Physics, Total Body Irradiation ( TBI)
description
Transcript of Physics, Total Body Irradiation ( TBI)
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Thisprotocoliscurrentlybeingreviewed.
Bytheendofthisprotocol,youwillhave:
1. Gainedabasicknowledgeofbonemarrowtransplantation
2. LearntthereasonswhyTBIisprescribed
3. Understoodsomeofthesideeffectsonthepatient
4. UnderstoodthetreatmentconsiderationsforTBI
5. Beengivenexamplesofdifferenttreatmenttechniques
WhyisTBIused?
Totalbodyradiation(TBI)wasfirsttrialledinthe1920s.Itwasadministeredatalowdose(0.10.25Gy)severaltimesaweektotreatmalignanciesofthelymphoma.Today,TBIisstillprescribedatalowdoseforspecificdiseasessuchasnonHodgkin'sLymphoma1.Inmodernradiationtherapycentres,TBIismorecommonlyusedinpreparationforabonemarrowtransplant.
BoneMarrowTransplantation
Bonemarrowtransplantation(BMT)iswidelyusedasatreatmentforhaematologicalmalignanciessuchasleukaemia,aswellasseverecombinedimmunoandenzymedeficienciesdisordersandhaemopoieticsystemdisorderssuchasaplasticanaemia
RadiationOncology,Physics,TotalBodyIrradiation(TBI)
ID: 000490 Approved:24 May 2010 Last Modified: 16 Feb 2012 Review Due:31 Aug 2012
Target Audience: ThisprotocolisaimedatprovidinginformationonTotalBodyirradiation(TBI)forthefollowing:
n MedicalPhysicsRegistrarsn RadiationOncologyRegistrarsn RadiationTherapists
Overview: n ThisprotocolisdesignedtoprovideanoverviewoftheclinicalindicationsforTBI,thetheoryandpractiseofTBI,thedifferentsetupuptechniquesused,andtheconsiderationsfortreatment.ItwillalsobrieflydescribetheprocessofbonemarrowtransplantationandsomeofthesideeffectsexperiencedbythepatientafterreceivingacourseofTBI.
n LowdoseTBIisbeyondthescopeofthisprotocol.
Key References: n AAPMreport17(outdatedbutworthreading)n Galvin,J.M.,Report:AAPM2001Meetingsn ESTRO,EULEPandEBMT,ProceedingsoftheInternationalMeetingonPhysical,
BiologicalandClinicalAspectsofTotalBodyIrradiation,Radiotherapy&Oncology,Supplement18(1),1990
Additional Resources: n AustralianBoneMarrowDonorRegistryn AustralianBoneMarrowTransplantFoundationn CurrentOpinioninOncology,Supplement21(1),pp146,2009
ProtocolObjectives:
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2.
However,notallpatientsaresuitableforaBMT.ConsiderationsforaBMTincludethephysicalhealthofthepatient,diagnosisandthestageofthedisease3.
ApatientwhoisreceivingaBMTiscalledarecipient,andthehealthybonemarrowisgivenbyadonor.Therecipientismatchedwithasuitabledonorbytissuetyping.Thisisdonebyestablishingthehumanleucocyteantigentype,whichisawhitebloodcellmarker,fromabloodsample3.
Insomecases,thedonorsbonemarrowundergoesaprocesstoremoveharmfulTlymphocytes,knownasTcelldepletion.TheseTcellscausegraftversushostdisease,wherethedonorscellsrecognisetherecipientscellsasforeignandmountanimmuneresponsetorejectthem.
Thebestpossibledonorisanidenticaltwinhoweverthereisonlya2535%chancethatafamilymemberwillprovideagoodmatch.Atransplantthatcomesfromanotherpersonisreferredtoasanallogeneictransplant,orasyngeneictransplantifthedonorisanidenticaltwin4.
Anautologoustransplantisonewherethepatientsownstemcellsareused.Thismaybedoneifthediseaseisinremissionordoesnotinvolvethebonemarrow.Thepatientsstemcellsaretakenandstored,thenreturnedtothepatientafterchemotherapyand/orradiationtherapy4.
Clinicalindicationsfortotalbodyirradiation
TBIisusedinaradiationoncologysettingasaconditioningregime.ItistypicallyprescribedforpatientsrequiringaBMT,withtheaimofincreasingthesuccessofthetransplantintherecipient5,6.
Thisisachievedthroughleukaemiacellkill,eradicatingtherecipientsbonemarrowandprovidingasufficientdegreeofimmunosuppressiontoavoidgraftrejectionintherecipient7,8.Thedonorshealthybonemarrowistheninfusedintotherecipientoverseveralhours3.
