Skyshine photon doses from 6 and 10 MV medical linear accelerators

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1 JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 13, NUMBER 1, 01 Skyhne photon doe from 6 nd 10 MV medcl lner ccelertor Edurdo de Pv, nd Luz A. R. d Ro Inttuto de Rdoproteção e Dometr IRD/CNEN/MCT, Recreo do Bndernte Ro de Jnero RJ, Brzl epv@rd.gov.br Receved 3 My, 011; ccepted 30 Augut, 011 The kyhne rdton phenomenon cont of the ctterng of prmry photon bem n the tmophere bove the roof of medcl lner ccelertor fclty, genertng n ddtonl doe t ground level n the vcnty of the tretment room. Thu, wth repect to rdoprotecton, th tuton ply n mportnt role when the roof degned wth lttle heldng nd there re buldng next to the rdotherpy tretment room. In lterture, there re few reported kyhne-meured doe nd thee contn poor greement wth emprcl clculton. In th work, we crred out meurement of kyhne photon doe rte produced from eght dfferent 6 nd 10 MV medcl ccelertor. Ech meurement w performed outde the room fclty, wth the bem potoned n the upwrd drecton, t horzontl dtnce from the trget nd for 40 cm 40 cm mxmum photon feld ze t the ccelertor ocenter. Meured doe-equvlent rte reult were compred wth clculton obtned by n emprcl expreon, nd dfference between them devted n one or more order of mgntude. PACS number: j, Bn Key word: kyhne, heldng, medcl ccelertor I. Introducton When the r bove the roof of rdotherpy fclty room ctter the prmry photon bem from medcl lner ccelertor, th try rdton known kyhne rdton. (1) The kyhne doe re prtculrly mportnt when the celng of teletherpy tretment room degned wth lttle or no heldng. There re few reported experment on econdry kyhne rdton, (,3) o new meurement of th type of rdton ply n mportnt role on the undertndng of the kyhne phenomenon telf. The m of th work to compre the meured kyhne photon doe rte n the vcnty of eght dfferent teletherpy tretment room houng 6 MV nd 10 MV medcl ccelertor wth emprcl clculton prevouly reported. (,3) Conderton of the lekge rdton from the ccelertor hed trnmtted through the lterl wll to the meurement of kyhne doe rte lo dcued. II. Mterl nd Method The geometry employed n kyhne photon doe rte meurement crred out n th work hown n Fg. 1. The photon bem drected upwrd (180 gntry poton), wth mxmum Correpondng uthor: Dr. Edurdo de Pv, Servço de Fíc Médc em Rdoterp e Medcn Nucler, Inttuto de Rdoproteção e Dometr IRD/CNEN/MCT, Av. Slvdor Allende /n, Recreo do Bndernte, CEP , Ro de Jnero, Brzl; phone: ; fx: ; eml: epv@rd.gov.br 15 15

