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

Abstract

PurposeThe purpose of this study was to directly calculate kQmsrB,fmsr correction factors for four cylindrical ICs for a 0.35 T MR-linac using the Monte Carlo (MC) method. MethodsA 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 kQmsrB,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. ResultsThe maximum beam quality dependence in the kQmsrB,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 kQmsrB,fmsr correction was < 2 % when the detector was aligned along the direction of the magnetic field at 0° and 180° 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° and 270° angles. ConclusionsA 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.