AsuccessfulBMTisachievedwhenthedonorsbonemarrowattachestothecavitiesintherecipientslargebonesandbeginstoproducenormalbloodcells3.
Effectonthepatient
Thepatientmayexperiencesideeffectsfromthechemotherapyandradiationtherapy,suchashairloss,nausea,vomiting,hairlossanddiarrhoea.Inadditiontothis,complicationsmayarisefromtheBMT,suchasgraftversushostdisease,rejectionorinfection.ThepatientmayevenrelapsefollowingaBMT3.
Prescription
FractionationinTBIisusedtoexploitthedifferencesinrepaircharacteristicsbetweenleukaemicandnormallungcells9.ManydosefractionationregimesarecurrentlyinuseinAustralasiaandinternationally.Bierietal(2001)conductedastudythatassessedthe5yrsurvivalrateforpatientsprescribedwith10,12and13.5Gy.Allpatientsweregivenabidailyfractionation(bd),over3days.The5yrsurvivalrateforeachofthoseprescriptionswere62,55and46%respectively.
Fractionationwasfoundtoinducelesstoxicityinthepatientsnormaltissues(lung,liver,lensetc)thanaprescriptionof10Gyinasinglefraction.However,insituationswithagraftTcelldepletion,ahigherrateofgraftfailuresafterfractionatedregimeswasobserved,indicatingthatthe1012Gyfractionatedschedulescouldbecomedetrimental.Increasingthedosetoovercomethereducedefficacyofthelowerdoseschedulewouldinturnincreasetoxicity7.Ahigherdosegiveninlargerfractions,eg16Gyin8bd,mayreducetheriskofleukaemicrelapseatthecostofincreasedmorbidity6,10.
12Gyin6bdiscommonlyconsideredastandardregime11,whereasintheUK14.4Gyin8bdisincreasinglyprescribed6.Otherfractionationscheduleshavebeenclinicallyusedforexample,9Gyin3dailyfractionsand12Gyin4dailyfractionsforpaediatriccases12.
Thedoseisnormallyprescribedtothepatientsmidlineattheumbilicusorpelvisregion.ICRU50(1993)recommendsadoseaccuracyof+7%to5%howevermanyRadiationOncologistsarewillingtoacceptupto10%accuracyasTBIisconsideredaspecialtechnique.
Underdosageincreasestheriskofarelapsewhilstoverdosage,particularlyincriticalstructures,increasestheriskofmorbidity.Theeffectofoverdosageinthelimbshasnotyetbeenstudied6.
Energy,beamspoileranddosehomogeneity
TBITechniques
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Photonbeamenergiesbetween4MVand18MVarecommonlyusedinTBI.Thewidthofthepatientisaconsiderationwhenselectingbeamenergyduetothetissuelateraleffect13.Fora52cmseparationattheshouldersofalargeadult,theentrancedosecanbeupto25%higherthanmidlinedosefora6MVbeamat500cmSSD.Reducingtheseparationto30cmreducesthedosedifferentialtoapproximately10%14.ThismaybeachievedthroughuseofanAP/PAfieldarrangement,ratherthanabilateral.Also,raisingtheenergyto15MVreducesthisdifferentialtolessthan15%.
Forpatientswiththickness35cm,higherenergiesshouldbeconsidered13.
However,increasingthebeamenergyalsoincreasestheskinsparingeffectinherentinphotonbeams,withthedepthofmaximumdose(dmax)progressingfurtherintothepatient.Henceabeamspoilerisusedtoincreasetheentrancedose,sonamedbecauseitspoilsthebeam.
ThebeamspoileristypicallymadeofperspexandmaybemountedontheTBItreatmentcouchorstandaloneasamoveablescreen.Thethicknessissuchthattheentrancedoseisraisedtowithin90%oftheprescribeddose13.
Electronsaregeneratedinthelinearaccelerator(linac)headandattypicalTBItreatmentdistances(>300cm)progressivelylosetheirenergy,whilstmoreelectronsaresimultaneouslygeneratedinair.Thespoilerservestoabsorborscatterelectronsgeneratedinthelinacheadandairittheninturnsbecomesasourceofelectronsgeneratedbythephotoninteractions.Theseelectronshaveawideangulardistributionandhavetheeffectofincreasingdoseinthebuildupregion.Thepatientistypicallypositioned1030cmawayfromthespoilerthisseparationdistanceaffectstheprofileattheentrysurface15.
Thespoilerisalsousedtohomogenisethedosetothepatient.Ideally,thepatientwouldreceivetheprescribeddoseuniformlyacrossthewholebody.Thisisverydifficulttoachieveclinicallyduetothevaryingwidthsofthepatientscontours13.Therearemanydifferentmethodsusedtocompensateandcorrectforthevariationsincontourandanatomy,aswellasshieldcriticalstructures.