2 16 de Pv nd d Ro: Skyhne doe from medcl ccelertor 16 Fg. 1. The geometry ued n kyhne photon doe rte meurement n the vcnty of rdotherpy tretment room. 40 cm 40 cm photon feld ze t ocenter nd wth no phntom. The photon bem form old ngle Ω defned : (4) 4rcn 4h (1) whch depend on the feld ze (40 cm) nd the ource-x dtnce h (100 cm). A ponted out n the work of Gomn et l., (4) Eq. (1) gve the correct wy to determne the old ngle when the rdton bem reemble n nverted pyrmd of be. The term d tht pper n Fg. 1, the dtnce n meter from the trget to pont meter bove the roof, nd d the horzontl dtnce n meter from the ocenter to the pont M where the doe rte meured. The urvey meter ued n the meurement of equvlent doe rte H were onzton chmber, nmely SmrtION model 10 R, erl number 468 nd 4779 (Thermo Fher Scentfc, Wlthm, MA) nd Inovon model 451P, erl number 541 nd 54 (Inovon Softwre Soluton Inc., Cheterfeld, MI), ll of them nnully clbrted n the Secondry Stndrd Dometry Lbortory (SSDL) of Brzl. The uncertnty octed wth urvey meurement w provded by the clbrton procedure nd w better thn 10% for confdence level of 95%. In th tudy, the dt of kyhne rdton meurement from eght dfferent medcl lner ccelertor were nlyzed. The equpment were even 6 MV mchne nd one 10 MV ccelertor, produced by Vrn Medcl Sytem nd Semen Medcl Soluton Mnufcturer. They were ntlled n dfferent rdotherpy fclte. The pont M (1.5 m bove the ground urfce) where meurement were conducted were loclzed n uncontrolled re wth ntegrl occupncy, nmely prkng lot or grden. For ech fclty, the doe rte w meured only t the pont M, hown n Fg. 1, whch the neret pont locted outde the tretment room tht n oberver hypothetclly locted two meter bove the celng n centrl x of the bem cn ee. Lekge rdton cn trvere the lterl wll of the tretment room nd lo rech the locton of meurement contmntng the kyhne rdton level. So the lekge doe rte hould be ubtrcted from the totl doe rte t M. The lekge contrbuton to the doe rte cn be evluted by ettng up 0 cm 0 cm feld ze, when no rdton cttered on the r bove the roof ext. Another contrbuton to the doe rte t pont M the prmry rdton cttered on the nde wll of the room, but we umed t to be neglgble. Journl of Appled Clncl Medcl Phyc, Vol. 13, No. 1, 01

3 17 de Pv nd d Ro: Skyhne doe from medcl ccelertor 17 In prevou tude, (,3,5) the followng emprcl expreon w ued to clculte the doeequvlent rte H (n nsv/h) due to kyhne rdton: H d B. D.d () where D 0 the X-ry borbed doe output rte (n Gy/h) t the ccelertor ocenter nd B the trnmon coeffcent through the roof (whch n ll ce nvetgted n th tudy were mde of common concrete of.35 g/cm 3 denty) nd cn be clculted by: B 1 ( t TVL1 ) TVL e 10 (3) In the equton bove, TVL 1 nd TVL e re the frt nd econd tenth-vlue lyer of common concrete, whch re, repectvely, 37 cm nd 33 cm for n endpont energy of 6 MV, nd 41 cm nd 37 cm for n endpont energy of 10 MV, (5) nd t the brrer thckne. Tkng nto conderton the thcknee of the celng of the tretment room nlyzed n th work, we obtned the trnmon coeffcent B n the rnge The X-ry borbed doe output rte for the eght ccelertor rnged from 00 to 50 cgy/mn t ocenter. The vertcl dtnce d w 5.5 meter for the room t ll fclte, nd the horzontl dtnce d w n the 8.8 m 15 m rnge. Followng the generlly ccepted recommendton to lwy ue the wort tuton n the clculton of brrer heldng (.e., the lrget doe whch mply hgher brrer thcknee), no phntom w plced crong the rdton pthwy nd mxmum 40 cm 40 cm feld ze w ued long ll meurement. Therefore, the old ngle remned contnt nd equl to terdn. III. Reult & DISCUSSION The meurement of kyhne rdton level t pont M outde the tretment room were performed ccordng to the geometry depcted n Fg. 1. The meured equvlent-doe rte H functon of the horzontl dtnce d from the ocenter for eght dfferent medcl lner ccelertor howed n Fg.. In Fg., the error br gned to the clbrton procedure cnnot be een becue they re mller thn the ymbol nd the full lne re merely to gude the eye. In Fg., crcle tnd for 6 MV meured doe rte, trngle tnd for 10 MV meured doe rte, nd qure tnd for emprcl clculton. (,3,5) Dhed lne repreent the permble doe rte lmt for uncontrolled re. (6) In the meurement crred out n th mple work we ntended to ue lwy the wort tuton n the ement of the photon doe rte due to kyhne. Th conervtve umpton permtted u to et up the unchnged 40 cm 40 cm photon bem feld ze long the meurement. In our nvetgton, we lo dd not conder horzontl dtnce mller thn the dtnce d howed n Fg. 1. Thee pont re nde hdow cone where the doe rte re expected to be lower, reult lredy oberved n the work by Gomn et l. (3) In th tudy, we lo nvetgted the doe rte due to lekge rdton from the ccelertor hed tht trnmtted through the lterl wll of the tretment room nd cn be ummed up to the kyhne doe rte t pont of nteret. Th lekge contrbuton w evluted by ettng Journl of Appled Clncl Medcl Phyc, Vol. 13, No. 1, 01

