Lecture 8 - Radiation Safety - Bohndiek Lab
Transcript of Lecture 8 - Radiation Safety - Bohndiek Lab
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RadiationSafety
14thFeb2018
LearningOutcomes
• Afterthislecture,youshouldbeableto:
– Understanddifferentradiationprotectionquantities– Explainthedifferencebetweenradiationdoseandradiationrisk– Describeimportantfactorsinradiationprotection
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Chargedparticleinteractions
Quantifyingradiationdose
Radiationrisks
RevisionI
• Atdiagnosticenergies,nuclearinteractionsarerare• Heavychargedparticles(e.g.protons)loseonlyasmall
fractionoftheirenergyineachcollision• Lightchargedparticles(e.g.electrons)losemostoftheir
energyinasinglecollision• Allformsofionisingradiationeventuallyresultina
distributionoflowenergyelectronshencetheseareofcentralimportanceinradiationbiology
• HeavychargedparticlesundergomultipleCoulombscatteringeventswithnegligibledeflection
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RevisionII
• Stoppingpoweristheaveragerateofenergylossinamedium
• theenergydepositionofchargedparticlesdifferssignificantlyfromthatofphotons:
• Lightchargedparticlescanloseenergyviabothcollisionsandradiativemechanisms
Protons
Photons-dE/dx
x
Chargedparticleinteractions
Quantifyingradiationdose
Radiationrisks
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Kineticenergyreleasedinmedia (KERMA)measurestheoverallenergylostbyionising radiation
Foramonoenergeticbeam:K = kNE µ
ρ
!
"#
$
%&med
keV
cm2g-1
cm-2
constantk=1.6x10-13 GykeV-1
K = k Φ E( )E=0
Emax
∫µ E( )ρ
#
$%
&
'(med
EdE
photonscm-2
Forapolyenergeticbeam:
Absorbeddoseistheenergygainedfromionisingradiationperunitmassofmaterial
ε = Rin − Rout + Q∑Energyimparted
RadiantenergyincidentonthevolumeSumofallchargedandunchargedparticleenergies,excludingrestmassenergies
Radiantenergyleavingthevolume
SumofallrestmassenergiesInanynucleartransformationsthatoccurwithinthevolume
1 Gy = 1 J1 kg
SIUnitofDose:
D =dεdm
AbsorbedDose: DT =1mT
Ddm∫MeanAbsorbedDose:
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Inradiationprotection,weareinterestedinthedamagedonebyradiationsouseequivalentdose
• 1Gyofprotonsorneutronscausesmoredamagetotissuethan1Gyofphotonsorelectrons
• Defineequivalentdose:orinagivenorganortissue:
H = DwR
Radiationtype Energy wR factorPhotons All 1
Electrons All 1
Protons >2MeV 5
Neutrons <10keV,>20MeV 5
10– 100 keV,2– 20MeV 10
100keV – 2 MeV 20
Atomicnuclei All 20
1 Sievert = 1 Gray × wR
ICRP2007
HT = DT ,RwR
Effectivedoseaccountsforthedifferentsusceptibilityoftissuetypestoradiation(unitstillSv)
E = wtHtt=0
N
∑
Tissuetype wfactor(each)
Bonemarrow,colon,lung,stomach, breast,remainder 0.12
Gonads 0.08
Bladder,liver, thyroid,oesophagus 0.04
Bonesurface,brain,salivaryglands,skin 0.01
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Howmuchisamicrosievert(μSv)?
