Comparing dose in the build-up region between compensator- and MLC-based IMRT

Transcription

1 JOURAL OF APPLIED CLIICAL MEDICAL PHYSICS, VOLUME 13, UMBER 5, 2012 Comparing dos in th build-up rgion btwn compnsator- and MLC-basd IMRT Khosrow Javdan, 1,2 Goffry G. Zhang, 1a Sarah Hoff, 1 Vladimir Fyglman, 1 Knnth Forstr 1 Radiation Oncology, 1 Moffitt Cancr Cntr, Tampa, Florida; Dpartmnt of Chmical and Biomdical Enginring, 2 Univrsity of South Florida, Tampa, Florida goffry.zhang@moffitt.org Rcivd 23 August, 2011; accptd 23 April, 2012 Th build-up dos in th mgavoltag photon bams can b a limiting factor in intnsity-modulatd radiation thrapy (IMRT) tratmnts. Excssiv surfac dos can caus patint discomfort and tratmnt intrruptions, whil undrdosing may lad to tumor rpopulation and local failur. Dos in th build-up rgion was invstigatd for IMRT dlivry with solid brass compnsator tchniqu (compnsator-basd IMRT) and compard with that of multilaf collimator (MLC)- basd IMRT. A Varian Trilogy linar acclrator quippd with an MLC was usd for bam dlivry. A spcial solid brass stp-wis compnsator was dsignd and built for tsting purposs. Two stp-and-shoot MLC filds wr programmd to produc a similar modulatd stp-wis dos profil. Th MLC and compnsator dos profils wr masurd and adjustd to match at th isocntr dpth of 10 cm. Build-up dos in th 1 5 mm dpth rang was masurd with an ultrathin window, fixd volum paralll plat ionization chambr. Mont Carlo simulations wr usd to modl th brass compnsator and stp-and-shoot MLC filds. Th masurd and simulatd profils for th two IMRT tchniqus wr matchd at th isocntr dpth of 10 cm. Diffrnt componnt contributions to th shallow dos, including th MLC scattr, wr quantifid. Man spctral nrgis for th opn and filtrd bams wr calculatd. Th compnsator and MLC profils at 10 cm dpth wr matchd bttr than ± 1.5%. Th build-up dos was up to 7% lowr for compnsator IMRT compard to MLC IMRT du to bam hardning in th brass. Low-nrgy lctrons contribut 22% and 15% dos at 1 mm dpth for compnsator and MLC modalitis, rspctivly. Compnsator-basd IMRT dlivrs lss dos in th build-up rgion than MLC-basd IMRT dos, vn though a compnsator is closr to th skin than th MLC. PACS numbr: dk, ng Ky words: build-up dos, IMRT, Mont Carlo, compnsator, MLC Conflict of Intrst: Th authors dclar thr ar no conflicts of intrst. I. Introduction Suprficial dos, including th dos in th build-up rgion for mgavoltag bams, has bn an ara of intrst in clinical radiation thrapy sinc th incption of xtrnal bam radiothrapy. (1) Among many tratmnt sits and tchniqus, th dos in th build-up rgion is of intrst for had and nck IMRT tratmnts. (2) Excssiv dos in th build-up rgion can caus rythma and moist dsquamation. An advrs skin raction can lad to a tratmnt brak which multipl studis hav shown to b associatd with wors local control du to tumor cll rpopulation. (3) a Corrsponding author: Goffry G Zhang, Radiation Oncology, Moffitt Cancr Cntr, Magnolia Driv, Tampa, FL 33612, USA; phon: (813) ; fax: (813) ; mail: goffry.zhang@moffitt.org 1 1

