Monte Carlo simulation for proton dosimetry in magnetic fields: Fano test and magnetic field correction factorskBfor Farmer-type ionization chambers.

Objective. In this contribution we present a special Fano test for charged particles in presence of magnetic fields in the MC code TOol for PArticle Simulation (TOPAS), as well as the determination of magnetic field correction factorskBfor Farmer-type ionization chambers using proton beams.Approach. Customized C++ extensions for TOPAS were implemented to model the special Fano tests in presence of magnetic fields for electrons and protons. The Geant4-specific transport parameters,DRoverRandfinalRange,were investigated to optimize passing rate and computation time. ThekBwas determined for the Farmer-type PTW 30013 ionization chamber, and 5 custom built ionization chambers with same geometry but varying inner radius, testing magnetic flux density ranging from 0 to 1.0 T and two proton beam energies of 157.43 and 221.05 MeV.Main results. Using the investigated parameters, TOPAS passed the Fano test within 0.39 ± 0.15% and 0.82 ± 0.42%, respectively for electrons and protons. The chamber response (kB,M,Q) gives a maximum at different magnetic flux densities depending of the chamber size, 1.0043 at 1.0 T for the smallest chamber and 1.0051 at 0.2 T for the largest chamber. The local dose differencecBremained ≤ 0.1% for both tested energies. The magnetic field correction factorkB, for the chamber PTW 30013, varied from 0.9946 to 1.0036 for both tested energies.Significance. The developed extension for the special Fano test in TOPAS MC code with the adjusted transport parameters, can accurately transport electron and proton particles in magnetic field. This makes TOPAS a valuable tool for the determination ofkB. The ionization chambers we tested showed thatkBremains small (≤0.72%). To the best of our knowledge, this is the first calculations ofkBfor proton beams. This work represents a significant step forward in the development of MRgPT and protocols for proton dosimetry in presence of magnetic field.

Proton beam electron return effect: Monte Carlo simulations and experimental verification.

Proton therapy (PT) is expected to benefit from integration with magnetic resonance (MR) imaging. However, the magnetic field distorts the dose distribution and enhances the dose at tissue-air interfaces by the electron return effect (ERE). The objectives were (a) to provide experimental evidence for the ERE in proton beams and (b) to systematically characterise the dependence of the dose enhancement ratio (DER) on magnetic field strength, orientation, proton energy and voxel size by computer simulations. EBT3 films were irradiated with 200 MeV protons with and without a 0.92 T transverse field of a permanent magnet to determine the DER at effective measurement depths of 0.156 and 0.467 mm from an air interface. High-resolution Monte Carlo simulations were performed to reproduce the irradiation experiments and to calculate the DER for proton energies between 50-200 MeV and magnetic field strengths between 0.35-3 T as function of distance from the air interface. Voxel sizes of 0.05, 0.5 and 1 mm were analysed. DERs of (2.2 ± 0.4)% and (0.5 ± 0.6)% were measured at 0.156 and 0.467 mm from the air interface, respectively. Measurements and simulations agreed within 0.15%. For a 200 MeV proton beam, the maximum DER in 0.05 mm voxels increased with magnetic field strength from 2.6% to 8.2% between 0.35 and 1.5 T, respectively. For a 1.0 T magnetic field, maximum DER increased from 3.2% to 7.6% between 50 and 200 MeV, respectively. Voxel sizes of 0.5 and 1 mm resulted in maximum DER values of 2.6% and 1.4%, respectively. The ERE for proton beams in transverse magnetic fields is measurable. The local dose enhancement is significant, well predictable, decreases rapidly with distance from the air interface, and is negligible beyond 1 mm depth. Its impact on air-filled ionisation chambers and porous tissues (e.g. lung) needs to be considered.