Beam modeling of a proton pencil beam scanning beam line integrated with a low-field open MR scanner

Objective: The integration of an MRI scanner into a proton beam line imposes challenges to the dose delivery, since the magnetic field (MF) of the scanner distorts the beams and hence the dose distribution. This study aims to develop a Monte Carlo (MC)-based beam model of the pencil beam scanning nozzle in the OncoRay facility to accurately model the dose delivery in the presence of an open MR scanner.
Materials and Methods: Measurements of proton beam spot size in air at varying distance from the nozzle were used to model beam optics using the Geant4-based MC code TOPAS. The beam energy and energy spread were obtained from the fit of depth-dose profiles measured in water.
A 3D map of the magnetic fringe field of the 0.22 T (vertical field) open MR scanner was generated with COMSOL and used in TOPAS. The simulated beam deflection was compared to measurements 210 cm downstream of the beam isocenter. Horizontal spot scanning positions from 0 to 200 mm and beam energies of 125 and 220 MeV were considered.
Results: The simulated spot sizes without MF agreed with experimental measurements within the experimental uncertainty. With MF, spot deflections of 183.6 (125 MeV) and 135.0 (220 MeV) mm were observed at the central spot position in the horizontal axis compared to their respective experimental values of 226.5 and 164.6 mm. For scan positions from 50 to 200 mm, relative differences between simulations and experimental results were within 6% (125 MeV) and 3.7% (220 MeV).
Conclusions: In the absence of MF, Initial validation of the beam modeling shows good reproducibility of spot sizes. Simulations with MF show large deviations for the beam deflection for central beam spots. The observed disagreement is likely related to deficiencies in the MF map, which has not been experimentally validated. Future work will include the validation of the MF map and a detailed evaluation of beam deflection and changes in spot sizes in the MF.