Medical Radiation Dosimetry: Theory of Charged Particle Collision Energy Loss

643 Pages·2014·5.67 MB·English

by  Brian J McParland (auth.)

Brian J. McParland
Medical Radiation
Dosimetry
Theory of Charged
Particle Collision
Energy Loss
Medical Radiation Dosimetry

Brian J. McParland
Medical Radiation Dosimetry
Theory of Charged Particle Collision
Energy Loss
BrianJ.McParlandBASc,MSc,
PhD,CPhys,CSci,FCCPM,FIPEM,FInstP
Amersham,Buckinghamshire,UK
ISBN978-1-4471-5402-0 ISBN978-1-4471-5403-7(eBook)
DOI10.1007/978-1-4471-5403-7
SpringerLondonHeidelbergNewYorkDordrecht
LibraryofCongressControlNumber:2013951430
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Preface
Background
Medicalradiationdosimetryisthedisciplineofevaluating,throughmeasurementor
calculation,theabsorbeddosereceivedbyapatientasaconsequenceoftherapeutic
or diagnostic exposures to ionising radiation. A high level of accuracy in the
calculation and measurement of this dosimetry is essential for the optimisation of
patient treatment and diagnostic efficacy, the assurance of patient safety and the
continueduseofmedicalexposuresinretrospectiveepidemiologicalstudiesofthe
effects of ionising radiation. The achievement of this accuracy requires detailed
theoretical knowledge of the means through which the radiation interacts with
matter.Astheabsorbeddoseistheimmediateconsequenceofthetransferofkinetic
energy by an energetic charged particle to a medium as it decelerates through
collisionswithatoms,thetheoreticalunderstandingofthisphysicalmechanismof
collisionalenergylossisfundamentaltoallmedicaldosimetryevaluationsandisthe
subjectofthisbook.
The use ofionising radiationsinmedical practice is extensive.Throughout the
world approximately 2.5 billion people are exposed annually for the detection or
diagnosisofdiseaseorabnormalpathology(IAEA2002).Thatis,overoneinthree
of all humans receive medical ionising radiation each year for diagnoses with
theresultofanassociatedannualworldwidepercaputeffectivedose1reportedby
theInternationalAtomicEnergyAgency(IAEA)tobeabout0.4mSv,2,3Inaddition
1Theeffectivedoseisameasureofradiocarcinogenicandhereditarydamageriskresultingfrom
non-uniformexposuresatlowradiationabsorbeddoses(ICRP1991)andhastheunitofsievert
(Sv).Atypicalannualeffectivedoseresultingfromtheexposuretonaturalsourcesofradiationis
about2.5mSv.
2This is estimated to be about 78 % due to non-dental x-ray procedures, 21 % due to dental
radiologicalexaminationsand1%duetonuclearmedicineexaminations.
3Thereisawidevariationinthiseffectivedosedependinguponthedegreeandcomplexityof
medical care (UNSCEAR 2000) with countries with the highest levels of health care being
associated with a per caput annual effective dose of 1.3 mSv whereas those with the lowest
beingassociatedwith0.02mSv.
vii
viii Preface
to these diagnostic exposures, about 5.5 million patients worldwide each year
undergo therapeutic irradiations which involve absorbed doses of the order of
50 Gray or more, magnitudes at which the concept of the effective dose becomes
invalid. The features and consequences of these two medical applications of
ionising radiation differ beyond just the magnitudes of the associated absorbed
doses. The biological risks associated with the low absorbed radiation doses of
diagnosticimagingarecancerandhereditarydamageandaretermedstochasticas
the probability of incidence is a function of absorbed dose whereas the ultimate
severity or consequence of the effect, should it occur, is not. These risks are the
consequence of radiation-induced mutations or oncogene activations. Hence,
theneedforaccuratedosimetryisnecessaryfortheestimatingoftheseprobabilistic
risks.Ontheotherhand,theprobabilisticregimefadesawayatthegreaterabsorbed
doses of radiotherapy. These absorbed doses lead to cell death and resulting
