Dose distribution verification for GYN brachytherapy using EBT Gafchromic film and TG-43 calculation

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Original

research

article

Dose

distribution

verification

for

GYN

brachytherapy

using

EBT

Gafchromic

film

and

TG-43

calculation

Somayeh

Gholami

_,

_Hamid

_Reza

_Mirzaei

_,

_Ali

_Jabbary

_Arfaee

_,

Ramin

Jaberi

_,

_Hassan

_Ali

_Nedaie

_,

_Seied

_Rabi

_Mahdavi

_,

Eftekhar

Rajab

Bolookat

_,

_Ali

_S.

_Meigooni

a_{DepartmentofMedicalPhysicsandBiomedicalEngineering,FacultyofMedicine,TehranUniversityofMedical}

Sciences,Tehran,Iran

b_{RadiationOncologyDepartment,CancerInstitute,TehranUniversityofMedicalSciences,Tehran,Iran}

c_{RadiationOncologyDepartment,ShohadaeTajrishHospital,CancerResearchCenter,ShahidBeheshtiUniversity}

ofMedicalSciences,Tehran,Iran

d_{DepartmentofMedicalPhysics,IranUniversityofMedicalSciences,Tehran,Iran} e_{ComprehensiveCancerCentersofNevada,LasVegas,NV,UnitedStates}

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Articlehistory:

Received23October2015 Receivedinrevisedform 20January2016 Accepted26June2016 Availableonline18July2016

Keywords: QAbrachytherapyphantom Filmdosimetry GZP6HDRsystem SelectronLDRsystem TG-43

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b

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Aim:Verificationofdosedistributionsforgynecological(GYN)brachytherapyimplantsusing EBTGafchromicfilm.

Background: Onemajorchallengeinbrachytherapyistoverifytheaccuracyofdose distri-butionscalculatedbyatreatmentplanningsystem.

Materialsand methods: A new phantomwas designed and fabricated using 90 slabsof 18cm×16cm×0.2cmPerspextoaccommodateatandemandOvoidassembly,whichis normallyusedforGYNbrachytherapytreatment.Thisphantomdesignallowstheuseof EBTGafchromicfilmsfordosimetricverificationofGYNimplantswithacobalt-60HDR sys-temoraLDRCs-137system.Gafchromicfilmswereexposedusingaplanthatwasdesigned todeliver1.5Gyofdoseto0.5cmdistancefromthelateralsurfaceofovoidsfromapair ofovoidassemblythatwasusedfortreatmentvaginalcuff.Foraquantitativeanalysisof theresultsforbothLDRandHDRsystems,themeasureddosevaluesatseveralpointsof interestswerecomparedwiththecalculateddatafromacommerciallyavailabletreatment planningsystem.ThisplanningsystemwasutilizingtheTG-43formalismandparameters forcalculationofdosedistributionsaroundabrachytherapyimplant.

Results:Theresultsoftheseinvestigationsindicatedthatthedifferencesbetweenthe cal-culatedandmeasureddataatdifferentpointswererangingfrom2.4%to3.8%fortheLDR Cs-137andHDRCo-60systems,respectively.

∗ _{Correspondingauthorat:RadiationOncologyDepartment,ShohadaeTajrishHospital,CancerResearchCenter,ShahidBeheshti} Uni-versityofMedicalSciences,Tehran,Iran.Tel.:+982161192500;fax:+982161192501.

E-mailaddress:[email protected](H.R.Mirzaei).

http://dx.doi.org/10.1016/j.rpor.2016.06.005

Conclusion: TheEBTGafchromicfilmscombinedwiththenewlydesignedphantomcould beutilizedforverificationofthedosedistributionsarounddifferentGYNimplantstreated witheitherLDRorHDRbrachytherapyprocedures.

©2016GreaterPolandCancerCentre.PublishedbyElsevierSp.zo.o.Allrightsreserved.

1.

