Clinical Dosimetry in Photon Radiotherapy – a Monte Carlo Based Investigation
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Aus dem Medizinischen Zentrum fur¨ Radiologie Sektion fur¨ Medzinische Physik Geschaftsf¨ uhrender¨ Direktor: Prof. Dr. K. J. Klose des Fachbereichs Medizin der Philipps-Universitat¨ Marburg In Zusammenarbeit mit dem Universitatsklinikum¨ Gießen und Marburg GmbH, Standort Marburg Clinical Dosimetry in Photon Radiotherapy – a Monte Carlo Based Investigation Inaugural Dissertation zur Erlangung des Doktorgrades der Humanbiologie (Dr. rer. physiol.) dem Fachbereich Medizin der Philipps-Universitat¨ Marburg vorgelegt von Jorg¨ Wulff aus Munchen¨ Marburg, 2010 fur¨ Klemens Angenommen vom Fachbereich Medizin der Philipps-Universitat¨ Marburg am 15.01.2010 Gedruckt mit der Genehmigung des Fachbereichs 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¨ (Fachbereich Physik) 6 Contents 1. INTRODUCTION AND THEORETICAL FOUNDATION 8 1.1. Necessity for Improving Accuracy in Dosimetry ............. 8 1.2. Outline .................................. 9 1.3. Physics of Ionizing Radiation ....................... 10 1.3.1. Electron and Positron Interactions . 10 1.3.2. Photon Interactions ........................ 11 1.3.3. Definition of Dosimetric Quantities . 12 1.4. Clinical Radiation Dosimetry ....................... 14 1.4.1. General Concepts ......................... 14 1.4.2. Ionization Chamber Dosimetry . 16 1.4.3. Cavity Theory .......................... 17 1.4.4. Dosimetry Protocols ....................... 21 1.4.5. Non-Reference Conditions .................... 23 1.4.6. Other Types of Detectors ..................... 25 1.5. Monte Carlo Simulations of Radiation Transport . 26 1.5.1. General Introduction and Historical Background . 26 1.5.2. The EGSnrc Code System .................... 27 1.5.3. Simulation of Photon and Electron Transport . 28 1.5.4. Variance-Reduction Techniques - General Concepts . 29 1.5.5. Ionization Chamber Calculations . 31 1.5.6. Simulation of Linear Accelerators . 33 2. METHODS FOR INVESTIGATION OF DOSIMETRY 37 2.1. Increasing Efficiency ........................... 37 2.1.1. Ionization Chamber Calculations in Photon Beams . 37 2.1.2. Fast kerma-Based Calculations . 44 2.1.3. Parallel Computing with the EGSnrc Monte Carlo Code . 46 2.2. Ionization Chamber Calculations for Reference Dosimetry . 46 2.2.1. Photon Spectra .......................... 46 2.2.2. Calculation of Perturbation and Beam-Quality Correction Factors 47 2.2.3. Uncertainty Estimation for Calculated Correction Factors . 51 2.3. Ionization Chambers under Non-Reference Conditions . 53 2.3.1. Modeling a Linear Accelerator Head . 54 2.3.2. Ionization Chamber in the 6 MV Field of a Linear Accelerator . 57 2.3.3. Ionization Chambers and Other Detectors Under Charged Parti- cle Dis-Equilibrium in the Penumbra of a Photon Beam . 57 3. RESULTS AND DISCUSSION 60 3.1. Increasing Efficiency ........................... 60 3.1.1. Ionization Chamber Related Calculations in Photon Beams . 60 3.1.2. Fast kerma-Based Calculations . 63 Contents 7 3.2. Calculations for Reference Dosimetry . 63 3.2.1. Perturbation Factors ....................... 64 3.2.2. Uncertainty-Estimation for Calculated Correction Factors . 73 3.2.3. Summary ............................. 77 3.3. Non-Reference Conditions ........................ 77 3.3.1. Modeling the Siemens KD Linear Accelerator . 77 3.3.2. Ionization Chambers in the 6 MV Field of a Linear Accelerator 83 3.3.3. Charged Particle Dis-Equilibrium in the Penumbra . 87 3.3.4. Summary ............................. 93 4. SUMMARY AND CONCLUSION 94 5. Abstract 97 6. Zusammenfassung 99 A. Einf ¨uhrungzur Monte-Carlo Simulation von Strahlungstransport 101 Bibliography 123 List of Tables 124 List of Figures 125 List of Abbreviations 127 Danksagung I Publikationsliste II 8 1. INTRODUCTION AND THEORETICAL FOUNDATION 1.1. Necessity for Improving Accuracy in Dosimetry Radiotherapy as an important form of cancer treatment aims at the eradication of tumour 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 knowledge of the delivered radiation dose to the patient. The tumour control and the normal tissue complication probability have a sigmoidal dependence on radiation dose. A hypothetical dose effect relation is schematically shown 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, which needs to be maintained in radiotherapy, amounts to 5% and includes all uncertain- ties of the dose delivery process (Papanikolaou et al., 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, based on epidemiological studies, will require a reduced uncertainty in the dose delivered during radiation treatment. 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% (1 standard deviation). A reduction to 1% is aimed at for the future (Papanikolaou et∼ al., 2004). The origin of this uncertainty can be retraced to theories of ionization chamber dosimetry applied in the current protocols and the data presently available. Modern radiation techniques employing small fields such as stereotactic radiotherapy provide good conformity to tumour volumes and allow sparing of organs at risk. In intensity modulated radiation therapy (IMRT) non-uniform fields are composed of many 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 techniques challenges the established protocols for dosimetry under reference conditions while aiming at the highest precision. Generally it is questionable if the mentioned 2% uncertainty holds for dosimetry under non-reference conditions with the application∼ of current dosimetry concepts. 1. INTRODUCTION AND THEORETICAL FOUNDATION 9 Figure 1.1.: Schematic illustration of tumour control probability (TCP) and the probabil- ity of normal tissue complication (NTCP) as a function of dose. The vertical line indicates a certain dose in the steep part of both effects responding to dose. Uncertainties in delivered dose might worsen the clinical outcome due to either reduction of TCP or increase of NTCP. 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- ation transport and their application to calculations for clinical dosimetry.1 In chapter 2 the developed methods for the efficient simulation of ionization chamber dosimetry are introduced. The methodology for the simulation of a clinical linear accelerator model is explained. Chapter 3 presents and discusses the results of the investigation of ionization chamber dosimetry under reference and a comparison to existing data in dosimetry pro- tocols. An analysis of systematic uncertainties is presented. The development of a linear 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 given in chapter 4. 1Please note that a more detailed introduction to Monte Carlo simulations of radiation transport is given in the appendix (in German language). 10 1.3. Physics of Ionizing Radiation 1.3. Physics of Ionizing Radiation In the following some basic principles and quantities in the context of ionizing radiation are given. This is not intended to be complete review, it rather serves as a brief intro- duction that covers the topics needed in the later chapters. For more details the reader is referred to appropriate textbooks, e.g. Attix (2004), Podgorsak (2006) or Reich (1990). 1.3.1. Electron and Positron Interactions When charged particles2 pass medium they interact with the absorber atoms through Coulomb interactions with atoms’ nuclei and orbital electrons. Collisions may be elastic when only a change of direction occurs or inelastic when further energy is transferred. Types of interaction can be distinguished. electron-orbital electron (collisional) interactions, where ionization with ejection • of the orbital-electron or excitation of the absorber atom follows; ejected electrons 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 ground state with the emission of characteristic x-rays or Auger-electrons electron-orbital and electron-nucleus (radiative) interaction, where scattering and • energy