macroscopic acute damage. These effects are deterministic due to their being a
result of the radiation-caused depletion of cells in an organ or tissue. Below an
absorbed dose threshold, no pathology is evident as the irradiated organ or tissue
can tolerate the loss ofa finitenumber of cells andstill retain function.However,
beyond this absorbed dose threshold, functional impairment becomes evident
clinically and the degree of damage increases with the radiation absorbed dose.
Deterministic effects are sought in radiation oncology as the number of sterilised
tumourcellsandtheconsequenttumourcontrolprobability4 increasewithabsorbed
dose.5 The balance of medical benefit (in terms of efficacious diagnosis, cure or
effective palliation) against the risk (characterised by late-term radiation-induced
cancers, ineffective tumour sterilisation or normal tissue damage) requires that
theradiationabsorbeddosetothepatientbeknowneffectivelysoastoenablethe
physician,whoisresponsibleultimatelyforpatientcare,tomakeareasonedmedical
andscientificjudgement.
Exclusion of Radiative Collisions
Medical ionising radiations are either charged (electrons, positrons, protons or
heavy ions) or uncharged (photons or neutrons), with the latter creating secondary
charged particles through interactions with atoms in the tissue. In both cases, the
outcomeofthemedicalirradiationwillbeafluxofenergeticchargedparticleswithin
tissueandwhichlosetheirkineticenergythroughinteractionswiththeatomswithin
thatmedium.Therearetwogeneralcategoriesofenergyloss,ofwhichonlyoneisthe
focusofthisbook.
4CompletedescriptionsofsuchradiobiologicaleffectscanbefoundinHallandGiaccia(2006).
5Acutedeterministiceffectshavebeenreportedinsomediagnosticimagingprocedures,suchas
fluoroscopy,althoughsuchoccurrencesareinfrequentandcanbeascribedtoimproperpractice
(Wagneretal.1999).
Preface ix
100
Carbon
10
)
2
m
c
V g Total
e
M
(
dEρdx Collision
1
Radiative
0.1
0.01 0.1 1 10 100 1,000
Electron kinetic energy (MeV)
Fig.P.1 Themasstotal,collisionandradiativestoppingpowersforelectronsincarbon,usedas
anelementalsurrogateforsofttissue.The‘total’curveisthesumofthe‘radiative’and‘collision’
energylossrates.Thedifferenceinmagnitudesbetweenthecollisionandradiativemasscollision
stopping powers for electron energies typical of medical applications (below 20 MeV)
demonstrateswhybremsstrahlungisnotconsideredhereformedicaldosimetryevaluations
Thefirstis thatofthe energyloss ofamovingchargedparticle asthe resultof
collisions with atomic electrons. Energy transfer to the medium through these
collisions leads to local energy deposition and to nonlocal energy deposition.
Thenonlocalenergydepositionisduetosecondaryrecoilelectronswithsufficient
kinetic energy to travel from the original interaction site and deposit their kinetic
energiesatadistance.
The second is the radiative energy loss of bremsstrahlung by which x-ray
photons are created through the electromagnetic interaction between the charged
particle and the nuclear Coulomb field. Any absorbed dose that ultimately results
from this interaction will be the consequence of a subsequent interaction of the
photonwithanatomandtheejectionofanatomicelectronwhichdeceleratesand
transfersenergytothemedium.
Of these two categories, the theory of collisional energy losses in medical
dosimetry is the core topic of this book whereas radiative energy loss theory is
not.Thejustificationforexcludingradiativelossesinatextbookdevotedtomedical
radiation dosimetry is that the bremsstrahlung production cross section is signifi-
cantonlyforlow-masschargedparticles(electronsorpositrons)withhighkinetic
energiesinteractingwithmediawithhighatomicnumbers.Theseconditionsarenot
metforelectronsorpositronsintissueatenergiestypicalofmedicalapplications.
To demonstrate this quantitatively, Fig. P.1 shows the mass stopping power