Proton dosimetry in a magnetic field: measurement and calculation of correction factors for a plane-parallel ionization chamber

Purpose: In magnetic resonance imaging-integrated proton therapy (MRiPT), the magnetic field-dependent change in the dosage of ionization chambers is considered by the correction factor k ⃗ B,M,Q, which needs to be determined experimentally or computed via Monte Carlo (MC) simulations. In this study, k ⃗B,M,Q was both measured and simulated with high accuracy for a plane-parallel ionization chamber at different clinical relevant proton energies andmagnetic field strengths.

Material&Methods: The dose-response of the Advanced Markus chamber (TM34045, PTW, Freiburg, Germany) irradiated with homogeneous 10×10 cm2 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 magnetic field (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, 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. k ⃗B,M,Q was calculated for the measurements for all energies and MF strengths based on the equation: k ⃗B,M,Q = MQ/M ⃗ BQ , where MQ and M ⃗ BQ were the temperature and air-pressure corrected detector readings with and without the MF, respectively. MC-based correction factors were determined as k ⃗B,M,Q = Ddet/D ⃗ Bdet , where Ddet and D ⃗ Bdet were the doses deposited in the air cavity of the ionization chamber model without and
with the MF, respectively.

Results: The detector showed a reduced dose-response for all measured energies and MF strengths, resulting in experimentally determined k ⃗B,M,Q values larger than unity. k ⃗B,M,Q increased with proton energy and MF strength, except for 0.5 T and 252.7 MeV. Overall, k ⃗B,M,Q ranged between 1.0065 and 1.0205 for all energies and MF strengths examined and the strongest dependence on energy was observed at 1 T. The MC simulated k ⃗B,M,Q values for all MF strengths showed a good agreement with the experimentally determined correction factors with a maximum deviation of 0.6% and trends within their standard deviations.

Conclusion: For the first time, measurements and simulations were compared for an Advanced Markus chamber for proton dosimetry within MFs. For all MF strengths, there was a good agreement of k ⃗B,M,Q between experimentally determined and MC calculated values in this study. By benchmarking the MC code for the calculation of k ⃗B,M,Q it can be used to calculate k ⃗B,M,Q for various ionization chamber models, MF strengths and proton energies to generate the data needed for a dosimetry protocol for MRiPT.