Absorbed Dose to Water Ionization Chambers

1 Page 1 NAT'LINST OFST^Hlilllil NIST PUBLICATIONS A111Q7 017QH4 REFERENCE NISTSpecialPublication250-74 AbsorbedDosetoWaterCalibrationof IonizationChambersina''CoGamma-Ray Beam RonaldoMinniti JileenShobe* StephenM.Seltzer Heather... Open at page → 3 Page 3 NISTSpecialPublication250-74 AbsorbedDosetoWaterCalibrationof IonizationChambersina''CoGamma-Ray Beam RonaldoMinniti JileenShobe* StephenM.Seltzer HeatherChen-Mayer SteveR.Domen IonizingRadiationDivision PhysicsLaborator... Open at page → 5 Page 5 Abstract Absorbed-dose-to-water calibrationsareimportanttothemedicalcommunitytofacilitate theaccuratedeterminationofdosesdeliveredtotumorsduringexternal-beamcancertherapy. Thefirstversionofthisdocumentpublishedin1990(NBS... Open at page → 7 Page 7 TABLEOFCONTENTS Listoffigures v Listoftables vii L Scope 1 2. Definitions 1 3. StandardsandFacilities 3 3.1 NISTabsorbed-dose-to-water standard 3 3.2 ^^Cosource 4 3.3 Calibrationfacilities 7 3.4 Thecalibrationservice 7 4... Open at page → 8 Page 8 8. UncertaintyStatement 30 9. References 34 AppendixANIST-BIPMcomparison 36 AppendixBNIST-NRCCcomparison 38 AppendixCSamplecalibrationreport 40 iv Open at page → 9 Page 9 ListofFigures Figure 1 Vertical-beam^^Cosource. 5 Figure2 Fieldsizeandbeamuniformityatthelevelofthedetector. 6 Figure3 Schematicdiagramofabsorbed-dose-to-water calibrationsetup. 8 Figure4 SchematiccrosssectionoftheDomenw... Open at page → 10 Page 10 ListofTables Table1 Specificationsoftypicalfarmer-typeionizationchambersusedatNIST. 1 3 Table2 Chambercharacteristics. ^ 16 Table3 Uncertaintyanalysisfortheprimary-standardmeasurementofthe 32 absorbed-dose-to-water rate,... Open at page → 11 Page 11 1. Scope ThisdocumentisintendedtogiveadetailedreportontheserviceofferedbyNISTforthe caUbrationofionizationchambersintermsofthequantityabsorbeddosetowater.Ionization chambersareirradiatedinsideawaterphantomusing*^"Cogamma... Open at page → 12 Page 12 R)canbeexpressedasasimpleequation: 4C K=X(2.58x10"'—) kg Ve;1-g (3) whereW/eisthemeanenergyperunitchargeexpendedindryairbyelectrons,andgisthemean fractionoftheinitialkineticenergyofsecondaryelectronsliberatedbyphotonstha... Open at page → 13 Page 13 proficiencytesting:AtestconductedbyNISTtodemonstratethatlaboratoriesparticipating in thetestareabletotransferameasurementofabsorbeddosetowaterthatistraceabletoNISI. temperature-pressure correctionfactorA:tp:Thecorrection... Open at page → 14 Page 14 calorimeter,andthegraphitecalorimeter,ratherthantheaveragethatmcludedtheNi-saturated systemasusedin1991.TheabsorbeddoserateinwaterfromtheNIST^°Cosourceintrinsicto this1994blendedstandardwas0.99956ofthatofthe1991blendedst... Open at page → 15 Page 15 5 Open at page → 16 Page 16 Figure2.Fieldsizeandbeamuniformityatthelevelofthedetectorwiththejawsset withanaluminumblock.Thisviewlooksuptowardthesource.Thecrosshairs representthegeometricalcenterofthebeam.Thefieldsizewasdeterminedbyuseofa CMS,Inc.sc... Open at page → 17 Page 17 3.3 Calibrationfacilities NISTmaintainsavertical444TBq(nominal12kCiNov,1989)^''Cosourceinthelowintensity