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RadiationSafety

BethanyGillettbethany.gillett@addenbrookes.nhs.uk

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