Background

Asradiationtherapytechniquesbecomemorecomplex,the quality assurance (QA) techniques have to be changed to provideasoundandpracticalmethodofverificationof treat-ment delivery in order to reduce errors during radiation therapy.1 _{A QA progressneeds to be adopted forcomplex}

brachytherapy treatment techniques, particularly when it involvestreatmentwithacomplexgynecologic(GYN)system. GYNbrachytherapytreatmentsforthecervix,vaginaland endometrialcancershavebeencommonlyusedforover100 years.2_{Accuracyofdosecalculationplaysavitalroleinthe}

brachytherapytreatmentplanning.Experimentalverification ofdoseaccuracyisoneofpossibleQAproceduresfor determi-nationofover-doseorunder-doseareainthebrachytherapy planningvolume.However,thistechniquefacesseveral chal-lengesrelatedtothemeasurementsofdosedistributionsina high-gradientregion.Oneoftheimportantcriteriaforthese experimentalsetupsistohave adosimeterwithhigh spa-tialresolutionsuchasGafchromicfilms.Gafchromicfilmsare beingusedas2Ddosimetersbyseveralinvestigatorsinvarious typesofapplications.3

Different investigators had demonstrated the useful-nessofthe EBT Gafchromicfilmsforbrachytherapy source dosimetry.4 _{Thesefilmsrequirenochemicalprocessingand}

theyareinsensitivetoambientlight.Therefore,theycanbe cuttotheshapeoftheexperimentalgeometryforthebest representationofadosimetricsetup.

2.

Aim

Verification of dose distributions for gynecological (GYN) brachytherapyimplantsusingEBTGafchromicfilm.

3.

Materials

and

methods

EBTGafchoromicfilmdosimetryfortwodifferent

brachyther-apy systems for GYN treatment were considered in this

study:theGZP6highdoserate(HDR)60Cobrachytherapyunit thatwasintroducedbyNuclearPowerInstituteofChinafor brachytherapyprocedures,5_{andtheSelectronlow-dose-rate}

(LDR)137_{Csremoteafterloadingsystemdistributedby}

Nucle-tron(NucletronBV,Veenendaal,TheNetherlands).6

3.1. GZP6HDRsystem

GZP6afterloadingunit(NuclearPowerInstituteofChina)with HDR 60_{Co sources has one stepping source and five}

non-steppingsource-braids.7_{TheGZP6treatmentplanningsystem}

isabletoproducedosedistributionsinthetransverseand lon-gitudinalplanes.Itcalculatesdose usingthe superposition

dose calculation technique. This system could be used

for intracavitary treatment of cervical, vaginal, endome-trial, rectal, esophageal and nasopharyngeal malignancies. The sources in this system were consisted of 60_{Co active}

pellets (3.5mm long and 1.5mm diameter) sealed by tita-nium capsulesandsphericalstainlesssteelinactive pellets (1.5mmdiameter)whichwerecoveredbyasteelspring.8

TG-43 recommended dosimetric characteristics of the sources in this system have been evaluated by several different investigators.7–9

Inthisstudychannels3and4ofGZP6HDRsystemwere utilizedtodeliver1.5Gydoseto0.5cmdistancefromthe lat-eralsurfaceofovoidswithin4.62min.Eachofthechannels 3and4containsonestationaryactivepellet(theyarenearly identicalintheirsourcestrengthandgeometry)andtheycan beusedintheovoids.

3.2. SelectronLDRsystem

SelectronCs-137LDRsystemhassomeactivepelletspherical sources(suppliedbyAmershamCorporation,Louisville,CO) andinactiveordummypelletsinanapplicatorset.10

Differ-entcombinationsofactiveandinactivepelletsareusedfor GYNcancer.TheNucletronPLATOtreatmentplanningsystem (TPS)calculatesthedosedeliveredbytheunitatthepointof interest.10_{TheTPSdeterminesthedosedistributionaround}

differentcombinationsofsourcesandspacers byassuming eachactivepelletasapointsource,usingthesuperposition dosecalculationtechnique.

The Selectron unit consists of spherical Cs-137 pellets composedof1.5mmactivesourcecoreofceramic, encapsu-lated in0.5mmsteel,withatotaldiameterof2.5mm.11 _In

addition, thissystemcontainssomenon-activepelletsasa

dummy,withthesame dimensionsandchemical

composi-tionastheactivepellets.6_{Liuetal.havepublishedtheTG-43}

recommendeddosimetriccharacteristicsofthesourcesinthis system.12

Inthisstudy,fromasetof8pellets,numbers2–7are con-sidered active,andthe remaindersare non-active,inorder todeliver1.5Gyofdoseto0.5cmdistancefromthe lateral surfaceofovoidswithin1.01h.