• AveragedosereceivedfromlivingintheUK– 6μSvperday
• AnnualdosefromradioactivefalloutintheUK– 10μSv
• ReturnflighttoSpain– 20μSv
• ChestX-ray– 20μSv
• Annualdosetomedicalphysicist– 100μSv
• CTscan– 5,000μSv
Themeasurementofradiationdoseisdosimetry
DCVoltageSource
IonCurrent
+
-
Anode
Cathode
Incidentparticle
Airvolume~6cm3
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Fluence ofphotonsisnotproportionaltoabsorbeddoseunlesselectronequilibriumexists
• Chargedparticleequilibrium(CPE)requiresthatthenumberofchargedparticlesenteringthemeasurementvolumeisequaltothoseleavingit
• Conditions:– Separationofboundariesofvolumemustbeatleasttherangeofany
secondarychargedparticle– Atomiccompositionofmediumishomogeneous– Densityofmediumhomogeneous– Uniformfieldofx-rayspassingthroughthemedium(negligibleattenuation)– Noinhomogeneouselectricormagneticfieldsarepresent
• Usuallymetinmodernexposurechambersforx-raybeamsindiagnosticradiology
ThetraditionalunitofexposureistheRoentgen
• TheRoentgenisdefinedonlyinair,underconditionsofelectronequilibrium:1R=2.58x10-4Ckg-1
1cm3 air=0.001293g(atSTP)
2.08x109 ionizations 2.58x10-4 Ckg-1
Exposure:1Roentgen
• STP:themassofairinanionisation chamberis~7.8mg• A1Rexposurewillthereforeliberate2.0x10-9 Cinsidethechamber,
correspondingto1.2x1010 ions
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MeasuredX-rayexposureinaircanbedirectlyrelatedtoabsorbeddoseinamediumunderCPE
Empirically,33.97eV isneededtoproduceanionpairinair:
Dair = 2.58×10−4 C/kg ×33.97J/C×X= 0.00876J/kg×X= 8.76mGy×X
Sprawls,RadiationQuantitiesandUnits
Ffactor
Ifx-raysareincidentuponanothermedium:
Dmed = Dair
µenρ( )
med
µenρ( )
air
= 0.876
µenρ( )
med
µenρ( )
air
X
= fX
Howareradiationprotectionquantitiesrelated?
PhysicalQuantitiesFluenceKerma
AbsorbeddoseStoppingpower/LET
ProtectionQuantitiesOrganabsorbeddoseOrganequivalentdoseEffectivedose
OperationalQuantitiesEquivalentdose(ambient,directional,personal)
Calculatedusingweightingfactorsandanthropomorphicphantoms
Comparedbymeasurementand
calculation
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TwomainmechanismsofDNAdamagecanarisefromradiationexposure
http://www.cna.ca/curriculum/cna_bio_effects_rad/
IonisingradiationcreatesionswhichbreaksthesugarphosphatebackboneorhydrogenbondsofthebasepairsoftheDNA,releasingelectrons.
IonisingradiationinteractswithwaterinthebodytoproducefreeradicalswhichsubsequentlyinteractwithDNA.Theseeffectsaremorecommonthandirecteffects.
Cellsurvivalcurvedemonstratesrelativebiologicaleffectiveness(RBE)
• Petridishescontainingclonogenic cellsexposedtosuccessivelyhigherdosesofionisingradiation– thesurvivingfractioncanthenbecalculatedbycomparingtoacontrolplate.
• Notetheshoulderofthecurve– thisdemonstratesabilityofcellstorepairatlowdoses
• Thecurvedemonstratesrelativebiologicaleffectiveness– ratioofdosesgivingidenticalbiologicaleffect(remembertheradiationweightingfactor…)
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Chargedparticleinteractions
Quantifyingradiationdose
Radiationrisks
Stepbystepprocess
Exposure
Ionisation
Chemicalchanges(freeradicals)
Molecularchanges(DNA)
Subcellulardamage
Cellularlevel
directaction
Celltransformation(stochasticeffect)
Celldeath(tissuereaction)
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Radiationeffectsmaybeclassedasstochasticordeterministic
• Stochasticeffectscanoccuratanydose(random)
• Effectsaregovernedbychance• Nothresholddose,tominimise
risks,keepdosesaslowasreasonablypracticable(ALARP)
• Usuallylowprobability• Assumptionprobabilityincreases
linearlywithdose• Cancerandheritableeffects• Thisisthemodelweacceptbut
thereisdebate(hormesis!)
Geneticeffectsandcancerarestochasticeffects
• Descendantsofsurvivorsofatombombandradiotherapypatientshaven’tshowngeneticeffects
• However,thisisnotprooftherisksaren’tthere:• Uncertainty• Diversenatureofseverehereditarydisease• Highnaturalprevalenceofseverephysicalandmentalgeneticallyrelatedhandicap
• Riskofhereditaryillhealthinsubsequentchildrenandfuturegenerationsestimatedtobe1in500,000for1mGyexposuretogonads.
