Proton dosimetry in a magnetic field: Measurement and calculation of magnetic field correction factors for a plane-parallel ionization chamber
Background: The combination of magnetic resonance imaging and proton therapy offers the potential to improve cancer treatment. The magnetic field (MF)-dependent change in the dosage of ionization chambers in magnetic resonance imaging-integrated proton therapy (MRiPT) is considered by the correction factor (Formula presented.), which needs to be determined experimentally or computed via Monte Carlo (MC) simulations. Purpose: In this study, (Formula presented.) was both measured and simulated with high accuracy for a plane-parallel ionization chamber at different clinical relevant proton energies and MF strengths. Material and methods: The dose-response of the Advanced Markus chamber (TM34045, PTW, Freiburg, Germany) irradiated with homogeneous 10× 10 cm2 quasi mono-energetic fields, using 103.3, 128.4, 153.1, 223.1, and 252.7 MeV proton beams was measured in a water phantom placed in the MF of an electromagnet with MF strengths of 0.32, 0.5, and 1 T. The detector was positioned at a depth of 2 g/cm2 in water, with chamber electrodes parallel to the MF lines and perpendicular to the proton beam incidence direction. The measurements were compared with TOPAS MC simulations utilizing COMSOL-calculated 0.32, 0.5, and 1 T MF maps of the electromagnet. (Formula presented.) was calculated for the measurements for all energies and MF strengths based on the equation: (Formula presented.), where (Formula presented.) and MQ were the temperature and air-pressure corrected detector readings with and without the MF, respectively. MC-based correction factors were determined as (Formula presented.), where (Formula presented.) and Ddet were the doses deposited in the air cavity of the ionization chamber model with and without the MF, respectively. Furthermore, MF effects on the chamber dosimetry are studied using MC simulations, examining the impact on the absorbed dose-to-water ((Formula presented.)) and the shift in depth of the Bragg peak. Results: The detector showed a reduced dose-response for all measured energies and MF strengths, resulting in experimentally determined (Formula presented.) values larger than unity. For all energies and MF strengths examined, (Formula presented.) ranged between 1.0065 and 1.0205. The dependence on the energy and the MF strength was found to be non-linear with a maximum at 1 T and 252.7 MeV. The MC simulated (Formula presented.) values agreed with the experimentally determined correction factors within their standard deviations with a maximum difference of 0.6%. The MC calculated impact on (Formula presented.) was smaller 0.2 %. Conclusion: For the first time, measurements and simulations were compared for proton dosimetry within MFs using an Advanced Markus chamber. Good agreement of (Formula presented.) was found between experimentally determined and MC calculated values. The performed benchmarking of the MC code allows for calculating (Formula presented.) for various ionization chamber models, MF strengths and proton energies to generate the data needed for a proton dosimetry protocol within MFs and is, therefore, a step towards MRiPT.
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