3.3. EBTGafchromicfilmdosimetry

Thenewphantomwasdesignedandfabricatedfrom90slabs of18cm×16cm×0.2cmPerspextoaccommodateGYN appli-cators(tandem andovoid).ThecompositionofthePerspex takentobeH,8%;C,60%;O,32%,withadensityof1.18g/cm3_.13

Thisconfigurationenablesustoverifythedosedistributions around the applicatorwitha high spatialresolution. Fig.1

showstheschematicdiagramofthisphantomwithitslayers andtheovoidapplicatorwhichisplacedinthephantom.

Fig.1–Schematicsofdifferentlayersofthephantomand Oviodapplicator.

The thin layer of the slabs was carefully machined to accommodatelayersofGafchromicfilmsinbetweentheslabs forradiation dosimetry.Theirradiatedfilms were cutwith scissorsinvarioussizesandshapetomatchthecurvatureand locationoftheapplicator.Largesize(3cmdiameter)ovoids wereselectedforbothGZP6andSelectronmachines.Withthis assembly,EBTGafchromicfilms14_{wereexposedusingaplan}

with1.5Gydosedeliveriesto0.5cmdistancefromthelateral surfaceoftheovoidsforbothunits.

Foraquantitativeevaluation,onlyapairoftheovoidshas beenusedinorder tobeable toreproducethe experimen-talsetupandgeometryoftheassemblyforrepetitionofthe measurement,andalsofordifferentsourcetypes.

Fig.2showstheschematicdiagramofthepointsofinterest andtheapplicator.Thesepointsofinterestinclude,point“D” representingvaginalmucosa,locatedat0.5cmdistancefrom thelateralsurfaceoftheovoidand“F”thatwaslocatedat1cm distancefromthelateralsurfaceoftheovoid.

Thedifferencesbetweentheplannedandmeasureddata demonstratethe accuracyofthe planned dosedistribution

Fig.2–SchematicdiagramoftheOvoidapplicatorsand calculatedpointsrelativetotheapplicator.DandFshow pointsofinterestfordosecalculation(graphsarenottothe scale).

around applicators.These measurements were repeatedat least3timestoimprovethestatisticalfluctuationofthefinal data.

SamplesofEBTGafchromicfilmswerecalibratedto deter-minethesensitivitycurve(i.e.responsevs.dose).Acalibrated teletherapy cobalt-60 system was chosen for calibration process of the Gafchromic films. For 137_{Cs dose}

calcula-tion because of energy dependence of Gafchromic films,15 response curve and the energy dependence ofGafchromic filmfordifferentradiationenergiesfromChiu-Tsaostudywas considered.16_{Calibrationcurveanditsrelativeenergy}

depend-encecoefficientwereusedfortheanalysisoftheirradiated filmsaroundtheGYNapplicatorfor60_Coand137_Cssources

withinthenewphantomdesign.130piecesofEBTfilmswith size of 2cm×1.5cm were selected for calibrating samples usingateletherapycobalt-60systemintherangeof25–800cGy dosewith25cGyinterval.Thesefilmswereexposedat1cm depth ina30cm×30cm×10cmslabPerspexlayerswitha 20cm×20cm radiationfield size.After24hfromexposure, filmswerescannedusingaflatbedMicrotekscanner(model: ScanMaker9800XL,MicrotekLaboratories,Inc.,Dayton,OH). Sincethepeakabsorptionoftheradiochromicfilmisinthe redregionofthevisiblespectrum(636nm),extractionofthe redchannelfromtheRGBimagecanimprovescanner sensitiv-ity.AssuggestedintheguidelineoftheGafchromicfilms,the irradiatedfilmsforbothcalibrationandmeasurementprocess werescannedinthelandscapeorientation.Forfilmcalibration andbothUnitexperimentalmeasurements,filmswereused fromthesamebatch.

Inaddition,wecompliedwiththerecommendationon pro-jectingthefilmorientationonsmallpiecesthatwereusedfor radiationdosimetry.Bothcalibrationsandexperimentalfilm scanningwereperformedusing100dpiresolutions.

ThepixelvalueswereobtainedfromIMAGEJsoftware (Java-basedimageprocessingprogram,NationalInstituteofHealth). Thescannersallowtheacquisitionoftransmissionscansin upto16bitspercolor.InEq.(1)netopticaldensity(NOD)was calculatedfromsubtractionofODirrandODunirrwhich repre-sented the opticaldensitiesofun-irradiated andirradiated film,respectively:

NOD=ODirr−ODunirr=

−log₁₀ (PVirr−PVdark) (PVblank−PVdark)

−

−log₁₀(PVunirr−PVdark) (PVblank−PVdark)

(1)

wherethePVirr,PVunirr,PVdarkandPVblankrepresentthepixel values measured inthe irradiated film,un-irradiated film, zero-lighttransmittedimagesoftheopaquesheetscanand blank,respectively.

