twbeams. Conventional abutment100-cm SSD leads to a sizable unthe surface for the CIAO compute100% of the prescribed dose.

urements very close to the surface. f

dose of 2 Gy was prescribed fromhe surface to a depth of 3 cm. Therumpet e-IMRT Pre-Planningoftware was tasked to find a solu-ion to achieve this objective. Theame dose was given to the sameepth on both sides of the junctionsing the CIAO technique. The so-

ution was simulated on Eclipse.he measured dose as determinedy the calibrated ion chamber arrayas compared with the plannedose. The maximum dose on the

unction of the fields was measuredt 5 depths by adding solid water tohe surface of the ion chamber ar-ay. The measured dose for theIAO technique compared favor-

bly with the computed dose. Theose at the surface in the junctionetween the two CIAO fieldsompared favorably with theentral-axis dose, which showshe homogeneous nature of theIAO method. The dose at the sur-

computed dose ratios for abut-ce distance (SSD) with a 2-mm

e triangle and the small blacked and measured doses, respec-tric abutment operation (CIAO)is the computed dose for two

ed with a 2-mm gap. The CIAOlts are more homogeneous thanconventionally delivered 12-MeVtwo 12-MeV electron fields at

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rescribed dose than that achiev-ble through the abutment of twoingle-energy 12-MeV beams at00-cm SSD.

UMMARY

Intensity-modulated electronherapy” has been a catchphrase forome time within the radiotherapyommunity. However, no majorlanning system has actually pre-ented clinicians with a straightfor-ard, easy-to-use tool. Going be-ond the 5 nominal energies thatight be used in a typical IMET

rocedure, Trumpet e-IMRT Pre-lanning Software is able to createhousands of custom dose distribu-ions with user-defined peak dose,ose heterogeneity, R50, and prac-ical range. To the above advan-ages, the Pre-Planning Softwareoncept also provides an importantimplifying aid for the clinical man-gement of abutting electron fields.onventional management of

butting electron fields is consider-ble because field junctions must beoved periodically to minimize the

mpact of dose heterogeneities.his requires close coordinationetween the physician, dosimetry,

hysics, and radiation therapists.

ithout adding overwhelming ad-itional costs to patients or timeommitments from staff members,he CIAO technique in our clinicas offered not only improved do-imetry but also a means of using aingle junction position through-ut treatment.

CKNOWLEDGMENTS

e wish to thank Eric Fleming ofBA Dosimetry for his assistanceith measurement verification andharles Meakin, MD, and Robertoline, MD, for their clinical sup-

ort. A word of thanks is also giveno Susan Harbit, CMD, Kevinpear, CMD, and the entirety ofhe physics, dosimetry, and therapytaff of Gaston Memorial Hospitalor their indispensable help in thelinical implementation of thisechnique.

EFERENCES

1. International Commission on RadiationUnits and Measurements. Prescribing, re-cording, and reporting electron beam ther-apy (Report No 71). Bethesda, Md: Interna-tional Commission on Radiation Units andMeasurements; 2004.

2. Gerbi BJ, Antolak JA, Deibel FC, et al. Rec-ommendations for clinical electron beam

dosimetry: supplement to the recommenda-

tions of Task Group 25. Med Phys2009;36:3239-79.

3. Harms WB, Purdy JA. Abutment of highenergy electron fields. Int J Radiat OncolBiol Phys 1991;20:853-8.

4. Khan F. The physics of radiation therapy.3rd ed. Philadelphia, Pa: Lippincott Wil-liams & Wilkins; 2003:297-356.

5. Kalend AM, Zwicker RD, Wu A, SternickES. A beam-edge modifier for abutting elec-tron fields. Med Phys 1985;12:793-8.

6. Kurup RG, Wang S, Glasgow GP. Fieldmatching of electron beams using plasticwedge penumbra generators. Phys Med Biol1992;37:145-53.

7. Feygelman V, Mandelzweig Y, Baral E.Matching electron beams without second-ary collimation for treatment of extensiverecurrent chest-wall carcinoma. Med Dosim1994;19:23-7.

8. Ulin K, Palisca M. The use of scattering foilcompensators in electron beam therapy. IntJ Radiat Oncol Biol Phys 1996;35:785-92.

9. Lachance B, Tremblay D, Pouliot J. A newpenumbra generator for electron fieldsmatching. Med Phys 1997;24:485-95.

0. McKenzie AL. A simple method for match-ing electron beams in radiotherapy. PhysMed Biol 1998;43:3465-78.

1. Richert JD, Hogstrom KR, Fields RS, Mat-thews KL II, Boyd RA. Improvement offield matching in segmented-field electronconformal therapy using a variable-SCD ap-plicator. Phys Med Biol 2007;52:2459-81.

2. Gentry JR, Steeves R, Paliwal BA. Inverseplanning of energy-modulated electronbeams in radiotherapy. Med Dosim 2006;

31:259-68.

ohn R. Gentry, MS, is from Gaston Memorial Hospital, Gastonia, North Carolina. Lawrence Klonowski, MS, is from Ingallsospital, Harvey, Illinois.ohn R. Gentry, MS, Gaston Memorial Hospital, 2525 Court Drive, Gastonia, NC 28054; e-mail: gentry.jr@gmail.com.