verticalbeamfacilitylocatedwheretheabsorbed-dose-to-water rateisknownasafunctionof fieldsize,chamberdepthinwater,an... Open at page → 18 Page 18 termsofabsorbeddoseto Figure3.Schematicdiagramofabsorbed-dose-to-water calibrationsetupshowing collimatorsize(s),detectordepthinphantom(x),source-to-detector distance(SDD)(z), andfieldsizeatthedetector (/). waterperunitc... Open at page → 19 Page 19 Themeasuredvalueof/includescorrectionstoaccountfordeviationsoftheambienttemperature andpressurefromstandardconditionsandacorrectionforelectronic-recombination effects.With theassumptionthatthechamberisopentotheatmosphere... Open at page → 20 Page 20 Collimated Beam Water: High-Purity Once-Distilled Expanded Polystyrene Hill / AluminumFoil 30cmX30cmX30cm AciylicContainer Electrode Figure4.SchematiccrosssectionoftheDomenwatercalorimetershowingtheessential featuresform... Open at page → 21 Page 21 Figure5.Schematicdiagramshowingtheconstructional detailsofthetemperatureprobe consistingofanembeddedthermistorneartheendofalongthincapillary.Dimensions areinmillimeters. wherethefactorV2istheresultofusingtwothermistorsto... Open at page → 22 Page 22 whererandTqarethevaluesofthethermistorresistanceatagiventemperatureTandTo respectively(theunitoftemperature isexpressedinKelvin). Equation6canbereducedtothelinearform, y =/^-X+0, (7) wherey =]nr,x=\/T,and^=InTo - /^/To,a... Open at page → 23 Page 23 15 20 25 30 Temperature,°C Figure6Calibrationdataofthermistorresistancesasafunctionoftemperature. Thecurvesaremarkedforthermistor#1andthermistor#2. 5.Secondary(transfer)standardionizationchambers Farmer-typeionizationcha... Open at page → 24 Page 24 TheExradinchambersareinherentlywaterproofwhiletheNEchambersarenot.Inorderto performmeasurementswiththeNEchambersinwateratwostepassemblyisrequired.First,a 1mmthickacrylicsleeveisthreadedontothechambersteminthesamemanneras... Open at page → 25 Page 25 Figure7.Waterproofingassemblyforchamberswhicharenotinherentlywaterproof. DrawingcourtesyofMed-Tec. Figure8.Water-phantomchamber-mounting apparatus.DrawingcourtesyofMed-Tec. 15 Open at page → 26 Page 26 Table2.Chambercharacteristics.Reproducibilitymeasurementsweremadeusingapotential differenceof300V. Chamber r/r Reproducibility Reproducibility - (%) (%) air water ExradinA12 0.01 0.05 1.0006 1.0006 NE2571 0.01 0.03 1.003... Open at page → 27 Page 27 1.365 b1.360 I 1.355 3 o 1.350 _3 ^1.345 < I I 1M II II 1 I I I I I I I I I I I I I I I I I I I I I I IM I I I IM 0122436486072 Time(min) 8496108120 Figure9StabilizationprofileoftheNE2571chamber.Measurementsweremadeevery... Open at page → 28 Page 28 Anobservationworthmentioning isinregardtotheNEchambers:OneoftheNISTNE chambersbegantoshowsomeerraticbehaviourduringoneoftheabovementionedcahbrations. Takingthesleeveoff,itwasfoundthatwaterhadseepedintothewaterproofingsle... Open at page → 29 Page 29 110 50— 40 ^ ^ ^ ^ ^ 1 ^ ^ ^ 1 0123456789 10 Depth(cm) Figure10.Depth-dosecurvefortheNIST^TosourcedeterminedbytheCMS,Inc. scanningwaterphantom. field.Forchambercalibrationsmadeintermsofabsorbeddosetowaterasquarefieldisus... Open at page → 30 Page 