Theselectedregionofinterestwasa2mm×2mminthe centerofthefilmpiecetoavoidopticaldensity(OD) measure-mentartifactsnearfilmedges.17

3.4. TG-43calculation

The task group 43 (TG-43) of the American Association

of Physicistsin Medicine(AAPM) has published a protocol regarding the brachytherapy dose calculation formalism.18

TG-43formalismcanalsobeusedforverificationof experi-mentaldosimetry.Variousinvestigatorshavedeterminedthe dosimetriccharacteristicsofdifferentbrachytherapysources usingthisrecommendation.19,20_{Dosimetriccharacteristicsof}

thesinglepelletsfromeachsystemhavebeenevaluatedusing theTG-43protocolsbydifferentinvestigators.12,21

ForbothSelectronandGZP6unitfilmdosimetrywiththe newphantomatmultiplelevelsrelativetotheGYNapplicators weredone.Wecompareddosecalculationoffilmdosimetry fromsomeinterestingpointswithtreatmentplanning. TG-43calculationwasconsideredforbothsources.Thephantom materialdensity(d=1.18g/cm3_{)wasconsideredforTG-43}

cal-culationsofbothsources.

TG-43formalismwasemployeddependingondosimetric parametersof60_Coand137_Cssources.

These parameters consist of air kerma strength, dose rateconstant, geometryfunction, radialdose functionand anisotropyfunction.TheyareindicatedinEq.(2)18_:

D·(r,)=SK·· GL(r,) GL(r0,0)·

g(r)·F(r,) (2)

DosimetriccharacterizationsofGZP660_CoHDR

brachyther-apyforTG-43calculationwereusedfromToosi’sstudy.22_So,

thesuperpositiontechniquewasused.Basedontheirstudy, doserateconstant valueofGZP6is1.04cGyh−1_U−1_{. Radial}

doseand2Danisotropyfunctionswereconsideredfromtheir MonteCarlosimulation.TG-43parametersofCs-137 Selec-tronLDRbrachytherapysourceswereconsideredfromSina’s MonteCarlosimulation.23_{So,doserateconstantvalueforthis}

unitis1.102cGyh−1_U−1_{.Weusedairkermastrength(S}

K)from treatmentplanningfordosecalculationinEq.(3)forCs-137 Selectronsources.For Cs-137Selectron, airkermastrength (SK)fromtreatmentplanningsystemwasconsideredin TG-43calculation.While,forGZP6system,airkermastrengthsof sourcesweremeasuredusingtheionizationchamber dosime-ter.

3.5. GZP6airkermastrength

ThevendorofGZP6hasprovidedapproximately600preplans withoutprovidingthealgorithmsthathadbeenusedinthese calculations.Theonlyparameterthatwasgiven,inaddition totheplans,wasthesourcestrengthsforindividualsources. Therefore,itwasdecidedtoverifytheaccuracyofthis param-eter.ToinvestigatetheairkermastrengthofGZP6unit,Monte Carlosimulationandpracticalmeasurementwereutilized.In ourcalculation,itisconsideredthatthedateofpracticalSK measurement,airkermafrom Monte Carlosimulation and treatmentplanningSKwasthesame.

Forsimulation,MCNP4CCodeisusedbasedonthe tech-niquethatwaspublishedbyToosietal.Self-absorptionofthe sourcecoreanditscapsuleweretakenintoaccountattheir study.7_{Forthispartofourwork,cylindricalringswith0.5cm}

thicknessand 0.5cmlengthwere simulatedinthe1–25cm transverse distance from the sourcecenter, in 1cm incre-ments.Inringcells,theF6tallywasusedtocalculateairkerma inunit ofMeV/g perphotoninthering cells.Source activ-ity,self-absorption,numberofdisintegrationfor60_Cosource,

decaycoefficient based on cobalt-60half timeand date of

measurement,squareofdistanceandotherappropriate con-versionfactorsweremultipliedbythevalueoftheresultfrom Tally F6tocalculateSK. Photonprimaryhistory was107 to reducestatisticalerrortolessthan1%.Forbothphotonsand electronsthecutoffenergywasdefinedas10keV.

