Session Item

Magnetic resonance guided radiation therapy (MRgRT) holds high promises for a new paradigm shift in cancer therapy, facilitating online plan adaptation. Acknowledging the opportunities to minimize geometrical uncertainties in proton beam radiation therapy, the development of MR guided proton radiotherapy (MRgPRT) has begun. Up to date, studies related to dose measurements in proton beams in the presence of magnetic fields are still very scarce. Fuchs et al. 2020 firstly presented experimental results on the magnetic field dependent detector response in proton beams. The main aims of the present work are to investigate the magnetic field dependent response of ionization chambers in proton beams; and to understand the underlying perturbation mechanisms using detailed Monte Carlo simulations.

One plane-parallel (PTW Roos 34001) and one cylindrical (PTW Farmer 30013) chamber was studied. Monte Carlo simulations were performed using GATE (version 9.2)/Geant4 (version 11.0.2). A 10 cm x 10 cm pencil beam scanning field with an energy between 97.4 and 252 MeV was used. The magnetic field perpendicular to the beam’s axis was varied between ±0.25 and ±1 T. The Roos chamber was orientated with its axis antiparallel to the beam’s axis (-x). The axis of the Farmer chamber was orientated perpendicular (+z) to both the beam’s axis and magnetic field (±y). The change of chamber response in magnetic field, characterized by kB,M = M/MB, was simulated for 2 cm depth. To facilitate the better understanding of the magnetic field influence, the contributions from protons and secondary electrons were differentiated. The role of the non-sensitive air volume adjacent to the chamber’s stem of the Farmer chamber was also elucidated.

For the Roos chamber, kB,M increases initially with magnetic field strength up to around B = 0.5 T (e.g. kB,M = 1.0066 ± 0.0007 at 252 MeV), after which kB,M decreases. With decreasing energy, the magnitude of this observed trend decreases.  For the Farmer chamber, kB,M shows the same behavior with a maximum of kB,M = 1.0038 ± 0.0006 at 252 MeV and +0.25 T. With reversed magnetic field (+y) kB,M decreases to a minimum of kB,M = 0.9953 ± 0.0004 at 97.4 MeV and -1 T. The results between the simulations with and without the correct modelling of the non-sensitive air volume of the Farmer chamber differs by 0.14 % at maximum. The detailed simulations demonstrate that the secondary electrons contribute primarily to the magnetic field dependent chamber response.  

Results from this study are consistent with the experimental results presented by Fuchs et al. Overall, the change of chamber response is limited to below 0.66 % for all studied configurations. Detailed simulations have provided useful insights on the chamber behavior in magnetic field. Further studies are necessary to extend the investigations to other measurement conditions, such as depths and field sizes, before the safe implementation of MRgPRT can be guaranteed.