4 18 de Pv nd d Ro: Skyhne doe from medcl ccelertor 18 Fg.. The equvlent doe rte functon of the horzontl dtnce from ocenter. Crcle = 6 MV meured doe rte; trngle = 10 MV meured doe rte; qure = clculton by men of Eq. (); dhed lne repreent the permble doe rte lmt for uncontrolled re. up 0 cm 0 cm feld ze t ocenter when no kyhne contrbuton ext. Meurement hve ndcted tht lekge component to doe t M repreent 1% of the totl doe. A cn be oberved n Fg., the mjorty of meured doe rte dt proved to be no greter thn the vlue 0.0 msv/week (or nsv/ec) of the permble doe lmt for uncontrolled re. (5,6) In Fg., we lo how emprcl clculton obtned by men of Eq. () nd ung the prmeter decrbed n the prevou ecton. In generl, thee etmton reulted n vlue one order of mgntude or more bove the meured one t dtnce from the lterl wll up to the pont M hown n Fg. 1, dfference lredy reported n other work. (,3) IV. Concluon The lrget photon feld ze, whch correpond to the hghet doe, were ued to nvetgte the contrbuton of the econdry kyhne rdton to the doe rte outde teletherpy tretment room houng 6 nd 10 MV lner ccelertor. Meurement hve ndcted tht only one vlue of kyhne doe rte w greter thn the permble heldng degn gol for the uncontrolled re nvetgted, due to poor heldng n tht room. A poor greement, of one order of mgntude or more, w found between meured kyhne photon doe rte nd emprcl etmton, ndctng tht uch clculton hould be ued wth cuton nd erve only rough etmton of kyhne doe rte contrbuton. Aprt from the lrge dcrepnce found between clculton nd meurement of photon doe rte, emprcl clculton hve the mert of howng the dependency between kyhne rdton nd tretment room geometrc quntte. Journl of Appled Clncl Medcl Phyc, Vol. 13, No. 1, 01

5 19 de Pv nd d Ro: Skyhne doe from medcl ccelertor 19 Reference 1. Ntonl Councl on Rdton Protecton nd Meurement. Rdton protecton degn gudelne for MeV prtcle ccelertor fclte. NCRP Report No. 51. Whngton, D.C.: NCRP; McGnley PH. Rdton kyhne produced by n 18 MeV medcl ccelertor. Rd Prot Mgmt. 1993;10(5): Gomn MS, McGnley PH, Rng MB, Phkkl AJ. Rdton kyhne from 6 MeV medcl ccelertor. J Appl Cln Med Phy. 010;11(3): Gomn MS, Phkkl AJ, Rng MB, McGnley PH. Provdng old ngle formlm for kyhne clculton. J Appl Cln Med Phy. 010;11(4): Ntonl Councl on Rdton Protecton nd Meurement. Structurl heldng degn nd evluton for megvoltge x- nd gmm-ry rdotherpy fclte. NCRP Report No Bethed, MD.: NCRP; Comão Nconl de Energ Nucler. Dretrze bác de rdoproteção. CNEN-NN-3.01 [n Portuguee]. Ro de Jnero, Brzl: CNEN; 005. Journl of Appled Clncl Medcl Phyc, Vol. 13, No. 1, 01