Proton dosimetry with a plane-parallel chamber: determination of magnetic field correction factors

Purpose: In magnetic resonance imaging-integrated proton therapy (MRiPT), the magnetic field-dependent change in the dosage of ionisation chambers is considered by the correction factor k_(B ⃑,M,Q), which can be determined experimentally or computed via Monte Carlo (MC) simulations. In this work, k_(B ⃑,M,Q) for a plane-parallel ionisation chamber was determined by measurements and MC simulations were used to reproduce these results with high accuracy.
Material/Methods: The dose-response of the advanced Markus chamber (TM34045, PTW, Freiburg, Germany) irradiated with homogeneous 10x10 cm² mono-energetic fields, using 103.3, 153.1, 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 and 1 T. The detector was positioned at a 2 cm water-equivalent depth 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 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)= M_Q/(M_Q^B ⃑ ), where M_Q and M_Q^B ⃑ were the temperature and air pressure corrected detector readings without and with MF, respectively. MC-based correction factors were determined as k_(B ⃑,M,Q)= D_det/(D_det^B ⃑ ), where D_det and D_det^B ⃑ were the doses deposited in the air cavity of the ionisation chamber model without and with 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 1 (Figure 1). k_(B ⃑,M,Q) increased with proton energy and MF strength, except for 0.32 T and 252.7 MeV. Overall, k_(B ⃑,M,Q) ranged between 1.006 ± 0.004 and 1.021 ± 0.010 for all energies and MF strengths examined and the strongest dependence on energy was found at 1 T. The MC simulated k_(B ⃑,M,Q) values for 0.32 and 1 T showed a good agreement with the experimentally determined correction factors and trends within their standard deviations. The maximum difference between experimentally determined and MC simulated k_(B ⃑,M,Q) values was 0.63%.
Conclusion: For the first time, measurements and simulations were compared for an advanced Markus chamber for the dosimetry of protons within MFs. For both 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 calculation of〖 k〗_(B ⃑,M,Q) it can be used to calculate 〖 k〗_(B ⃑,M,Q) for various ionisation chamber models, MF strengths and proton energies in order to generate data needed for a dosimetry protocol for MRiPT.