Magnetic removal of electron contamination in radiotherapy x-ray beams

Transcription

1 University of Wollongong Research Online University of Wollongong Thesis Collection University of Wollongong Thesis Collections 2006 Magnetic removal of electron contamination in radiotherapy x-ray beams Brad Oborn University of Wollongong, Recommended Citation Oborn, Bradley M, Magnetic removal of electron contamination in radiotherapy x-ray beams, MSc thesis, Department of Engineering Physics, University of Wollongong, Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library:

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3 Magnetic Removal of Electron Contamination in Radiotherapy X-ray Beams Brad Oborn

4 Magnetic Removal of Electron Contamination in Radiotherapy X-ray Beams A thesis submitted in partial fulfilment of the requirements for the award of the degree Master of Science Medical Radiation Physics from University of Wollongong by Bradley Michael Oborn (B. Med Rad Phys) Department of Engineering Physics 2006

5 Certification I, Bradley Michael Oborn, declare that this thesis, submitted in partial fulfilment of the requirements for the award of Master of Science, in the Department of Engineering Physics, University of Wollongong, is wholly my own work unless otherwise referenced or acknowledged. The document has not been submitted for qualifications at any other academic institution. Bradley Oborn 20 September 2006 i

6 Acknowledgements Firstly I would like to thank Professor Anatoly Rosenfeld for his continued assistance in my studies ever since I commenced my Honours degree in Medical Radiation Physics at the University of Wollongong in He has always been interested in my studies, and dedicates most of his time to his students. I feel privileged to have been taught by and associated with him. Secondly I must thank Associate Professor Martin Butson. Without him I would not have found this fascinating topic to study, and would not have developed the understanding I currently have of medical physics in radiotherapy. Martin has always been a great source of knowledge and I consider him as a mentor for more than just medical physics research. I also wish to thank the following Physics staff members at the University of Wollongong for their guidance and advice during this thesis; Dr Michael Lerch, Dr Dave Martin, and Dr George Takacs. Finally I wish to thank my wife Marisa for her patience with me during the last 2 years. ii

7 Abstract Removal of contamination electrons to lower patient skin dose from Linac produced radiotherapy x-ray beams is a serious issue in modern radiotherapy. Such removal can be achieved via the use of a magnetic field and is the subject of investigation in this thesis. The magnetic deflector consists of two separate and adjustable banks of permanent Neodymium-Iron-Boron magnets held in a simple Aluminium frame, which slots into the accessory mount of a conventional Varian Clinac 2100C Linear Accelerator. The deflector allows x-ray beams with field sizes of up to 30x30 cm 2 (source to surface distance of 100 cm) to pass through without interference, and weighs less than 20 kg. The deflector generates a maximum field of 0.21 T between the magnets along the central axis for a 10 cm magnet bank separation, and similarly 0.07 T for a 20 cm separation. Using the magnetic deflector, experimental measurements at the central beam axis show entry doses that approach that of the theoretical entry dose without electron contamination (Monte Carlo predictions) for 6 and 10MV x-ray beams. These range from 25% (6MV, 10x10 cm field size) to 55% (10MV, 20x20 cm field size with Perspex block trays) relative dose reduction at the phantom surface. Theoretical modelling has been performed which confirms the removal of the electron contamination for these typical clinical x-ray beam energies and field sizes. Pure electron beam path modelling has also been studied using this technique for determining the accuracy of the modelling technique. Results agree closely with experimentally observed values for 5 clinical electron beam energies between 6 and 20 MeV. The theoretical simulations are based around 3-dimensional modelling of the path of the contamination electrons as they travel through the magnetic field set up by the deflector (MATLAB). The magnetic field data used in modelling has been generated by a finite element package (Maxwell 3D). The experimental verification methods include the use of radiographic film and Attix parallel plate ion chambers with solid water phantoms for both qualitative and quantitative measurements. iii

