Radiation therapy.docx

Radiation therapy.docx

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Radiationtherapy

Radiationtherapy

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"Radiation（medicine）"redirectshere.ItisnottobeconfusedwithRadiation（pain）.

Radiationtherapyofthepelvis.Lasersandamouldunderthelegsareusedtodetermineexactposition.

AxesseRadiotherapy

Radiationtherapy（intheUSA）,radiationoncology,orradiotherapy（intheUK,CanadaandAustralia）,sometimesabbreviatedtoXRT,isthemedicaluseofionizingradiationaspartofcancertreatmenttocontrolmalignantcells（nottobeconfusedwithradiology,theuseofradiationinmedicalimaginganddiagnosis）.Radiotherapymaybeusedforcurativeoradjuvanttreatment.Itisusedaspalliativetreatment（wherecureisnotpossibleandtheaimisforlocaldiseasecontrolorsymptomaticrelief）orastherapeutictreatment（wherethetherapyhassurvivalbenefitanditcanbecurative）.Totalbodyirradiation（TBI）isaradiotherapytechniqueusedtopreparethebodytoreceiveabonemarrowtransplant.Radiotherapyhasseveralapplicationsinnon-malignantconditions,suchasthetreatmentoftrigeminalneuralgia,severethyroideyedisease,pterygium,pigmentedvillonodularsynovitis,preventionofkeloidscargrowth,andpreventionofheterotopicossification.Theuseofradiotherapyinnon-malignantconditionsislimitedpartlybyworriesabouttheriskofradiation-inducedcancers.

Radiotherapyisusedforthetreatmentofmalignantcancer,andmaybeusedasaprimaryoradjuvantmodality.Itisalsocommontocombineradiotherapywithsurgery,chemotherapy,hormonetherapy,Immunotherapyorsomemixtureofthefour.Mostcommoncancertypescanbetreatedwithradiotherapyinsomeway.Theprecisetreatmentintent（curative,adjuvant,neoadjuvant,therapeutic,orpalliative）willdependonthetumortype,location,andstage,aswellasthegeneralhealthofthepatient.

Radiationtherapyiscommonlyappliedtothecanceroustumor.Theradiationfieldsmayalsoincludethedraininglymphnodesiftheyareclinicallyorradiologicallyinvolvedwithtumor,orifthereisthoughttobeariskofsubclinicalmalignantspread.Itisnecessarytoincludeamarginofnormaltissuearoundthetumortoallowforuncertaintiesindailyset-upandinternaltumormotion.Theseuncertaintiescanbecausedbyinternalmovement（forexample,respirationandbladderfilling）andmovementofexternalskinmarksrelativetothetumorposition.

Tosparenormaltissues（suchasskinororganswhichradiationmustpassthroughinordertotreatthetumor）,shapedradiationbeamsareaimedfromseveralanglesofexposuretointersectatthetumor,providingamuchlargerabsorbeddosetherethaninthesurrounding,healthytissue.

Brachytherapy,inwhicharadiationsourceisplacedinsideornexttothearearequiringtreatment,isanotherformofradiationtherapythatminimizesexposuretohealthytissueduringprocedurestotreatcancersofthebreast,prostateandotherorgans.

Contents

∙1Mechanismofaction

∙2Dose

o2.1Fractionation

∙3Effectondifferenttypesofcancer

∙4Historyofradiationtherapy

∙5Typesofradiationtherapy

o5.1Externalbeamradiotherapy

▪5.1.1Conventionalexternalbeamradiotherapy

▪5.1.2StereotacticRadiation

▪5.1.3Virtualsimulation,3-dimensionalconformalradiotherapy,andintensity-modulatedradiotherapy

▪5.1.4ParticleTherapy

o5.2Brachytherapy

o5.3RadioisotopeTherapy（RIT）

∙6Sideeffects

o6.1Acutesideeffects

o6.2Latesideeffects

o6.3Cumulativesideeffects

o6.4Radiationtherapyaccidents

∙7Seealso

∙8References

∙9Furtherreading

∙10Externallinks

Mechanismofaction

RadiationtherapyworksbydamagingtheDNAofcancerouscells.ThisDNAdamageiscausedbyoneoftwotypesofenergy,photonorchargedparticle.ThisdamageiseitherdirectorindirectionizingtheatomswhichmakeuptheDNAchain.Indirectionizationhappensasaresultoftheionizationofwater,formingfreeradicals,notablyhydroxylradicals,whichthendamagetheDNA.Intheolder,mostcommonformofradiationtherapy,Intensity-modulatedradiotherapy（IMRT）（photons）,mostoftheradiationeffectisthroughfreeradicals.Becausecellshavemechanismsforrepairingsingle-strandDNAdamage,double-strandedDNAbreaksprovetobethemostsignificanttechniquetocausecelldeath.Cancercellsgenerallyareundifferentiatedandstemcell-like,theyreproducemore,andhaveadiminishedabilitytorepairsub-lethaldamagecomparedtomosthealthydifferentiatedcells.Thissingle-strandDNAdamageisthenpassedonthroughcelldivision,accumulatingdamagetothecancercell'sDNA,causingthemtodieorreproducemoreslowly.