2 2 Javdan t al.: Build-up dos in compnsator and MLC basd IMRT 2 Thus a significant clinical concrn is avoiding xcssiv surfac dos to potntially nsur bttr tratmnt complianc and outcoms. On th othr hand, physicians ar concrnd that dlibratly undrdosing skin to avoid an advrs raction may rsult in a local failur in som clinical sttings. Th dos in th build-up rgion is dtrmind by th photon nrgy spctrum and th angular distribution of th photons and lctrons. Ths paramtrs ar not modld wll in commrcial tratmnt planning systms (TPS). As a rsult, thos TPS ar known to b inaccurat in calculating dos in th build-up rgion, as rportd by Chung t al. (4) Ths authors found that th two commrcial TPS ovrstimatd surfac dos by 7.4% to 18.5%. In th IMRT planning procss, (5) th idal variabl flunc maps ar typically dtrmind first. To convrt this idalizd flunc into a physically dlivrabl on, th bam can b modulatd by ithr dividing it into a sris of sub-bams (sgmnts) cratd by a multilaf collimator (MLC), or by insrting a variabl-thicknss solid attnuator in th path of th bam (compnsator-basd IMRT). Bcaus of th uncrtainty in th build-up rgion calculations by th modl-basd tratmnt planning systms, th diffrncs in th suprficial doss btwn th MLC-basd and compnsator-basd IMRT cannot b accuratly ascrtaind simply by comparing th tratmnt plans. Thr ar svral contributing factors that can potntially caus th dos diffrnc btwn th two tchniqus in th build-up rgion. On is th scattr from th compnsator. It has bn prviously shown (6,7) that th closr th compnsator is to th patint, th largr is th scattr contribution to th build-up rgion. On th othr hand, th bam hardning by th compnsator may dcras th suprficial dos. Masuring dos in th build-up rgion poss uniqu challngs du to th lack of lctronic (quasi)quilibrium. Whil paralll plat chambrs ar fairly wll suitd for dos build-up masurmnts bcaus of th minimal volum avraging ffct, thy ar known to ovr-rspond at shallow dpths. (8) An xtrapolation chambr has bn known to provid bttr dosimtric rsults (9) whn compard to th paralll plat chambrs, but it is bulky and tim-consuming to us. Vlky t al. (10) and Rawlinson t al. (11) hav providd simpl ovr-rspons corrctions for th paralll plat chambrs. Bcaus of th challngs posd by masurmnts in th buildup rgion, Mont Carlo (MC) simulations ar considrd on of th mor robust mthods of dtrmining th dos nar th phantom surfac. (12) Th objctiv of our work was to invstigat th dos diffrncs in th build-up rgion btwn th MLC-basd and compnsator-basd IMRT dlivry tchniqus using a MC simulation program. Th study idntifid and valuatd th contaminant radiation (13,14) (scattrd photons and lctrons) within th had of a linar acclrator for th two dlivry tchniqus. Th ffct of this contaminant radiation on th dos in th build-up rgion was invstigatd for varying compnsator-to-surfac distancs (CSD), and th rsults wr compard btwn th two tchniqus. Finally, nrgy spctra for th two dlivry tchniqus wr ascrtaind from th simulations. II. Matrials and Mthods A. Compnsator and MLC filds A solid brass stp compnsator (Fig. 1) was fabricatd by a commrcial vndor (dotdcimal Inc., Sanford, FL) to dlivr an intnsity-modulatd stp-wis profil at 10 cm dpth in watr in a singl fild. Th compnsator was mountd on an opn port Plxiglas tray and insrtd into th acclrator accssory tray mount. A 6MV cm 2 bam was dlivrd using a Trilogy linar acclrator (Varian Mdical Systms, Palo Alto, CA). A Varian Millnnium 120 laf MLC was usd to dynamically gnrat a similar stp-wis dos profil. Th rquird MLC sgmnts wr cratd with Varian SHAPER program (v. 6.2). Du to th larg fild siz xcding th MLC laf xtnsion limits, th fild had to b split in two to dlivr th ntir