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Riskofdevelopingcancer
• Overallriskofdeveloping canceris• 4.1%perSv• foradultworkers• E.g.foran8mSvCTscan,theincreasedriskofcancerinductionis1in3000
• Naturalincidenceofcancer1in3• 5.5%perSvforwholepopulation• Difficulttoquoteriskofdevelopingfatalcancer–treatmentsareimprovingallthetime
Radiationeffectsmaybeclassedasstochasticordeterministic
• Tissuereactions(alsoknownasdeterministiceffects)
• Welldefinedthresholdatwhichtheeffectwilloccur
• Asdoseincreases,severityofeffectincreases• Non-linearrelationship• Mosttissuereactionshaverepairmechanisms
andtheratethedoseisdeliveredinfluencesthethresholddose
• Effectsnotseenbelow100mSv• E.g.skinreaction
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Deterministiceffectsareconsideredtooccurabovecertainacutedosethresholds
Tissue Acutedose(Gy) Effect Latency
Skin 26
ReddeningHairloss
1day10days
Lensofeye 0.55
DetectablelesionsCataracts
YearsMonths
Ovary 2.5 Reducedfertility Fewdays
Testis 0.15 Temporarysterility Months
Bonemarrow 0.5 Reducedwhitecells Fewdays
Observedchangesincellsdependsoncellturnovertime:
– Rapidlydividingcells,damagecanbeseenwithinafewhours– Slowlydividingcells,effectsobservedinmonthsorevenyears.
Deterministiceffectsfromdiagnosticprocedures
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Radiationriskisassessedfromlongtermstudies
• EvaluatedfromHiroshima,Nagasaki,Chernobylsurvivors(humanepidemiology),occupationalexposures(nuclearindustry),radiationtherapypatients,mousemodels
• Approximaterisk5%perSv• Therearemanyproblemswithassessingrisk,including:
– Highnaturalincidenceofcancer– Externalinfluences(lifestyle,diet)– Japanesedatahigherdoserates/dosesthanencounteredoccupationally
Therearenumeroussourcesofradiationexposure
Releases from nuclear industry0.1% (0.1 μSv)
Occupational exposure0.2% (6 μSv)
Fallout from atomic weapons0.2% (6 μSv)
Air travel, luminous watches, etc1% (30 μSv)
Medical irradiation15% (410 μSv)
Natural Background83% (2230 μSv)
Total annual dose = 2700 μSv
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Everydayrisksarehighcomparedtoradiationrisk
• Flu– 1in5000
• Roadaccident– 1in10,000
• Accidentathome– 1in25,000
• Hitbylightning– 1in107
Radiationrisksinpregnancy
• Embryoconsistsofrapidlydividingcells,weknowthesearemostsensitivetoradiation.
• Tissuereactions:– Principaltissuereactionsinafoetusexposedtoionisingradiationaredeath,
malformation,growthretardationandabnormalbraindevelopment.– Effectsareunlikelytooccurbelow100mGy.– Norisksforoccupational/diagnosticexposures.
• Stochasticeffects– Thoughttobeindependentofstageofpregnancyafterthefirstthreetofour
weeks.– Afoetaldoseof25mGywasfoundtodoublethenaturalincidencerateof
childhoodcancer(~1in500).– Lackofevidenceonlifetimecancerrisks.
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Dosesindiagnosticradiologyarerelativelylow;someproceduresrequirehigherdosetoimproveSNR
Exam EffectiveDose(mSv) Additionalcancerrisk
ConventionalX-ray
Chest 0.03 1in106
Mammogram 0.7
Dental 0.05(ave)
CT Head 2.0
Chest 8.0 1in4000
Abdomen 10.0
Interventional Angioplasty(heart)
7.5– 57.5 1in600
Occupationaldoses– annuallimitsinmSv
Wholebody
Skin Extremities Lensofeye
Employees 20 500 500 20
Trainees 6 150 150 15
Anyotherperson
1 50 50 15
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Highdosesleadtodeathduetoorganfailure
Wholebodydose(Gy) Organortissuefailure Timeatwhichdeathoccursafterexposure(days)
<10 Bonemarrow 30-60
5-15 Intestineandlungs 10-20
>15 Nervoussystem 1-5
>100 Nervoussystem Withinafewhours
TheLD50 forhumansisabout3Gy
Areallifeexample- Chernobyl
203peopleinwhomradiationsicknessconfirmed
RadiationDose(Gy) No.patients Deathswithin100days
1-2 105 0
2-4 53 1
4-6 23 7
6-16 22 21
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Areallifeexample- Litvinenko
• Poisonedwithpolonium-210• Deterministiceffect• Alphaemittercouldn’tbedetectedoutside
thebody• Depositsmainlyinsofttissue• Particularlyliver,spleenandbonemarrow• Alsotokidneysandskin,particularlyhair
follicles
Radiationprotectionaimstolimitbothdeterministicandstochasticeffects
• Justification• Optimization:AsLowAsReasonablyAchievable(ALARA)• Limits(E<20mSv/year)
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Alldosestoionisingradiationhavetobejustified
• Becauseoftherisksofionisingradiation– anyexposuresmustbejustified.