Oneofthemajorlimitationsofphotonradiotherapyisthatthecellsofsolidtumorsbecomedeficientinoxygen.Solidtumorscanoutgrowtheirbloodsupply,causingalow-oxygenstateknownashypoxia.Oxygenisapotentradiosensitizer,increasingtheeffectivenessofagivendoseofradiationbyformingDNA-damagingfreeradicals.Tumorcellsinahypoxicenvironmentmaybeasmuchas2to3timesmoreresistanttoradiationdamagethanthoseinanormaloxygenenvironment.[1]Muchresearchhasbeendevotedtoovercominghypoxiaincludingtheuseofhighpressureoxygentanks,bloodsubstitutesthatcarryincreasedoxygen,hypoxiccellradiosensitizerdrug'ssuchasmisonidazoleandmetronidazole,andhypoxiccytotoxins（tissuepoisons）,suchastirapazamine.

DirectdamagetocancercellDNAoccursthroughhigh-LET（linearenergytransfer）chargedparticlessuchasproton,boron,carbonorneonionswhichhaveanantitumoreffectwhichisindependentoftumoroxygensupplybecausetheseparticlesactmostlyviadirectenergytransferusuallycausingdouble-strandedDNAbreaks.Duetotheirrelativelylargemass,protonsandotherchargedparticleshavelittlelateralsidescatterinthetissue;thebeamdoesnotbroadenmuch,staysfocusedonthetumorshapeanddeliverssmalldoseside-effectstosurroundingtissue.TheyalsomorepreciselytargetthetumorusingtheBraggpeakeffect.Seeprotontherapyforagoodexample,withphotos,ofthedifferenteffectsofIMRTvs.chargedparticletherapy.Thecyclotron's,dielectricwallaccelerator（DWA）,[2]orStillRiverSystems'ssuperconductinghighfieldmagnet[3]（twonewcompactprotronreplacements）providetheenergysourceforchargedparticletherapy.Theseparticlescanbechargedtodifferentamountstoprovidethedesiredtissuepenetration.Thisprocedureavoidshealthytissuebecauseitreleasesitsenergyatthelastfewmilimeterscalibratedtobeatthetargettumorandstops.BecauseIMRThaslittlemassitcannotbecontrolledtoasfineadegreeaschargedparticlesandisstilldamaginghealthycellswhenitexitsthebody.Thisiscriticallyimportantinalmostallcaseswherethecloseproximityofotherorgansmakesanystrayionizationverydamagingexample:

（headandneckcancers）.Thisdamagecausessecondaryinductedcancers.[4]Thisx-rayexposureisespeciallybadforchildren,duetotheirgrowingbodies.Theyhavea30%chanceofasecondmalignancyafter5yearspostinitialRT.[5]

Dose

Theamountofradiationusedinphotonradiationtherapyismeasuredingray（Gy）,andvariesdependingonthetypeandstageofcancerbeingtreated.Forcurativecases,thetypicaldoseforasolidepithelialtumorrangesfrom60to80Gy,whilelymphomasaretreatedwith20to40Gy.

Preventative（adjuvant）dosesaretypicallyaround45-60Gyin1.8-2Gyfractions（forBreast,Head,andNeckcancers.）Manyotherfactorsareconsideredbyradiationoncologistswhenselectingadose,includingwhetherthepatientisreceivingchemotherapy,patientcomorbidities,whetherradiationtherapyisbeingadministeredbeforeoraftersurgery,andthedegreeofsuccessofsurgery.

Deliveryparametersofaprescribeddosearedeterminedduringtreatmentplanning（partofdosimetry）.Treatmentplanningisgenerallyperformedondedicatedcomputersusingspecializedtreatmentplanningsoftware.Dependingontheradiationdeliverymethod,severalanglesorsourcesmaybeusedtosumtothetotalnecessarydose.Theplannerwilltrytodesignaplanthatdeliversauniformprescriptiondosetothetumorandminimizesdosetosurroundinghealthytissues.

Fractionation

（ThissectiononlyappliestophotonRT.）Thetotaldoseisfractionated（spreadoutovertime）forseveralimportantreasons.Fractionationallowsnormalcellstimetorecover,whiletumorcellsaregenerallylessefficientinrepairbetweenfractions.Fractionationalsoallowstumorcellsthatwereinarelativelyradio-resistantphaseofthecellcycleduringonetreatmenttocycleintoasensitivephaseofthecyclebeforethenextfractionisgiven.Similarly,tumorcellsthatwerechronicallyoracutelyhypoxic（andthereforemoreradioresistant）mayreoxygenatebetweenfractions,improvingthetumorcellkill.Fractionationregimesareindividualisedbetweendifferentradiotherapycentresandevenbetweenindividualdoctors.InNorthAmerica,Australia,andEurope,thetypicalfractionationscheduleforadultsis1.8to2Gyperday,fivedaysaweek.Insomecancertypes,prolongationofthefractionscheduleovertoolongcanallowforthetumortobeginrepopulating,andforthesetumortypes,includinghead-and-neckandcervicalsquamouscellcancers,radiationtreatmentispreferablycompletedwithinacertainamountoftime.Forchildren,atypicalfractionsizemaybe1.5to1.8Gyperday,assmallerfractionsizesareassociatedwithreducedincidenceandseverityoflate-onsetsideeffectsinnormaltissues.

Insomecases,twofractionsperdayareusedneartheendofacourseoftreatment.Thisschedule,knownasa