Beam quality correction factors for ionization chambers in a 0.35 T magnetic resonance (MR)-linac - A Monte Carlo study

Purpose: The purpose of this study was to directly calculate k(Qmsr)(B,fmsr) correction factors for four cylindrical ICs for a 0.35 T MR-linac using the Monte Carlo (MC) method. Methods: A previously-validated TOPAS/GEANT4 MC head model of the 0.35 T MR-linac was employed. The MR-compatible Exradin A12, A1SL, A26, and A28 cylindrical ICs were modeled considering the dead volume in the air cavity. The k(Qmsr)(B,fmsr)correction factor was determined for initial electron energies of 5-7 MeV. The correction factor was calculated for all four angular orientations in the lateral plane. The impact of the 0.35 T magnetic field on the IC response was also investigated. Results: The maximum beam quality dependence in the k(Qmsr)(B,fmsr )exhibited by the A12, A1SL, A26, and A28 ICs was 1.10 %, 2.17 %, 0.81 %, and 1.75 %, respectively, considering all angular orientations. The magnetic field dependence was < 1 % and the maximum k(Qmsr)(B,fmsr) correction was < 2 % when the detector was aligned along the direction of the magnetic field at 0 degrees and 180 degrees angles. The A12 IC over-responded up to 5.40 % for the orthogonal orientation. An asymmetry in the response of up to 8.30 % was noted for the A28 IC aligned at 90 degrees and 270 degrees angles. Conclusions: A parallel orientation for the IC, with respect to the magnetic field, is recommended for reference dosimetry in MRgRT. Both over and under-response in the IC signal was noted for the orthogonal orientations, which is highly dependent on the cavity diameter, cavity length, and the dead volume.