3 3 Javdan t al.: Build-up dos in compnsator and MLC basd IMRT 3 Fig. 1. Brass compnsator mountd on an opn port Plxiglas tray insrtd into uppr wdg slot of an acclrator. From lft to right sid of th figur, th stp thicknss was 7.62, 5.08, 4.0, 0.6, 1.0, 2.0, and 3 cm. Stp width varid from 2.5 to 3.4 cm projctd at th isocntr. Th ffctiv jaw stting was cm 2 at th isocntr. profil. To crat a stp-wis profil using MLC tchniqu that was similar to th on gnratd by th compnsator-basd tchniqu, a ratio of th physical bam attnuation of th brass compnsator was usd to rlat th amount of bam-on tim pr sgmnt ndd to crat th profil. Th dos indx (fractional dos sgmnt pr total dos for a fild) for ach sgmnt was stimatd by using Eqs. (1) and (2). BS1 i i 1 x1i x1i (1) BS2 i i 1 x2ii x2i (2) whr BS1 i is th bam-on tim for sgmnt i in th first MLC fild, and BS2 i is th bam-on tim for sgmnt i in th scond MLC fild. For ach MLC sgmnt in th filds, th corrsponding brass stp thicknss is x1i for sgmnt i in th first fild and x2i for sgmnt i in th scond fild. Th ffctiv linar attnuation cofficint μ for brass was dtrmind to b cm -1 by substituting th masurd transmission ratio for a cm 2 fild into th xponntial attnuation formula μ = -ln(transmission)/x, whr x is th brass thicknss in cm. Equations (1) and (2) rlat th fractional bam-on tim for th MLC sgmnts to th physical thicknss, and thrfor attnuation, of th compnsator stps. In ordr for th two MLC filds to produc th sam stp-wis dos distribution as that of th compnsator stps, w nd to apply th sgmnt wighting to ach sgmnt and add thm togthr.

4 4 Javdan t al.: Build-up dos in compnsator and MLC basd IMRT 4 Th sgmnt wights SW1 i and SW2 i ar calculatd using Equations 3 and 4: MU1 SW1i BS1i MU 1 MU 2 MU 2 SW 2i BS2i MU1 MU 2 (3) (4) whr MU1 and MU2 ar th monitor units for filds 1 and 2, rspctivly, which can b writtn as MU1 x1i i 1 x1i i i 1 i 1 x2i (5) MU 2 j 1 x1i x2i i 1 i 1 x2i (6) Th stp-wis IMRT profils from th compnsator-basd and th MLC tchniqus wr masurd at 10 cm dpth using a commrcially availabl linar diod array dtctor systm, Profilr (Sun uclar Corporation, Mlbourn, FL). Th MLC-basd stp-wis profils wr matchd to th compnsator-basd IMRT profils by adjusting th dos indx for ach sgmnt and minor adjustmnts to th MLC positions. Aftr th rlativ profils wr matchd, th final stp was to obtain th sam ionization charg on th cntral axis with an ionization chambr. An ultra-thin window ( mm) fixd volum paralll plat ionization chambr (EXRADI Modl A10 Standard Imaging. Inc., Middlton, WI) was usd. Th chambr in th Plastic Watr phantom (CIRS Inc., orfolk VA) was positiond at th normalization point at isocntr at 10 cm dpth (90 cm SSD). Basd on th chambr radings, minor adjustmnt was mad to th monitor units for th compnsator dlivry. Aftr th absolut dos profils wr matchd, chambr masurmnts wr mad in th build-up rgion. B. Chambr masurmnts in th build-up rgion Chambr radings for ach profil stp wr collctd at 1, 3, and 5 mm dpths in Plastic Watr. Th latral shift for ach stp was mad by using th latral couch position radout at th acclrator consol. Th distanc for ach shift was doubl-chckd with a mtal rulr. Th chambr radings wr normalizd to th radings obtaind during th profils matching procdur dscribd abov. Th chambr radings wr corrctd using th Rawlinson mthod (11) to account for gomtry and wall matrial dnsity: CEl ( ) P( d) Pd ( ) W 4d ( ) 0.8 dmax (7) whr P / (d) is th corrctd dos at dpth d, P(d) is masurd dos at d, th nrgy dpndnt factor C(E) is 0.27 for a 6 MV bam, (11) l is th plat sparation, W is th innr wall diamtr, ρ is th wall matrial dnsity, d is th dpth to th front surfac of chambr, and d max is th dpth of maximum dos.

