Monte Carlo study of fluence perturbation effects on cavity

Monte Carlo study of fluence perturbation effects on cavity

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Monte Carlo study of fluence perturbation effects on cavity dose response in clinical protonbeams

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Monte Carlo study of fluence perturbation effects on cavity

Phys.Med.Biol.43(1998)65–89.PrintedintheUKPII:S0031-9155(98)86156-3

MonteCarlostudyof uenceperturbationeffectsoncavitydoseresponseinclinicalprotonbeams

HugoPalmansandFrankVerhaegen

DepartmentofBiomedicalPhysics,UniversityofGent,Proeftuinstraat86,B-9000Gent,Belgium

Received21July1997,in nalform4September1997

Abstract.Currentprotocolsforclinicalprotonbeamdosimetryhavenotimplementedanychamber-dependentcorrectionfactorsforabsorbeddosedetermination.ThepresentworkinitiatesaMonteCarlostudyofthesefactorswithemphasisonproton uenceperturbationeffectsandpreliminarycalculationsofperturbationeffectsfromsecondaryelectrons.

TheprotonMonteCarlocodePTRANwasmodi edtoallowsimulationofprotontransportinnon-homogeneousgeometriesofbothunmodulatedandmodulatedbeams.Thedosetowaterderivedfromthedosecalculatedinanaircavityagreeswellwithresultsfromanalyticalcalculationsassumingadisplacementofthepointofmeasurement.Forunmodulatedbeamssmalldifferences,limitedto0.8%,couldbepartiallyattributedtoprotonmultiplescattering.Effectsofreplacingwateraroundthecavitywithwallmaterialareexplainedbytheintroductionofawater-equivalentwallthickness.Formodulatedbeamsnosigni cantperturbationeffectsarise.Secondaryelectronspectraarecalculatedanalytically.PreliminaryelectrontransportcalculationswithEGS4showthatwallperturbationsoftheorderof1%couldresult.

Perturbationeffectscausedbytheenergytransportofsecondaryparticlesfrominelasticnuclearinteractionshavenotbeenstudiedhere.Inclusionofinelasticnuclearenergytransfersinthecavitydose,assumingtotallocalabsorption,indicatethatseparatescalingofthiscontributionwiththeratiooftotalinelasticnuclearcrosssectionscouldbeimportant.

1.Introduction

CurrentdosimetryprotocolssuchastheAAPMTG16protocol(AAPM1986)andtheECHEDprotocol(Vynckieretal1991,1994)fordosetowaterdeterminationwithionizationchambersinclinicalprotonbeamswithanenergyrangeof50–250MeVhavenotincludedanychamber-dependentcorrectionfactors.

Medinetal(1995)proposedanexpressionfortheabsorbeddosetowaterDw,QatthereferencepointinaprotonbeamcorrespondingtotheNDformalismoftheIAEACodeofPracticeforphotonandelectronbeamdosimetry(IAEA1987)inwhichaperturbationcorrectionfactorpQisde ned:

Dw,Q=MQND,Q0

(Wair)Q

(sw,air)QpQ

(Wair)Q0

(1)

inwhichQandQ0denotetheprotonbeamqualityandthecalibrationbeamqualityrespectively,MQistheionizationchamberreading,correctedforatmosphericnon-standardconditions,forrecombinationandforpolarityeffects,ND,Q0istheabsorbeddosetoaircalibrationfactorfortheionizationchamber,(Wair)Q/(Wair)Q0istheratioofthemeanenergyrequiredtoproduceanionpairinthetwobeamqualitiesand(sw,air)Qisthewater

c1998IOPPublishingLtd0031-9155/98/010065+25$19.50

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Monte Carlo study of fluence perturbation effects on cavity

66HPalmansandFVerhaegen

toairmassstoppingpowerratiointheprotonbeam.Inprinciple,restrictedstoppingpowersshouldbeusedtoaccountforthedistinctionbetweenenergylossestosecondaryelectronsthatareabletotransferenergyawayfromthegenerationpointandthosethatarenot,asforphotonbeams(SpencerandAttix1955).Thisraisesthequestionofthecut-offenergyandofthedealingwithtrack-endswhichwewillnotdiscusshereasitisoutofthescopeofthepresentwork.ThetotalperturbationcorrectionfactorpQconsistsoftheproductoffactorsforthenon-waterequivalenceofthewall(pwall),forthecentralelectrodeeffect(pcel gbl),fortheperturbationofelectron uenceduetoinsertionoftheaircavityinwater(pcav)andoptionally,ifthegeometricalcentreofthechamberisusedasreferencepoint,forthedisplacementeffect(pdispl).Wecanremarkthatthede nitionofpcavcouldbegeneralizedto uenceperturbationofallsecondarychargedparticles.

Regardingthe(Wair)pvalueforprotonsthereisstillarelativelylargeuncertaintyduetothedif cultyofdeterminingthisfactor.RecentcomparisonsofionizationchamberdosimetryandwatercalorimetrybySiebersetal(1995)andPalmansetal(1996)indicatethatthevalueadoptedbytheAAPMprotocol(AAPM1986)isclosertorealitythanthevaluerecommendedbytheECHEDprotocol(Vynckieretal1991,1994).However,onlytheproductof(Wair)pand(sw,air)pcanbedeterminedaccuratelybyuseofwatercalorimetry.

Regarding(sw,air)pforclinicalprotonbeams,importantworkhasbeendonebyMedinandAndreo(1992,1997a).Theyshowedthatthecontributionsfromsecondaryprotonstosw,airarelessthan0.1%,whereasthecontributionfromsecondaryelectronscouldchangethewatertoairstoppingpowerratiobyupto0.6%forprotonsintheclinicalenergyrange.

ItisgenerallyassumedthattheperturbationcorrectionfactorpQisveryclosetounityorthat,atleast,itsdeviationfromunityiswellwithinthetotaluncertaintyofthe naldose.Regardingprimaryproton uenceperturbations,thiscanbearguedtobeduetothelowscatteringcharacteristicsofprotons.Perturbationeffectscausedbyanincompletesecondaryelectronequilibriumarealsocommonlyneglected.Themaximumenergyofsecondaryelectronsisthreeordersofmagnitudesmallerthantheprotonenergysotherangeinwaterofmostsecondaryelectronsisverysmallanditisassumedthatonlysmalleffectsaretobeexpected.Furthermore,perturbationeffectscausedbysecondaryparticlesoriginatingfrominelasticnuclearinteractionsarealsoneglected.Forthemajorityofthechargedsecondaryparticlesthestoppingpowersaremuchhigherthanforprimaryprotonsresultinginveryshortranges.Onlyneutronsandsecondaryprotonshaverangeslargeeno …… 此处隐藏：39356字，全部文档内容请下载后查看。喜欢就下载吧 ……