Charged Particle Transport in Magnetic Fields in the EGSnrc Monte Carlo Code System

Abstract

The advent of magnetic resonance guided radiation therapy provides a promising technology for dealing with tumour motion and anatomical variations during treatment.These machines possess a variety of beam energies, geometrical configurations, and different magnetic field strengths. Although photon beams do not directly experience the influence of the magnetic field, electrons set in motion will curve and impact dose distributions. Clinical reference dosimetry protocols rely on correction factors which account for the change in detector response for different beam qualities in the absence of a magnetic field. The effect of the magnetic field poses challenges for dosimetry, as ion chambers and solid state detectors respond disproportionately to the actual change in the dose to the media in the presence of the magnetic field. This necessitates an adaptation of current dosimetry protocols through calculation of high precision magnetic field and beam quality correction factors which account for detector response variation.In this work, charged particle transport in magnetic fields is implemented in EGSnrc and is shown to pass the Fano cavity test at the 0.1 % level. Further good agreement with experimental ion chamber measurements is shown, and important effects such as air gaps and the unknown sensitive volume of the chamber are determined to cause several percent variation in the calculated ion chamber dose. Ion chamber magnetic field correction factors are then evaluated for over thirty cylindrical ionization chamber and a select number of parallel-plate chambers. Magnetic field correction factors for the majority of cylindrical chambers are within 1 % of unity, while parallel-plate chambers require correction factors on the order of several percent and, unlike cylindrical chambers, no optimal orientation is available to reduce the effect of the magnetic field. The %dd(10) x beam-quality specifier is shown to have a strong dependence of the magnetic field strength, and the TPR 20 10 is determined to be the optimal beam-quality specifier in magnetic fields. Collectively, this work contributes to the EGSnrc gold standard Monte Carlo code and to the evolving field of clinical reference dosimetry in magnetic fields.ii To my supervisor and mentor, Dr. David W. O. Rogers, I wish to extend my sincere gratitude for giving me the opportunity to work with, and learn from, him. From the first time that I met Dave, he has provided me with exceptional guidance and support. Dave's door has always been open when I needed help, and his insights and calm manner have always helped point the way forward. I have been truly lucky in my choice of supervisor and it is one of the best decisions I have ever made. I would also like to thank all of my friends, office mates, and colleagues at Carleton University. Martin Martinov has been an instrumental help throughout my research, and provided many helpful suggestions and constantly applied his excellent programming knowledge to aide everyone in the group. Also, I am very appreciative to all...

Reference dosimetry in MRI-linacs: evaluation of available protocols and data to establish a Code of Practice

“…The observed discrepancies of >1% between their values for the PTW 30013 chamber with orientation d (0.976) and other more recent values given in tables 5 and 6 might be explained by not considering the dead volume. Monte Carlo calculations of magnetic field correction factors which take into account the dead volume (see section 5.3 and table 4) have been done by Spindeldreier et al (2017), Malkov andRogers (2017, 2018) and Pojtinger et al (2019). However, since the simulated beam in the latter study was based on a spectrum from Ahmad et al (2016) the flatness might be different than in an actual MRI-linac beam.…”