5 5 Javdan t al.: Build-up dos in compnsator and MLC basd IMRT 5 C. Mont Carlo modling An EGSnrc-basd(15) MC simulation packag for clinical radiation tratmnt units, BEAMnrc(16), was usd. C.1 Modl validation Th prcntag dpth dos curvs as wll as th bam profils in a watr phantom from MC simulations wr compard with th masurd data for 5, 10, 20 and 40 cm opn squar fild sizs. Rprsntativ xampls of ths comparisons (for a and a cm2 fild at dmax and at a dpth of 10 cm), ar shown in Fig. 2. (a) (b) Fig. 2. Comparison btwn masurd and calculatd prcnt dpth dos curvs and bam profils for cm2 (a) and cm2 (b) filds. Th profils wr compard at th dpths of dmax and 10 cm in watr.

6 6 Javdan t al.: Build-up dos in compnsator and MLC basd IMRT 6 Aftr succssful validation, phas spac fils for diffrnt jaw opnings wr gnratd, in which th physical paramtrs for all th particls travrsing th plan of intrst blow th scondary jaws wr stord. Ths fils ar thn usd as radiation sourcs for MLC and compnsator simulations, obviating th nd to rsimulat th acclrator had ach tim. C.2 Gomtric modling of th compnsator and MLC filds Th componnt modul BLOCK in BEAMnrc was usd for th gomtric modling of th masurd compnsator physical dimnsions, including divrgnc. Th MLC was modld using th appropriat componnt modul in BEAMnrc. Th acclrator modls ar shown in Fig. 3. Th nominal widths of both th compnsator stps, and MLC sgmnts wr slightly adjustd to match th simulatd profils to th masurmnt at 10 cm dpth. Although small (a fraction of a millimtr), ths adjustmnts wr instrumntal to obtaining a good match. Phas spac fils calculatd ithr blow th MLC or th compnsator wr usd as radiation sourcs for phantom dos distribution calculations using DOSXYZnrc. (16) A flat watr phantom was st up downstram from th phas spac plans at ithr 32.4 cm CSD (90 cm SSD), or 48.4 cm CSD (106 cm SSD) to calculat th cntral axis dpth doss and contributions of contaminant radiation for ach MLC sgmnt and th compnsator. Appropriatly wightd contributions from ach MLC sgmnt wr addd togthr to dtrmin th cumulativ modulatd dos distribution undr th MLC. Fig. 3. Mont Carlo modl of a Varian Trilogy acclrator had gomtry. Major parts such as th targt (1), primary collimator (2), flattning filtr (3), transmission chambr (4), and jaws (5) ar shown in panl (a). MLC (6), th stp wdg (7), and th phantom top (8) ar shown in panl (b). C.3 Sgmnt wights Stp-wis dos profils wr calculatd using MC for both th MLC and brass compnsator tchniqus. Th xact wights of ach MLC sgmnt and th thicknss of ach brass stp wr again slightly adjustd as a part of matching with th masurmnts. Th modifications wr small, within th xpctd uncrtaintis of MC calculations. Finally, th calculatd doss for th MLC- and compnsator-basd modulation tchniqus wr matchd at 10 cm dpth. Th masurd and calculatd doss in th build-up rgion and dos profils at 10 cm dpth wr compard for th two dlivry tchniqus.

