Clinical dosimetry in photon radiotherapy [Elektronische Ressource] : a Monte Carlo based investigation / vorgelegt von Jörg Wulff

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AusdemMedizinischenZentrumfur¨ RadiologieSektionfur¨ MedzinischePhysikGeschaftsf¨ uhrender¨ Direktor:Prof.Dr.K.J.Klose¨desFachbereichsMedizinderPhilipps-UniversitatMarburgInZusammenarbeitmitdemUniversitatsklinikum¨ GießenundMarburgGmbH,StandortMarburgClinicalDosimetryinPhotonRadiotherapy–aMonteCarloBasedInvestigationInauguralDissertationzurErlangungdesDoktorgradesderHumanbiologie(Dr.rer.physiol.)demFachbereichMedizinderPhilipps-Universitat¨ Marburgvorgelegtvon¨JorgWulffausMunchen¨Marburg,2010fur¨ KlemensAngenommenvomFachbereichMedizinderPhilipps-Universitat¨ Marburgam15.01.2010GedrucktmitderGenehmigungdesFachbereichsDekan:Prof.Dr.M.RothmundReferent:Prof.Dr.Dr.J.T.HeverhagenKorreferent:Prof.Dr.Dr.G.Kraft(Darmstadt)Prufungsausschuss:¨Prof.Dr.H.Schafer¨Prof.Dr.Dr.J.T.HeverhagenProf.Dr.H.J.Jansch¨ (FachbereichPhysik)6Contents1. INTRODUCTIONANDTHEORETICALFOUNDATION 81.1. NecessityforImprovingAccuracyinDosimetry . . . . . . . . . . . . . 81.2. Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.3. PhysicsofIonizingRadiation . . . . . . . . . . . . . . . . . . . . . . . 101.3.1. ElectronandPositronInteractions . . . . . . . . . . . . . . . . 101.3.2. PhotonInteractions . . . . . . . . . . . . . . . . . . . . . . . . 111.3.3. DefinitionofDosimetricQuantities . . . . . . . . . . . . . . . 121.4. ClinicalRadiationDosimetry . . . . . . . . . . . . . . . . . . . . . . . 141.4.1. GeneralConcepts . . . . . . . . . . . . . .

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AusdemMedizinischenZentrumfur¨ Radiologie
Sektionfur¨ MedzinischePhysik
Geschaftsf¨ uhrender¨ Direktor:Prof.Dr.K.J.Klose
¨desFachbereichsMedizinderPhilipps-UniversitatMarburg
InZusammenarbeitmitdemUniversitatsklinikum¨ GießenundMarburgGmbH,
StandortMarburg
ClinicalDosimetryinPhotonRadiotherapy
–aMonteCarloBasedInvestigation
InauguralDissertation
zurErlangungdesDoktorgradesderHumanbiologie
(Dr.rer.physiol.)
demFachbereichMedizinderPhilipps-Universitat¨ Marburg
vorgelegtvon
¨JorgWulff
ausMunchen¨
Marburg,2010fur¨ KlemensAngenommenvomFachbereichMedizinderPhilipps-Universitat¨ Marburg
am15.01.2010
GedrucktmitderGenehmigungdesFachbereichs
Dekan:Prof.Dr.M.Rothmund
Referent:Prof.Dr.Dr.J.T.Heverhagen
Korreferent:Prof.Dr.Dr.G.Kraft(Darmstadt)
Prufungsausschuss:¨
Prof.Dr.H.Schafer¨
Prof.Dr.Dr.J.T.Heverhagen
Prof.Dr.H.J.Jansch¨ (FachbereichPhysik)6
Contents
1. INTRODUCTIONANDTHEORETICALFOUNDATION 8
1.1. NecessityforImprovingAccuracyinDosimetry . . . . . . . . . . . . . 8
1.2. Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3. PhysicsofIonizingRadiation . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.1. ElectronandPositronInteractions . . . . . . . . . . . . . . . . 10
1.3.2. PhotonInteractions . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3.3. DefinitionofDosimetricQuantities . . . . . . . . . . . . . . . 12
1.4. ClinicalRadiationDosimetry . . . . . . . . . . . . . . . . . . . . . . . 14
1.4.1. GeneralConcepts . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.4.2. IonizationChamberDosimetry . . . . . . . . . . . . . . . . . . 16
1.4.3. CavityTheory . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1.4.4. DosimetryProtocols . . . . . . . . . . . . . . . . . . . . . . . 21
1.4.5. Non-ReferenceConditions . . . . . . . . . . . . . . . . . . . . 23
1.4.6. OtherTypesofDetectors . . . . . . . . . . . . . . . . . . . . . 25
1.5. MonteCarloSimulationsofRadiationTransport . . . . . . . . . . . . . 26
1.5.1. GeneralIntroductionandHistoricalBackground . . . . . . . . 26
1.5.2. TheEGSnrcCodeSystem . . . . . . . . . . . . . . . . . . . . 27
1.5.3. SimulationofPhotonandElectronTransport . . . . . . . . . . 28
1.5.4. Variance-ReductionTechniques-GeneralConcepts . . . . . . . 29
1.5.5. IonizationChamberCalculations . . . . . . . . . . . . . . . . . 31
1.5.6. SimulationofLinearAccelerators . . . . . . . . . . . . . . . . 33