AirkermastrengthofGZP6unitwasmeasuredaccording

to the TECDOC-1274 technical document of the

Interna-tional AtomicEnergy Agency(IAEA).24 For measurementof air kerma, available 0.6cm3 _{Farmer type ionization}

cham-ber(FC65-GTNC/249ScanditronixWellhofer)wasusedwith 0.551g/cm3 _{buildupcapandanelectrometer(Scanditronix}

Wellhofer). The chamber was calibrated by the SSDL lab-oratory of Atomic Energy Organization of Iran (AEOI) for

standard field of a teletherapy cobalt-60 machine with

ND,W=4.83cGy/nc.Weusedthemultipledistancemethodand readingsofchamberinampereatspecifieddistancesalong theperpendicularbisectorofthelinesourcewerescored.The exactpositionofthesourcewaswelldefinedbyusinga mil-limetricrulerandsourceautoradiography.Apparentdistance betweenthesensitivevolumeofthechamberandthecenter ofthesourcewasmeasured.Thereis1mmoffsetindistance. Measurementswereperformedin1cmintervals.Ineach posi-tion,wemeasured3timesandmadetheaverageofvalues.To approachSK,chamberreadinginamperewasmultipliedby thechambercalibrationvalue(ND,W),temperatureand pres-surecorrectionfactor(KT,P),durationexposureandsquareof thedistancetothecenterofthesource.

3.6. Filmdosimetryuncertaintyestimation

TheoverallaccuracyofEBTfilmmeasurementswasderived usingthemethodproposedbyDevicetal.25Statistical uncer-taintyoffilmresponsetothesame valueofdoseisrelated tosomefactorssuchasnon-uniformitythicknessinthe sen-sitive layer ofthe films, uncertainties associated with the scanning procedure and output variations ofLinac during exposure.Inthisstudy,weconsideredtwosourcesof uncer-tainties. The first one is related to netOD measurements and theother isrelatedtofittingcoefficientsincalibration curves.Moreover,aspecialattentionwaspaidtothedistances betweenthephantom’sedgeand edgeofthefilm.The dis-tancebetweenthephantomslabs’edgeandtheapplicators is95␮m.Moreover,thereisa0.1mmgapbetweentheedge ofthemachinedfilmandthephantom’sedge.Also,thereisa 0.5mmerrorindeterminingthepositionofpointsofinterest. Thetotaldoseuncertaintywascalculatedthroughthe fol-lowingequation:

Total=

2

netOD+calib2 -fit+2phantom-film (3)

4.

Results

4.1. EBTGafchromicfilmcalibration

Fig.3showsthecalibrationcurveoftheEBTGafchromicfilm response,intermsofnetopticaldensity(NOD),asafunctionof absorbeddosetowaterintherangeof25–800cGy.The mea-sureddatawerecorrelatedusingthefollowingpolynominal

Table1–Comparisonbetweenthemeasuredandcalculateddosevaluesatdifferentpointsaroundatypicalapplicatorin GYNimplantofSelectron137_CsLDRsource.

Points

Dose-treatment planning(cGy)

Dose-TG-43(cGy) Dose-film(cGy) Differencebetween filmdosimetryand TG-43calculation(%) PointD-L 150 144.3 145.8 +1 PointD-R 150 144.3 144.9 +0.4 PointF-L 100 93.5 95.8 +2.4 PointF-R 100 93.5 95.4 +2

Fig.3–CalibrationcurveforEBTGafchromicfilmresponse (opticaldensity)asfunctionoftheabsorbeddose(cGy).The errorbarsinthisgraphrepresent±5%.Thesolidlineon thisgraphsimplyconnectsthedatapoints.

functioninEq.(4)withR2_{=0.9837.Calibrationcoefficientof} 137_{CssourcewasobtainedfromChiu-Tsao’sstudy.Forscanner}

withredlightrelationshipbetweentheNODresponseofthe EBTfilmandsourceenergyinMeV(parameterx)isconsidered asEq.(5).

Dose (Gy)=42.43NOD2_{−8.14NOD+5.24 (4)} NOD=−0.025x2+0.1165x+0.869 (5)

So, thereis 2%differencebetween 60_{Co with meanenergy}

1.25MeV and 137_{Cs with energy 0.662MeV. This difference}

isemployedfor137_{Csfilmdosimetry.There isatotalof5%}

uncertaintythatwasemployedfortheresultsofdose mea-surements.

