SU-E-T-73: Off-Axis Dose and Dose Equivalent Due to Secondary Neutrons from Uniform Scanning Proton Beams

Abstract

Purpose: The production of secondary neutrons is an undesirable byproduct of proton therapy and it is important to quantify the contribution from secondary neutrons to patient dose received outside the treatment volume. The purpose of the study is to investigate the off‐axis dose and dose equivalent from secondary neutrons experimentally using CR‐39 Plastic Nuclear Track Detectors (PNTD) at Procure Proton Therapy Center,Oklahoma City. Methods: CR‐39 PNTD is generally insensitive to therapeuticprotons, but has a sensitivity to the secondary neutrons similar to that of tissue. In this experiment, we placed several CR‐39 PNTDs laterallyinside a 60×20×35 cm3 phantom and in free air respectively at various depths and angles with respect to the primary beam axis. Three differentproton energies (78 MeV, 162 MeV and 226 MeV), 4 cm modulation width, 5cm diameter brass aperture, 38 cm snout to isocenter distance and 10 cm isocenter point in the phantom were used for the entire experiment andsimulation. Monte Carlo simulations were also performed based on the experimental setup using a simplified snout configuration and FLUKA. Results: The dose and dose equivalent is observed to be higher in air than in the phantom. Preliminary results show that the measured ratio of secondary neutron dose equivalent to primary proton dose (H/D) in CR‐39 PNTDranges from 1.04 ±0.13 mSv/Gy inside the phantom to 43.5 ± 3 mSv/Gy in free air for 78 MeV proton. The experiment and simulation results follow a similar decreasing trend with distance but the magnitude varies with a factor of about 2 to 10 depending on measurement location. Conclusions: We are currently investigating the contribution from external neutrons produced in the nozzle and from the internal neutrons produced inside the phantom to total patient dose outside the treatment volume as a function of location.