Dose distribution of secondary radiation in a water phantom for a spatially fractionated proton beam

Objective 3D mapping of scattered radiation doses in water after grid proton irradiation using a dedicated brass collimator was performed within the EURADOS experiment at the scanning gantry of the Cyclotron Centre Bronowice at IFJ PAN. The goal of this paper was to determine to what extent the dedicated grid collimators would increase secondary doses administered to patients compared to Pencil Beam Scanning (PBS) techniques. Methods A broad set of passive point-like detectors including thermoluminescence (TLD), radio-photoluminescence (RPL) and track detectors (PADC) positioned in a dedicated water phantom was applied to determine gamma and neutron doses. The dose assessment was supported by Monte Carlo (MC) radiation transport calculations. Results The results show that the highest absorbed dose values were measured in the proximal area of the proton beam field and in the Spread out Bragg Peak (SOBP). Applying the same treatment plan the beam formation with PBS + grid collimator caused a 5-fold increase in the absorbed out-of-field dose compared to irradiation with the PBS and in the case of fast neutrons, the increase was around a factor of 10. These doses are comparable to unwanted exposure to scattered radiation arising from conventional photon radiotherapy. Conclusions The spatial distribution of the measured dosimetric quantities supports the thesis that in spatially fractionated proton radiotherapy, in which the beam is formed using a grid collimator, the main source of gamma radiation and neutrons is the collimator. Despite this, grid collimators remain a viable option due to their adaptability to various proton systems.