Simulation study of secondary neutron reflection distribution in proton therapy room

Abstract

BackgroundThe protection of secondary neutrons in the complex secondary radiation of the 100~250 MeV proton therapy room has always been the focus of shielding design. Currently, iron plates embedded in concrete are often used to reduce neutron transmission, but this will cause significant neutron reflection (toward the patient). And neutrons have a high radiation weighting factor which may cause harmful effects on patients.PurposeThe study aims to investigate the reflection distribution of secondary neutrons in the proton therapy room and its impact on patients.MethodsFirst of all, a simple model of the proton therapy room was established, and Monte Carlo program FLUKA was used to calculate the effects of the secondary neutron reflection from different directions and at different embedding positions of the iron shield in the 245 MeV proton therapy room. Then, under the three irradiation modes of AP (Antero-posterior), PA (Posterior-anterior) and LAT (Lateral), the effective dose of the patient was calculated according to the energy of the reflected neutrons reaching the position of the patient whilst the neutron external radiation dose conversion coefficient given in report of ICPR (International Commission on Radiological Protection) Publication 116 was used.ResultsThe simulation results show that the main shielding wall contributes more than 95% to the reflection distribution of the secondary neutrons in the treatment room. The radiation dose level outside the treatment room is the lowest when the embedded depth of the iron plate in the wall is 0 (the iron plate is exposed in the room), but highest effective dose rates caused by reflected neutrons are 1.99 μSv‧s-1, 1.37 μSv‧s-1 and 1.00 μSv‧s-1, respectively, under the three irradiation modes of AP, PA and LAT. With the increase of embedding depth of iron plate, the effective dose rate of reflected neutron decreased gradually, but the decreasing trend becomes slower and slower.ConclusionsThe structure of the main shielding wall of the treatment room and the embedded position of the iron plate have a significant impact on the reflected secondary neutron energy spectrum and the patient's radiation health. This feature needs to be considered in the shielding design of the proton therapy room.

Keywords

• proton accelerator
• neutron protection
• monte carlo