8 List of Tables Table 1-1. Important published research on electron filters Table 1-2. Published research on magnetic deflectors Table 1-3. The Varian magnetic electron spreader Table 1-4. Important published research on helium air bag systems Table 1-5. Published research on electrostatic field deflectors Table 3-1. Properties of the materials modelled in Maxwell 3D Table 3-2. Simulation settings for pure electron beams Table 3-3. Comparisons between simulations and experimental measurements for pure electron beam deflection Table 3-4. Simulation settings for contamination electrons and positrons within the modelled x-ray beam List of Figures Figure 1.1. The common photon and beta particle interactions with atoms in matter... 5 Figure 1.2. Relative contributions of the Photoelectric interaction, Compton scattering, and Pair Production processes Figure 1.3. Typical Linac Bremsstrahlung x-ray spectrums for 8 and 18MV beams... 9 Figure 1.4. Typical Monte Carlo generated electron contamination energy spectrum produced by an 18MV Linac x-ray beam... 9 Figure 1.5. Simple separation of contamination electrons from the parent x-ray beam using a magnetic field Figure 1.6. Typical percentage depth-dose curve for a Linac generated x-ray beam Figure 1.7. Detailed diagram of the build-up region of a typical percentage depth-dose curve Figure 2.1. General coordinate system used in this project Figure 2.2. Measurement and simulation volumes Figure 2.3. Measurement and simulation planes Figure 2.4. Detailed diagrams of the magnetic deflector and it components Figure 2.5. Magnetic deflector in position for experimental measurements Figure 2.6. Geometric advantage of the magnetic deflector being located away from patient surface Figure 2.7. Relative contributions to electron contamination from each of the scattering components, and the magnetic field required to remove this Figure 2.8. Description of the electron contamination starting directions Figure 3.1. Magnetic field intensity and directions through a central slice of a permanent NdFe35 magnet (5x5x5 cm) as determined by Maxwell 3D Figure 3.2. Intensity plot of the magnetic field on the Central Endview Plane as determined by Maxwell 3D iv

9 Figure 3.3. Intensity plot of the magnetic field on the Central Sideview and Central Topview Plane as determined by Maxwell 3D Figure 3.4. Magnetic field directions in the Central Endview Plane Figure 3.5. Magnetic field directions around the magnets in the Central Endview Plane Figure 3.6. Consistent electron deflection directions for a 10 cm magnet bank separation in the Central Endview Plane (for an electron travelling down the page) Figure 3.7. Consistent electron deflection directions for a 10 cm magnet bank separation in the Central Topview Plane (for an electron travelling into the page) Figure D image of the consistent deflection direction volume in the region between the magnet banks (Magnet Bank Separation is 10 cm) Figure 3.9. A visual description of the mesh size (or number of elements) in the Simulated Volume Figure Simulated magnetic field magnitude values in the Central Endview Plane Figure Simulated magnetic field strength magnitude in the Central Topview Plane Figure Simulated y-direction components of the magnetic field strength values in the Central Endview Plane Figure Simulated y-direction components of the magnetic field strength values in the Central Topview Plane Figure Iso-field surface plots of the magnetic field strength extending from the magnetic deflector for a Magnet Bank Separation of 10 cm Figure Photograph of the Teslameter and schematic diagram of the probe Figure Experimental y-direction magnetic field measurements in the Central Measurement Plane for a Magnet Bank Separation of 10 cm Figure Comparison between Maxwell 3D and Experimental results in the Central Beam Axis and other nearby axes Figure Comparison between Maxwell 3D and experimental data in the Central Measurement Plane Figure Comparison between Maxwell 3D and experimental data in the x=+7.5 cm plane Figure Comparison between Maxwell 3D and experimental data in the x=-7.5 cm plane Figure Possible sources of error between the simulated measurements and experimental results Figure Contour plot of the magnetic field strengths in the Central Endview plane, and the corresponding lineplot along the Central Beam Axis Figure The experimental set-up used for taking radiographic film measurements Figure MeV electron beam deflection with a Magnet Bank Separation of 10 cm Figure MeV electron beam deflection with a Magnet Bank Separation of 10 cm Figure MeV electron beam deflection with a Magnet Bank Separation of 10 cm Figure MeV electron beam deflection with a Magnet Bank Separation of 10 cm Figure MeV electron beam deflection with a Magnet Bank Separation of 10 cm Figure MeV electron beam deflection in 3D to show the path taken with a Magnet Bank Separation of 10 cm Figure The 6 MeVelectron beam energy spectrum used for modelling v