2. METHODSFORINVESTIGATIONOFDOSIMETRY 37
2.1. IncreasingEfficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.1.1. IonizationChamberCalculationsinPhotonBeams . . . . . . . 37
2.1.2. Fastkerma-Based . . . . . . . . . . . . . . . . . . 44
2.1.3. ParallelComputingwiththeEGSnrcMonteCarloCode . . . . 46
2.2. IonizationChamberCalculationsforReferenceDosimetry . . . . . . . 46
2.2.1. PhotonSpectra . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.2.2. CalculationofPerturbationandBeam-QualityCorrectionFactors 47
2.2.3. UncertaintyEstimationforCalculatedCorrectionFactors . . . . 51
2.3. IonizationChambersunderNon-ReferenceConditions . . . . . . . . . 53
2.3.1. ModelingaLinearAcceleratorHead . . . . . . . . . . . . . . . 54
2.3.2. IonizationChamberinthe6MVFieldofaLinearAccelerator . 57
2.3.3. Chambers and Other Detectors Under Charged Parti-
cleDis-EquilibriuminthePenumbraofaPhotonBeam . . . . . 57
3. RESULTSANDDISCUSSION 60
3.1. IncreasingEfficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.1.1. IonizationChamberRelatedCalculationsinPhotonBeams . . . 60
3.1.2. Fastkerma-BasedCalculations . . . . . . . . . . . . . . . . . . 63Contents 7
3.2. CalculationsforReferenceDosimetry . . . . . . . . . . . . . . . . . . 63
3.2.1. PerturbationFactors . . . . . . . . . . . . . . . . . . . . . . . 64
3.2.2. Uncertainty-EstimationforCalculatedCorrectionFactors . . . 73
3.2.3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
3.3. Non-ReferenceConditions . . . . . . . . . . . . . . . . . . . . . . . . 77
3.3.1. ModelingtheSiemensKDLinearAccelerator . . . . . . . . . . 77
3.3.2. IonizationChambersinthe6MVFieldofaLinearAccelerator 83
3.3.3. ChargedParticleDis-EquilibriuminthePenumbra . . . . . . . 87
3.3.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4. SUMMARYANDCONCLUSION 94
5. Abstract 97
6. Zusammenfassung 99
A. Einfuhrung¨ zurMonte-CarloSimulationvonStrahlungstransport 101
Bibliography 123
ListofTables 124
ListofFigures 125
ListofAbbreviations 127
Danksagung I
Publikationsliste II8
1. INTRODUCTIONANDTHEORETICAL
FOUNDATION
1.1. NecessityforImprovingAccuracyin
Dosimetry
Radiotherapyasanimportantformofcancertreatmentaimsattheeradicationoftumour
cells with the use of ionizing radiation. A consistent quality assurance procedure is
mandatory to ensure the accurate dose delivery to a tumour volume and to avoid any
unnecessary harm to normal tissue. A central point of quality assurance is the exact
knowledgeofthedeliveredradiationdosetothepatient.
The tumour control and the normal tissue complication probability have a sigmoidal
dependenceonradiationdose. Ahypotheticaldoseeffectrelationisschematicallyshown
in figure 1.1. The characteristic dose effect curves for tumour control and normal tissue
complication with their steep gradients require the accurate knowledge of dose to the
patient. Any uncertainty on delivered dose may either result in an underdosage of the
tumour or a complication for normal tissue. The generally accepted total uncertainty,
whichneedstobemaintainedinradiotherapy,amountsto5%andincludesalluncertain-
tiesofthedosedeliveryprocess(Papanikolaouetal.,2004).
There is a large variation in the reported slopes in the dose effect curves, but it has
been reported that even a 1% dose accuracy improvement can result in a 2% increase in
cure rate for early stage tumours (Boyer and Schultheiss, 1988). Besides the quality of
an individual treatment, any attempt to improve the knowledge of dose effect relations,
basedonepidemiologicalstudies,willrequireareduceduncertaintyinthedosedelivered
duringradiationtreatment.
One crucial contribution to the overall uncertainty is the determination of dose under
reference conditions in a clinical therapy beam and is currently expected to be ∼2%
(1standarddeviation). Areductionto1%isaimedatforthefuture(Papanikolaouetal.,
2004). The origin of this uncertainty can be retraced to theories of ionization chamber
dosimetryappliedinthecurrentprotocolsandthedatapresentlyavailable.