4.2. SelectronLDRbrachytherapy

ThevalueofreferenceairkermarateforSelectronTG-43dose calculationisextractedfromtreatmentplanningandequalto 8876.74Gym2_h−1_{.Thequantitativeanalysisofthemeasured}

dataforSelectron137_{CsLDRsourceandTG-43dosecalculation}

isshowninTable1.ResultsofSelectronunitshowgood agree-ment (totaldifference isupto6%)betweenfilmdosimetry, TG-43dosecalculationandtreatmentplanning.

4.3. GZP6HDRbrachytherapy

Air kerma strength (SK) for GZP6 HDR brachytherapy sys-tem from Monte Carlo code was obtained 4414×10−6 _and

4077×10−6_Gym2_h−1_{forsourceofCH}

3andCH4,respectively.

The value of measured air kerma strength was

obtained by takinginto account the date ofmeasurement 4207×10−6_±1%Gym2_h−1 _{for CH}

3 and 3864.5×10−6±

1%Gym2_h−1_forCH

4.Fig.4showsresultsofSKmeasurement byamultipledistancemethod.Thereisabout5.4%difference betweenthemeasurementandsimulation.

Table2indicatestheresultsofTG-43dosecalculationsand filmdosimetry.Maximumdifferencebetweenfilmdosimetry andTG-43wasto3.8%.

Fig.4–Plotoftheairkermarate×d2vs.distanced(cm)fromthesourcecenter.Theerrorbarsonthisdatapointsrepresent 5%statisticalfluctuationoftheFarmerdosimetry.

Table2–Comparisonbetweenthemeasuredandcalculateddosevaluesatdifferentpointsaroundatypicalapplicatorin GYNimplantofGZP660_CoHDRsource.

Points

Dose-treatment planning(cGy)

Dose-TG-43(cGy) Dose-film(cGy) Differencebetween filmdosimetryand TG-43calculation(%) PointD-L 150 148.5 143.6 −3.3 PointD-R 150 148.5 142.8 −3.8 PointF-L 100 98 95 −3 PointF-R 100 98 95.5 −2.5

5.

Conclusions

AlthoughtheAmericanAssociationofPhysicistsinMedicine TaskGroupreports56and59providereasonableguidanceon specificprocessofQAinbrachytherapy,26,27 _improved

guid-anceisneededforanytreatmentdeliverysystemtominimize each individual treatment failure. In this project, the dose distributions around the GYNapplicators for twodifferent machines,SelectronLDRsourceandGZP6HDRsystemwere measuredusingEBTGafchoromicfilmforasampleGYNsetup. Theresultshowsgoodagreementbetweenourfilmdosimetry, TG-43calculationandtreatmentplanning(totaldifferenceis upto6%).Inthis study,ithasbeendemonstratedthat Per-spexcanbeusedasphantommaterialinbrachytherapy.The Perspexmaterialisdenserthan water,soitproducesmore attenuationofprimaryradiationbutontheotherhand,this iscompensatedbyanincreaseinscatterradiationunderfull scatteringconditions.28

These investigations have verified that using the

Gafchromic film dosimetry technique together with the

new phantom; one could examine the accuracy of dose

calculationatdifferentdistancestotheGYNapplicators.This systemhasshownthatinahighdosegradientregionaround compoundapplicators,userscouldverifytheaccuracyofdose distributionwithatechniquethat isclinicallyrelevantand practical.Recently,therearesomestudiesrelatedtoproper phantomsforQAofbrachytherapysystems.29,30 _Comparing

them with this phantom, different models of Gafchromic filmscanbeusedwhichareveryconvenientandreliablefor routinequalityassurance.Thisassemblycanbeusedathigh gradientregionsforperiodicQAprocedures.

Conflict

of

interest

None.

Financial

disclosure

None.

Acknowledgements

I hereby acknowledge Clinical Research Development Unit (CRDU)ShohadaTajrish hospitalwhich wassupportedthis research.

The Radiotherapy Department of Shahid Beheshti

Uni-versityatShohada hospitalsponsoredthe purchaseofthe phantommaterialsandfilmsusedintheinvestigations.The

authorswouldliketopresenttheirappreciationforDrMehdi Ghorbaniforhisinvaluablecontributionsatdifferentphases ofthisproject.Inaddition,theeditorialcommentsand sug-gestionsbyDrCourtneyKnaupforenhancingthequalityof themanuscriptaregreatlyappreciated.

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