• Benefitsmustoutweightherisks.• Thisincludesdiagnostic,therapeuticandoccupationally.• Researchiscomplicated!
Alldosesshouldalsobeoptimised
• QualityassuranceX-rayequipment.• Ensurepatientgetsthesmallestdosefortheintendedclinical
outcome• Staff
– Time(dosedirectlyproportionaltotime)– Distance(1/r2)– Shielding(leadPPE)
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Doselimitation
• Doselimitsforstaffandmembersofthepublic(notforpatients)
• Personalmonitoring
• Limitationmeansthatdosesmustbekeptbelowspecifiedlegallevels:DoseLimits(staffandpublic– notpatients)
• Limitsrepresentafinalrestrictiontokeepdosestoareasonablelevel- notsufficientinitself
• ShouldalwaysaimtokeepdosesAsLowAsReasonablyPracticable(theALARPprinciple)
Thermoluminescentdosemetersareusedforpersonalmonitoring
• ElectronicbandstructureofTLmaterialsallowradiationenergytobetrappedintrappingcentresprovidedbyimpurities
• Controlledheatingallowstrappedelectronstoreleasestoredenergyaslightastemperatureincreasesdeepertrapsaredepopulated
• LightoutputismeasuredbyPMtubeandaplotwithtimegiveaglowcurve• TLmaterialscanhaveseveralglowcurvepeaksduetothevariousdepthsof
trappingcentre
• LiF:Mg:TiisapopularTLmaterialfordosimetrybecause:– Itisnearlytissueequivalent– thelightemissionis400nmmatchingthepeakresponseof
commonPMtubes– Mainglowpeakisat200°C,highenoughtominimizefading
butlowenoughtostopinfraredemissions– Glowcurveisshapedtoenableeasyseparationoflow
temperaturepeaks– NotadverselyaffectedbyambientconditionsexceptUV
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PersonalMonitoring
• TLDsreportresultinmSv• Hp07skinequivalentdose• Hp3eyeequivalentdose• Hp10wholebodyeffectivedose• Staffwearmonitorsforwholebody(waist),eye,fingerrings,
legmonitors,collarmonitors....
Patientdosecalculations
Measurementofeffectivedosecanneverbemadedirectly,insteadmeasureentrancesurfacedoseandestimateusingMonte-Carlosimulations
DoseAreaProduct(generalX-ray)DoseLengthProduct(CT)
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Roomshielding
• ToensuredosestomembersofthepublicremainwithinlimitsweensureX-rayroomsadequatelyshielded.
• Publicdoselimit1mSvperyear– weworktodoseconstraintof0.3mSvperyear
• LeadshieldingforX-rayrooms,concreteforlinacbunkers.
RadioactiveMaterials
• Notonlyexternalexposurerisk,alsointernalexposure• Needtoavoidcontaminationbecausethiswouldincrease
likelihoodofingestion• Ifspillontoskinneedtowashimmediately– canget
significantskindoses• Contaminationmonitoringimportant
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Goodworkingpractice
• Reduceexposure:– Time(directlyproportional)– Distance(1/r2)– Shielding(exponentialattenuation– typicallywithX-raysweshieldourselves,
withradioactivematerialsweshieldthesource)
• Reducetheriskofcontamination– workoverdriptrayswithabsorbentmaterial)– WearPPE
• Alwayskeepbelowdoselimits– Weworkwellbelowtheselimitsinthehealthsector,annualeffectivedoses
<1mSv.Staffgroupwithhighestoccupationalexposuresairlineworkers.
Chargedparticleinteractions
Quantifyingradiationdose
Radiationrisks