Modern radiation techniques employing small fields such as stereotactic radiotherapy
provide good conformity to tumour volumes and allow sparing of organs at risk. In
intensitymodulatedradiationtherapy(IMRT)non-uniformfieldsarecomposedofmany
small elementary fields and a larger part of total dose to the patient is delivered in these
small field segments (Bortfeld, 2006). The application of these advanced radiotherapy
techniqueschallengestheestablishedprotocolsfordosimetryunderreferenceconditions
whileaimingatthehighestprecision. Generallyitisquestionableifthementioned∼2%
uncertainty holds for dosimetry under non-reference conditions with the application of
currentdosimetryconcepts.1. INTRODUCTIONANDTHEORETICALFOUNDATION 9
Figure1.1.:Schematicillustrationoftumourcontrolprobability(TCP)andtheprobabil-
ityofnormaltissuecomplication(NTCP)asafunctionofdose. Thevertical
line indicates a certain dose in the steep part of both effects responding to
dose. Uncertaintiesindelivereddosemightworsentheclinicaloutcomedue
toeitherreductionofTCPorincreaseofNTCP.
1.2. Outline
Starting with some general and brief introduction to the physics of ionizing radiation,
the concepts of clinical ionizing radiation are presented in chapter 1. This chapter also
gives an introduction to the numerical Monte Carlo methods for the simulation of radi-
1ation transport and their application to calculations for clinical dosimetry. In chapter 2
the developed methods for the efficient simulation of ionization chamber dosimetry are
introduced. The methodology for the of a clinical linear accelerator model is
explained. Chapter3presents anddiscusses theresults oftheinvestigation ofionization
chamber dosimetry under reference and a comparison to existing data in dosimetry pro-
tocols. Ananalysisofsystematicuncertaintiesispresented. Thedevelopmentofalinear
accelerator model and its validation is described. The calculations under non-reference
dosimetry in the field of the linear accelerator model as well as in idealized conditions
of charged particle dis-equilibrium are presented. A general conclusion and summary is
giveninchapter4.
1Please note that a more detailed introduction to Monte Carlo simulations of radiation transport is given
intheappendix(inGermanlanguage).10 1.3. PhysicsofIonizingRadiation
1.3. PhysicsofIonizingRadiation
Inthefollowingsomebasicprinciplesandquantitiesinthecontextofionizingradiation
are given. This is not intended to be complete review, it rather serves as a brief intro-
ductionthatcoversthetopicsneededinthelaterchapters. Formoredetailsthereaderis
referredtoappropriatetextbooks,e.g. Attix(2004),Podgorsak(2006)orReich(1990).
1.3.1. ElectronandPositronInteractions
2When charged particles pass medium they interact with the absorber atoms through
Coulombinteractionswithatoms’nucleiandorbitalelectrons. Collisionsmaybeelastic
when only a change of direction occurs or inelastic when further energy is transferred.
Typesofinteractioncanbedistinguished.
• electron-orbital electron (collisional) interactions, where ionization with ejection
oftheorbital-electronorexcitationoftheabsorberatomfollows;ejectedelectrons
carrying enough energy for traveling a certain distance away from the point of
interaction are called δ- or knock-on electrons; the ionized atom will return to its
groundstatewiththeemissionofcharacteristicx-raysorAuger-electrons
• electron-orbital and electron-nucleus (radiative) interaction, where scattering and
energylossbyproductionofradiativephotons(Bremsstrahlung)results
• softinteractionwiththewholeatom,wherevirtuallynoneoronlyasmallamount
ofenergyislost,stillbeingthemostnumeroustypeofinteraction
EnergylossesperunitlengthxaredescribedbythestoppingpowersS =dE/dxofa
material,ormorefrequentlyusedasmassstoppingpowerS/ρwithmedium’sdensityρ.
The total stopping power consists of collisional and radiative contributions (see above).
The radiative photons travel far before being absorbed and as will be discussed below
(see eq. 1.5) local absorbed dose is directly proportional to the collisional part of the
stopping powers. A brief look at the underlying equation for the description of the col-
lisional stopping powersS for electrons and positrons is helpful for later discussions.col
FollowingICRUReportNo.37(ICRU,1984)S isgivenbycol
2 2 h iS 2πr mc 1 Zcol e 2 ±= ln(E/I) +ln(1+τ/2)+F (τ)−δ (1.1)
2ρ u β A
where r is the classical electron radius, m is the mass of the electron, c is the velocitye
of light, u is the atomic mass, β is the ratio of particle velocity to the velocity of light,
Z is the atomic number. A is the atomic weight,E is the kinetic energy of the electron,
±I is the mean excitation energy of the absorber atom, F is an auxiliary function for
electrons(-)andpositrons(+),τ istheratioofkineticenergyE oftheelectrontoitsrest
energyandδ isdensity-effectcorrection.
2Inthecontextofphotondosimetry,chargedparticlesareconsideredtobeelectronsorpositrons. Within
the following no distinction between electrons and positrons is drawn, i.e. electrons are used as a
synonymforboth.