7 7 Javdan t al.: Build-up dos in compnsator and MLC basd IMRT 7 C.4 Dos in th build-up rgion as a function of compnsator-to-surfac distanc (CSD) MC simulations wr don to approximat th build-up dos diffrnc with th CSD changs. This is only an approximation of th diffrncs on would s with a diffrnt acclrator; as for an accurat stimation, a complt simulation of that acclrator would b rquird. To approximat th ffcts du to th CSD chang, but not an SSD chang, all th rsults wr corrctd using th Maynord s F factor. (17) C.5 Dpth dos and contributions of contaminant radiation Th componnt modul (CM), CHAMBER, was usd as a phantom to facilitat th dpth dos, as wll as contaminant radiation dos calculations in BEAMnrc. Total dos and contaminant radiation doss wr calculatd in watr at 1.0, 3.0, 5.0, 10.0, 15.0, 53, and 100 mm dpths. Th contaminant dos contribution was from scattrd photons and lctrons from th acclrator had and th MLC. To invstigat th ffct of bam hardning by th compnsator, th calculatd nrgy flunc distributions wr compard for a 2 15 cm 2 opn fild, and th sam bam filtrd by a 2 cm thick slab of brass. III. Rsults A. Modl validation Th Mont Carlo modl was validatd by matching th calculatd and masurd 6 MV bam profils and prcnt dpth dos curvs for th small and larg opn filds at svral dpths in watr. Th prcntag dpth dos curvs and profils from MC simulations matchd th masurd data within ± 1% for th low gradint dos rgion. In rgions of build-up or pnumbra, th distanc btwn th calculatd and masurd profils was within 1 mm, with an xampl shown in Fig. 2. B. Stp-wis dos profil match at 10 cm dpth To collct th sam ionization charg at th normalization point, a total of 729 MUs (600 MU for fild MU for fild 2) wr dlivrd with th MLC sgmnts and 262 MUs with th compnsator. Matchd dos profils at th isocntr (10 cm dpth) ar shown in Fig. 4. Th disagrmnt btwn th masurmnts and calculatd profils did not xcd ± 1.5%. Fig. 4. Th matchd stp-wis profils producd by an MLC and a solid compnsator. Th cntr of 6 MV bam travrss th thinnst part (0.6 cm) of th brass compnsator.

8 8 Javdan t al.: Build-up dos in compnsator and MLC basd IMRT 8 C. Dos comparison in th build-up rgion Th MC calculations and masurd data agr within ± 2% for compnsator thicknsss of 1, 2, and 3 cm. Th doss in th build-up rgion at 1, 3, and 5 mm dpths ar prsntd in Tabls 1(a), (b), and (c). Th dos was consistntly lowr with th compnsator-basd IMRT dlivry compard to th MLC-basd dlivry, with th maximum diffrnc of 7% at 5 mm dpth undr th 2 cm thick brass compnsator. Th p-valus for th paird t-tst btwn th calculatd and masurd doss for th compnsator stps wr 0.11, 0.18, and 0.63 for 1, 3, and 5 mm dpths, rspctivly. Th corrsponding p-valus for th MLC stps wr 0.27, 0.16, and Th diffrnc btwn masurd and calculatd doss for compnsator stps and MLC stps was not statistically significant (p > 0.05). Th dos in th build-up rgion was slightly lowr for th compnsator compard to th MLC tchniqu undr th 1.0 and 3.0 cm thick stps. Th t-tst p-valus btwn th calculatd doss for compnsator and MLC stps wr 0.009, 0.002, and for 1.0, 3.0, and 5.0 mm Ta b l 1(a). Doss (cgy) at 1 mm dpth in Plastic Watr and with th compnsator and th MLC as a function of quivalnt stp thicknss of th compnsator. Doss wr calculatd with Mont Carlo and masurd with paralll plat ionization chambr at 90 cm SSD, with th dos at isocntr bing 100 cgy. STEP COMP MLC (cm) Calculatd Masurd Calculatd Masurd ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.2 Ta b l 1(b). Doss (cgy) at 3 mm dpth in Plastic Watr and with th compnsator and th MLC as a function of quivalnt stp thicknss of th compnsator. Doss wr calculatd with Mont Carlo and masurd with paralll plat ionization chambr at 90 cm SSD, with a dos at isocntr bing 100 cgy. STEP COMP MLC (cm) Calculatd Masurd Calculatd Masurd ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.2 Ta b l 1(c). Doss (cgy) at 5 mm dpth in Plastic Watr with th compnsator and th MLC as a function of quivalnt stp thicknss of th compnsator. Doss wr calculatd with Mont Carlo and masurd with paralll plat ionization chambr at 90 cm SSD, with a dos at isocntr bing 100 cgy. STEP COMP MLC (cm) Calculatd Masurd Calculatd Masurd ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.2