Criticalstructuresandtissuecompensators
Materialssuchasperspex,ricebags,sandbagsorthegelatinelikebolusareregularlyusedtohomogenisethepatientscontours(figure1)andassistinshieldingcriticalstructuressuchasthelung,liverandkidneys.
(a) (b)
Figure1:Anexampleofa)bolusbagsandb)perspexblocksusedinsomedepartmentstoshieldthepatient'sheadinbilateraltreatment
Stripsoflead,cerrobendorlowmeltingpointalloyblocksmayalsobeusedtofurtherprotectcriticalstructures13.Often,thepatientsownarmsandhandsareusedasshielding:inabilateraltreatment,thepatientsarmsmaybepositionedalongtheirside,providingfurtherlungshielding(figure2).
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(a) (b)
Figure2:Patientinlateralpositionwith(a)armscrossedoverchestforgreaterlateralchestexposure,and(b)witharmsbysideto'shield'lateralchest.
Whencriticalstructuressuchaslungsandliverareshielded,anelectronbeammaybeusedtoboostthedosetothoseregionstoreducetheincidenceofrelapse16.
Doserate
LatetermcomplicationsfromaBMTandTBIconditioningregimeincludeinterstitialpneumonitis,cataracts,renaldysfunctionandgraftversushostdisease.Whilstradiationalonemaynotaccountforthese,manystudieshaveinvestigatedtherelationshipbetweendoserateandspecificcomplications.
Doserateisasignificantconsiderationintheonsetofrenalcomplications,withdoserates
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SomecentresimagethepatientwithaCTscanoftheentirebodyorcertainlevelswithinthepatienttoobtaininformationforatreatmentplanningsystem(ifused)ortomanuallycalculateseparationanddeviseshieldingpositions.Xraysmayalsobeusedtomarkoutlungshieldingpositions.
Itisimportanttonotethatwhendevisingshielding,thepatientshouldbescannedorxrayedinthetreatmentpositionduetotheshiftinanatomywhenlyingsupineorsideways,orstanding.
MonitorUnits(MU)maybegeneratedusingatreatmentplanningsystemormanualcalculation.Insomecentres,MUisnotcalculatedandthetreatmentdeliveryisbasedontheionchamberreadingatthepatientsgroin,correctedforambientpressure,temperatureandpatienttemperature.
Dosimetry
InvivodosimetryforTBIisofrelevanceinreportingthedosedeliveredandmostimportantly,thedosehomogeneityduringeachtreatmentfraction.Itisalsousedtoverifypatientpositionandthereproducibilityofthesetup5.ThereareseveralfactorstoconsiderwhenchoosingadosimetertoperforminvivodosimetryforaTBIpatient.Theseconsiderationsareconsistentwiththerequirementsofaninvivodosimeterforanytypeofpatientmeasurement.
Someconsiderationsinclude:
n inherentbuildupinthedosimetern accuracyn reproducibilityn doserate,n fieldsize,n angular,n SSDandn temperatureindependencen linearityn easeofuseandreadoutn postirradiationfadingn andphysicalsize20
Thermoluminescencedosimeters(TLDs)areoftenusedinTBIastheyconformwellwiththerequirementsofinvivodosimetersandhaveasmalluncertaintyofupto2.5%21.ThethicknessaTLDchipisrepresentativeofthesensitivelayersoftheskin.HoweverTLDsarelabourintensivetoprepare,readoutandcalibrate,andrequiresomephysicalspaceforthesupportinghardware20.
SemiconductorsarealsowidelyusedforTBIdosimetry,withmuchresearchstillbeingconductedtocontinuallyimprovetheirphysicalandresponsecharacteristics.Metaloxidesemiconductorfieldeffecttransistors(MOSFETs)havebeenusedforTBIdosimetry.Reproducibilitywithin3%oftheentranceandexitdose,andagreementwithin3.9%ofTLDreadingshavebeenachieved20,22.Semiconductordiodesallowforimmediatedosereadings,howevercaremustbetakenduetotheirangularandenergydependence23.
Otherdosimetersincludeopticallystimulatedluminescence(OSL),whichhastheadvantageofbeingeasiertohandlethanTLDs,andtheselfdevelopingGAFchromicfilm(figure5).GAFchromicEBTfilmhasbeenfoundtoagreewithTLDresultswithin6.7%foratypicalpatientmeasurement24.
(a) (b)
Figure5:(a)CutpiecesofGAFchromicEBT2filmusedforinvivodosimetry(b)pointdensitometerusedtoreadoutfilm.
