Dose distribution of secondary radiation in a water phantom for a proton pencil beam - EURADOS WG9 intercomparison exercise

Systematic 3-D mapping of out-of-field doses induced by a therapeutic proton pencil scanning beam (PBS) in a 300 × 300 × 600 mm3 water phantom was performed using a set of thermoluminescence detectors (TLD): MTS-7 (7LiF:Mg, Ti), MTS-6 (6LiF:Mg, Ti), MTS-N (natLiF:Mg, Ti) and TLD-700 (7LiF:Mg, Ti), radiophotoluminescent detectors (RPL) GD-352M and GD-302M and polyallyldiglycol carbonate (PADC)-based (C12H18O7) track-etched detectors. Neutron and gamma-ray doses, as well as LET distributions were experimentally determined at 200 points within the phantom. In parallel, GEANT 4 Monte Carlo code was applied to calculate neutron and gamma radiation spectra at the position of each detector. For the cubic proton target volume 100 × 100 × 100 mm3 (Spread Out Bragg Peak with modulation of 100 mm) the scattered photon doses along the main axis of the phantom perpendicularly to the primary beam were approximately 0.5 mGy/Gy at a distance of 100 mm and 0.02 mGy/Gy at 300 mm from the centre of the target. For neutrons, the corresponding values of dose equivalent were found to be ~0.7 mSv/Gy and ~0.06 mSv/Gy respectively. The measured neutron doses were comparable with the out-of-field neutron doses from the similar experiment with 20 MV X-rays, whereas photon doses for the scanning proton beam were up to 3 orders of magnitude lower. 1