30 Forthefield-sizetestthecollimator-dialsettingswerevariedfrom10x10to11.5x11.5in incrementsof0.25.Fieldsizes(50%contouratthelevelofthedetector)associatedwiththese settingsvaryfrom14cmx14cmto16.1cmx16.1cm.InFig.11thereading... Open at page → 31 Page 31 NISThadusedasquaremetaljigwithsidesof50.8mmtosetthejawsonitsCoTheratronF teletherapyunit.Thejigwasinsertedintotheopening,andthejawswerethenclosedaroundit tosetthejawopeningsandhencethefieldsize.Theunithasasetoffourknobst... Open at page → 32 Page 32 Itwasfinallydecidedthatthefieldsizeatthetimeofthewater-calorimetermeasurementscould notbereliablyassured.Plottingfieldsizevs.calibrationcoefficientsforallNISTabsorbed-dose ionchambers,afieldsize(determinedbysettingthedia... Open at page → 33 Page 33 1.075— I 4.0 4.5 5.0 5.5 6.0 Depthinwater(cm) Figure12.NetCurrentasafunctionofchamberdepth,normalizedtothecurrentat5cm depth.Source-to-surface distance(SSD)ismaintainedat95cm. 6.4 Dependenceondepthofwaterabovechamber For... Open at page → 34 Page 34 6.6 Percentagedepthdose Apercent-depth-dose studywasperformedwiththechamber/phantomcombinationandthe resuhswerethesameaswhentheCMSscanningwaterphantomw£isusedtocharacterizethe source,(seeFig.10). 6.7 Measurementreproduci... Open at page → 35 Page 35 6.8 Electricalleakagemeasurements ThequalityofthesechamberscoupledwiththeNISTmeasurementsystemresultsinelectrical leakagemeasurements thatdonotexceed0.02%ofthedosereadings.Anyuncertaintyinthe leakagewouldaddnegligibleunc... Open at page → 36 Page 36 6.10Dependenceonwaterpurityandevaporation Calibrationswereperformedwithbothtapwateranddistilledwaterfillingthephantom. Itis recognizedthatwaterqualityplaysanimportantroleinwatercalorimetry,butitappearstohave noeffectonth... Open at page → 37 Page 37 WhenachamberisreceivedatNIST,itisfirstinspectedforanyphysicaldamagethatmayhave occurredduringshipment. Ifanydamageisdetected,thecustomerisnotifiedimmediately,and correctiveactionsaretakeninaccordancewiththecustomers'requ... Open at page → 38 Page 38 7.2 Absorbed-dose-to-water measurements Ifthechambertobecahbratedisnotinherentlywaterproof,everyeffortismadeto accommodatethechamberowner'swaterproofingsleeve.Ifthewaterproofingsleeveprovided willnotworkintheNISTphantom,... Open at page → 39 Page 39 doserate,theNISTchamberisusedtoverifysetupparameters. Usually,threeleakagemeasurements,followedbyfiveradiationmeasurements,andthenthree moreleakagemeasurementscompriseonedataset.Todeterminethenetcharge, ,thesix leakageme... Open at page → 40 Page 40 AppendixCincludesasampleofatypicalabsorbed-dose-to-water calibrationreport.Thereport containstheabsorbed-dose-to-water calibrationcoefficient,A/d,w,determinedwithnegativefiill voltageappliedtothewallofthechamber.Alsolist... Open at page → 41 Page 41 whichissummarizedinFig.6ofthatreference.Asaresultofthatstudyitwasconcludedthatas longasthethermistorsarecalibratedbeforeandaftereachgroupofmeasurementsdonewiththe watercalorimeter,anevaluationofTypeBfortheuncertaintyof0.... Open at page → 42 Page 42 Table3.Uncertaintyanalysisfortheprimary-standardmeasurementofthe absorbed-dose-to-waterrate,D^ .Therelativeuncertaintiesareexpressedin%. UncertaintyComponents TypeA TypeB heatdefect 0.30 reproducibilityofmeasurementgroup... Open at page → 43 Page 43 Table4.Uncertaintyanalysisforthecalibrationofanionizationchamberintermsof absorbeddosetowater.Therelativeuncertaintiesareexpressedin%. UncertaintyComponents Nn,w TypeA TypeB charge 0.10 0.10 time 0.05 air-densitycorrecti... Open at page → 44 Page 44 9.References 1. SeltzerS.M.andBergstromP.M.,ChangesintheU.S.PrimaryStandardsfortheAir- KermafromGamma-RayBeams,J.Res.Natl.Inst.Stand.Technol.108,359-381, (2003). 2. BoutillonM.andPerroche-RouxA.M.,Re-evaluationofWvaluefo... Open at page → 45 Page 45 11. Dyk,J.V.andMacDonald,J.C.F.,Penetrationofhighenergyelectronsinwater,Phys. Med.Biol.17,pp.52-65,(1972). 12. Klevenhagen,S.C.,PhysicsandDosimetryofTherapyElectronBeams,MedicalPhysics Publishing,Madison,WI,(1993). 13. A... Open at page → 46 Page 46 AppendixA NIST-BIPMcomparison AnindirectcomparisonofstandardsofabsorbeddosetowateroftheNISTandoftheBIPMwas carriedoutinthe^°CoradiationbeamattheBIPMinOctober1997^^.Theprimarystandardof theNISTisawatercalorimeterandtheBIP... Open at page → 47 Page 47 HavingobtainedbothN/:andf^o.wcalibrationcoefficientsforthetwotransferchambers, itisinterestingtocomparetheseinthebeamsofthetwolaboratories.TheratioA^d,w/ atthe BIPMforoneofthechamberswas1.1020.AttheNISTitwas1.1037,wheret... Open at page → 48 Page 48 AppendixB NIST-NRCCcomparison Inearly1998,threetransferionizationchamberswereusedtocomparetheair-kermaand absorbed-dose-to-water calibrationcoefficientsmeasuredbytheNRCCandtheNIST.Eachof thelaboratoriesusesawatercalorime... Open at page → 49 Page 49 TableB.l Tn< itiitp CcGv/nC^ (cGv/nC) ''D,w '^ 'K NE2571() NRCC 41229 45103 1.0940 NIST 40952 45331 11069v.\\J\J7 NRCC/NIST 10065 09950 NE2571() NRCC 41311 4.5186 1.0938 NIST 4.1069 4.5429 1.1062 NRCC/NIST 1.0059 0.9947... Open at page → 50 Page 50 AppendixC Samplecalibrationreport Thefollowingpagesshowasamplereportonly.Portionsofthereportmaybecustomizedtofit acustomer'sspecificneed. Open at page → 51 Page 51 NationalInstituteofStandardsandTechnology REPORTOFABSORBED-DOSE-TO- WATERCALIBRATION FOR LaboratoryName Address Address RadiationDetectionChamber:Exradin,ModelA12,SNABC Calibrationsperformedby[ScientistName] ReportReview... Open at page → 52 Page 52 NationalInstituteofStandardsandTechnology REPORTOFABSORBED-DOSE-TO-WATER CALIBRATION FOR LaboratoryName Address Address RadiationDetectionChamber:Exradin,ModelA12,SNABC Chamberorientation:Thecavitywaspositionedinthecente... Open at page → 53 Page 53 ExplanationofTermsUsedintheCalibrationProceduresandTables AbsorbedDosetoWater;Theabsorbed-dose-to-water rateattheNISTcahbrationpositionis . 60 measuredbyawatercalorimeter.TheCogamma-rayrateiscorrectedtothedateofcahbratio... Open at page → 54 Page 54 field.Forgammaradiation,thebeamcodeidentifiestheradionucUde. ChangeinTerminology: TheRadiationInteractionsandDosimetryGroupoftheNISTIonizingRadiationDivisionhasmadea changeinitsterminologyincalibrationandspecialtestrepor... 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