Dosimetersareplacedatsomeofthefollowingpositions:head,neck,sternalnotch,chest,abdomen,pelvisandankles.Dosimetersmayalsobeplacedbetweenthethighsnearthegroinasasubstituteforpatientmidline.Anionchambermayalso
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beusedatthispositiontoallowdirectcomparisontoprescribeddose25(figure6).
(a) (b)
Figure6Anexampleofanionchamber(a)placedatgroinwithGAFchromicEBT2filmattached,and(b)connectedtoelectrometertomonitordosedelivered.
TBIonCobalt60
TBImaybeperformedonaCobalt60unit.Variousmethodsincludeusingastationarybeaminconjunctionwithamovingcouch26,orplacingthepatientinastretcheronthefloor27.ThepatientassumesaproneandsupinepositionfortheAP/PAfields.
RadiationSafety
MostTBItreatmentsareperformedwithahighenergylinearaccelerator,withthegantryat90oor270oandthecollimatorat45owithjawsfullopentogivethemaximumfieldwidthpossible.Giventheextendedtreatmentdistance,therequirednumberofMUtodelivertheprescribeddoseforTBIcanbeupto36timesmorethanifthepatientwereatisocentre28.
WhilstscatterfromtheisocentreisnotaconcernforTBI,moreradiationwillbedirectlyincidentontheprimarybarrierbehindthepatient.AnextensiontotheNCRPbarrierdesignformulahasbeenproposed,whichseparatesdirect,leakageandscatterforthelinearacceleratorworkloadcomponents28.
References
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2. Gratwohl,A.1990."Bonemarrowtransplantation:indicationsandtechnique."RadiotherOncol18Suppl1:39.3. "AustralianBoneMarrowDonorRegistry(ABMDR)."Linktoexternalarticle 4. "ArrowBoneMarrowTransplantFoundation."Linktoexternalarticle 5. Briot,E.,A.DutreixandA.Bridier.1990."Dosimetryfortotalbodyirradiation."RadiotherOncol18Suppl1:16
29.6. Gilson,D.andR.E.Taylor.1997."Totalbodyirradiation.ReportonameetingorganizedbytheBIROncology
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8. Bieri,S.,C.Helg,B.Chapuis,etal.2001."Totalbodyirradiationbeforeallogeneicbonemarrowtransplantation:ismoredosebetter?"IntJRadiatOncolBiolPhys49(4):10711077.
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13. Khan,FM.2003.ThePhysicsofRadiationTherapy:Lippincott,Williams&Wilkins,USA.4thEd.14. Galvin,J.M.2001."AAPM2001MeetingReports."Linktoexternalarticle 15. Kassaee,A.,Y.Xiao,P.Bloch,etal.2001."Dosesnearthesurfaceduringtotalbodyirradiationwith15MVX
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MemorialSloanKetteringCancerCenterexperience."RadiotherOncol18Suppl1:6881.17. Cheng,J.C.,T.E.SchultheissandJ.Y.Wong.2008."Impactofdrugtherapy,radiationdose,anddoserateon
renaltoxicityfollowingbonemarrowtransplantation."IntJRadiatOncolBiolPhys71(5):14361443.
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18. Oya,N.,K.Sasai,S.Tachiiri,etal.2006."Influenceofradiationdoserateandlungdoseoninterstitialpneumonitisafterfractionatedtotalbodyirradiation:acuteparotitismaypredictinterstitialpneumonitis."IntJHematol83(1):8691.Linktoexternalarticle
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22. Scalchi,P.andP.Francescon.1998."Calibrationofamosfetdetectionsystemfor6MVinvivodosimetry."IntJRadiatOncolBiolPhys40(4):987993.
23. Williams.J.R,Thwaites.D.R.2000.RadiotherapyPhysics:Inpractice:OxfordUniversityPress,USA.24. Su,F.C.,C.ShiandN.Papanikolaou.2008."ClinicalapplicationofGAFCHROMICEBTfilmforinvivodose
measurementsoftotalbodyirradiationradiotherapy."ApplRadiatIsot66(3):389394.25. Lancaster,C.M.,J.C.CrosbieandS.R.Davis.2008."Invivodosimetryfromtotalbodyirradiationpatients
(20002006):resultsandanalysis."AustralasPhysEngSciMed31(3):191195.26. Zabatis,Ch,T.Koligliatis,S.Xenofos,etal.2008."Dosimetryintranslationtotalbodyirradiationtechnique:a
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Thecurrencyofthisinformationisguaranteedonlyupuntilthedateofprinting,foranyupdatespleasecheckwww.eviq.org.au
02Apr2013
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