9 9 Javdan t al.: Build-up dos in compnsator and MLC basd IMRT 9 dpths, rspctivly. Th corrsponding valus for th masurd doss wr 0.003, 0.001, and Th diffrnc btwn th compnsator and MLC doss corrsponding to diffrnt stps was statistically significant (p < 0.05). D. Contribution of contaminant radiation Tabl 2 shows th contribution of contaminant radiation from th solid compnsator and th stp-and-shoot MLC calculatd as a prcntag of th total dos at slctd dpths at th cntral axis. Th photon dos contribution was 78%, 91%, 95%, and 99% of th total dos for th compnsator and 82%, 92%, 95%, and 99% of th total dos for th MLC tchniqu at 32.4 cm CSD and at 1.0, 3.0, 5.0 and 15.0 mm dpths in watr, rspctivly. At 48.4 cm CSD, th photon contribution was 82%, 93%, 96%, and 99% of th total dos for th compnsator tchniqu and 83%, 92%, 95%, and 100% of th total dos for th MLC, at th sam st of dpths. Th contaminant lctron dos contribution was 22%, 9%, 5%, and 1% of th total dos for th compnsator tchniqu and 15%, 6%, 3%, and 0% of th total dos for th MLC dlivry, at 32.4 cm CSD and at 1.0, 3.0, 5.0, and 15.0 mm dpths in watr, rspctivly. At 48.4 cm CSD, th contaminant lctron dos contribution was 18%, 7%, 4%, and 1% of th total dos for th compnsator and 15%, 6%, 4%, and 0% of th total dos for th MLC tchniqu at th sam dpths. MLC scattr dos contribution to th total dos was 3%, 2%, 2%, and 1% of th total dos at 32.4 cm CSD, at 1.0, 3.0, 5.0, and 15.0 mm dpths in watr, rspctivly. At 48.4 cm Ta b l 2. Th total dos and prcntag contribution from th photons, contaminant lctrons, and MLC scattr ar shown as a function of dpth for th two IMRT tchniqus at (a) 32.4 cm CSD (90 cm SSD) and (b) 48.4 cm CSD (106 cm SSD). IMRT Dpth Total Dos Photons Contaminant Elctrons MLC Componnt (cm) cgy % % % Solid ± Compnsator ± (a) ± ± ± ± ± Stp-and-shoot ± MLC ± (a) ± ± ± ± ± Solid ± Compnsator ± (b) ± ± ± ± ± Stp-and-shoot ± MLC ± (b) ± ± ± ± ± ±

10 10 Javdan t al.: Build-up dos in compnsator and MLC basd IMRT 10 CSD, th MLC scattr dos contribution was 2%, 2%, 1%, and 1% of th total dos for th sam dpths. Th photon bam spctra for an opn bam and a bam attnuatd with a 2 cm thick brass compnsator ar shown in Fig. 5. Th avrag nrgy incrasd from 1.57 ± 0.10 MV for th opn bam to 2.17 ± 0.10 MV for th bam filtrd by th 2 cm of brass compnsator. Fig. 5. ormalizd planar nrgy flunc distribution of 6 MV bam for 2 15 cm 2 fild with and without a 2 cm brass slab. IV. DISCUSSIO This study was limitd to tsts conductd with a simpl stp-wis brass compnsator and a corrsponding MLC sgmnt arrangmnt producing a similar flunc profil. This corrsponds to fairly simpl IMRT filds, and th rsults should not b automatically xtrapolatd to th IMRT bams with highr dgr of modulation, particularly whn a significant numbr of MLC sgmnts ar lss than 2 cm in width. Th sourcs of th dosimtric diffrncs at shallow dpths btwn th MLC and compnsator modulators can b only quantifid with MC calculations, whr th contributions of th diffrnt particls can b valuatd sparatly. Howvr, Mont Carlo simulations also hav limitations. As th dos calculation grid nds to bcom finr to accuratly calculat th stp dos gradints in th build-up rgion, th numbr of particls intracting in ths thin slabs diminishs. Undr ths circumstancs, to prform a calculation with small uncrtainty a prohibitivly larg numbr of historis would b rquird to achiv th rquisit statistics. Th input fil for th individual MLC sgmnt simulations in this work was rstartd 7 tims to achiv bttr statistics. Th simulation showd that th dos in th build-up rgion was lowr undr th 2 cm thick compnsator compard to an MLC sgmnt arrangmnt providing th sam dgr of modulation. This dos rduction was du to bam hardning producd by th compnsator, as vidncd by th photon spctra comparison (Fig. 5). In support of having slctd th 2 cm thick compnsator for dos comparisons, in a sparat but rlatd study, Opp t al. (18) hav analyzd prviously plannd cass using IMRT with brass compnsators. In that work, a histogram of transmission factors (plottd by compnsator thicknss) for 10 cass (with a total of 50 brass compnsators) was gnratd. It is clar that