10 Figure MeV electron beam deflection via the magnetic deflector Figure MeV electron beam deflection via the magnetic deflector Figure MeV electron beam deflection via the magnetic deflector Figure MeV electron beam deflection via the magnetic deflector Figure MeV electron beam deflection via the magnetic deflector on the base film Figure MeV electron beam deflection via the magnetic deflector on the -X film Figure The make-up of the dose delivered to a patient surface Figure Radiographic film image of a 6MV 10x10 cm x-ray field and qualitative plot profiles Figure Radiographic film image of a 10MV 20x20 x-ray field with 3 cm of extra Perspex and qualitative plot profiles Figure Comparison of a 6MV 10x10 cm x-ray beam with and without the magnetic deflector at 10 cm Magnet Bank Separation Figure Comparison of a 10MV 20x20 cm x-ray beam with and without the magnetic deflector at 20 cm Magnet Bank Separation and 3 cm of extra Perspex Figure D diagram of the deflection of electron contamination in the 10MV 20x20 cm case Figure The electron and positron contamination energy spectrums used for 10MV x-ray beam modelling Figure Theoretical film from electron contamination for a 10MV 20x20 cm x-ray beam without the magnetic deflector Figure MV electron contamination deflection via the magnetic deflector on the base film Figure MV electron contamination deflection via the magnetic deflector on the -X film Figure MV electron contamination deflection via the magnetic deflector on the +X film. These are primarily positrons Figure True (top) and theoretical (bottom) film comparison for the 10MV case Figure Central Beam Axis %Depth-Dose profile for a 10x10 cm field size, 6MV x-ray beam with a Magnet Bank Separation of 20 cm Figure Central Beam Axis %Depth-Dose profile for a 15x15 cm field size, 6MV photon beam with a Magnet Bank Separation of 20 cm Figure Central Beam Axis %Depth-Dose profile for a 20x20 cm field size, 6MV photon beam with a Magnet Bank Separation of 20 cm Figure Central Beam Axis %Depth-Dose profile for a 20x20 cm field size, 10MV x-ray beam with a Magnet Bank Separation of 20 cm and 3 cm of Perspex Figure Comparison between the overall reductions in entry dose for a 6 and 10MV beam with a Magnet Bank Separation of 20 cm vi

11 Contents Certification Acknowledgements Abstract List of Tables List of Figures i ii iii iv iv 1 INTRODUCTION Radiotherapy 1 a. Reason for radiotherapy 1 b. Ability of X-ray radiation to treat/destroy cells 2 c. Side effects of radiation therapy Photon, Electron, and Positron Interactions in Matter 3 a. Photon interactions in matter 3 b. Electron and Positron interactions in matter 4 c. Positron annihilation X-ray Production: the Linac 7 a. Bremsstrahlung Radiation Electron Contamination in Linac X-ray Beams 8 a. Mechanisms that generate electron contamination 8 b. Typical energy spectrum of contamination electrons 'Magnetic' Electron Contamination Removal 10 a. The Lorentz force 10 b. Electron motion in magnetic fields 10 c. Electron contamination removal Previous Research on Electron Contamination Removal 12 a. Introduction 12 b. Electron filters 12 c. Magnetic deflectors 13 d. Magnetic electron spreaders 15 e. Helium air bag systems 16 f. Electrostatic field deflectors 17 vii

12 1.7 Properties of Linac X-Ray Beams 18 a. Build-Up region properties and theory Thesis Aims 20 a. Overview 20 b. Experimental aims 20 c. Theoretical modelling aims 20 2 MATERIALS AND METHODS Definitions 21 a. General coordinate system 21 b. Measurement and simulation volumes, planes and axes The magnetic deflector 25 a. Deflector design 25 b. Device Magnets 28 c. Construction methods 28 d. Location on Linac Simple electron path modelling in a magnetic field 32 a. Overview Modelling of electron contamination removal in x-ray beams 33 a. Introduction 33 b. Electron contamination initial conditions: Starting positions 33 c. Electron contamination initial conditions: Starting energies 33 d. Electron contamination initial conditions: Starting directions 34 3 RESULTS Magnetic fields generated by the magnetic deflector 35 a. Introduction 35 b. Visualisation of the relative magnetic field intensity using Maxwell 3D 38 c. Visualisation the magnetic field directions around the magnetic deflector 40 d. Quantitative simulations of the magnetic field strengths 46 e. Manual measurements of the magnetic field strength 53 f. Comparison between Maxwell 3D and experimental results 55 g. Discussion Experimental pure electron beam deflection 63 a. Overview 63 viii

13 b. Results 63 c. Discussion Theoretical pure electron beam deflection using the magnetic deflector 72 a. Overview 72 b. Theoretical Results 73 c. Discussion Experimental electron contamination removal in x-ray beams using the magnetic deflector 81 a. Verification of electron contamination without the magnetic deflector 81 b. Results 81 c. Verification of electron contamination removal with the magnetic deflector 85 d. Discussion Theoretical electron contamination removal in x-ray beams using the magnetic deflector 90 a. Introduction 90 b. Positron contamination 90 c. Theoretical Results 92 d. Discussion Percentage depth-dose measurements 99 a. Percentage depth-dose profiles in the central axis 99 b. Discussion DISCUSSION AND CONCLUSION 106 a. The magnetic deflector 106 b. Reduction in skin dose with the magnetic deflector 106 c. Clinical use of a magnetic deflector 107 d. Future work REFERENCES 108 ix