11 11 Javdan t al.: Build-up dos in compnsator and MLC basd IMRT 11 th most probabl compnsator thicknss in th modulation rgion was about 2 cm, as shown in Fig. 6. Th closr th compnsator is to th patint, th highr skin dos th patint would gt, du to th scattrd low nrgy photons and lctrons. In all acclrator configurations, th brass compnsator is always closr to th patint than th MLC is. Howvr, MC simulations in this study indicat that for th modulation lvl quivalnt to 2 cm of brass, th compnsator dos was 5%, 6%, and 7% lowr than th MLC dos at 1, 3, and 5 mm dpths, rspctivly. Extndd SSD masurmnts and simulations in this study suggst that th us of acclrators othr than Varian, which can afford largr compnsator to surfac distanc, would furthr rduc th dos in th build-up rgion whn using compnsators. Fig. 6. Compnsator thicknss analysis basd on 50 rtrospctiv IMRT filds (Opp t al.(18), with prmission). V. Conclusions Low-nrgy scattrd photons and lctrons ar th major contributors to dos in th build-up rgion nar th surfac. Th dos from contaminant lctrons sharply dcrass in magnitud with dpth, whil th photon contribution incrass with dpth in th build-up rgion. This trnd was similar for both 32.4 and 48.4 cm CSDs (90 and 106 cm SSDs), xcpt th dos in th build-up rgion was rducd at xtndd CSD, particularly with th compnsator. Th bam hardning ffct in compnsator-basd IMRT rducs th numbr of low-nrgy photons in th tratmnt bam which, in turn, rducs th shallow dos. Evn with a smallr distanc to patint skin compard to that of MLC-basd IMRT, compnsator-basd IMRT still dlivrs lowr build-up dos, which could b bnficial for skin sparing in crtain radiothrapy tratmnts. Rfrncs 1. Dvic S, Hgyi G, Vuong T, Muanza T, Podgorsak EB. Comparativ skin dos masurmnt in th tratmnt of anal canal cancr: convntional vrsus conformal thrapy. Md Phys. 2004;31(6): Palinck L, D Wagtr C, VanEsch A, Duthoy W, Dpuydt T, Dv W. Comparison of build-up dos btwn Elkta and Varian linar acclrators for high-nrgy photon bams using radiochromic film and clinical implications for IMRT had and nck tratmnts. Phys Md Biol. 2005;50(3): Bs S, Hndry J, Jrmic B. Effcts of prolongation of ovrall tratmnt tim du to unplannd intrruptions during radiothrapy of diffrnt tumor sits and practical mthods for compnsation. Int J Radiat Oncol Biol Phys. 2007;68(3): Chung H, Jin H, Dmpsy JF, t al. Evaluation of surfac and build-up rgion dos for intnsity-modulatd radiation thrapy in had and nck cancr. Md Phys. 2005;32(8):

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