Monte Carlo study of the neutron ambient dose equivalent at the heavy ion medical machine in Wuwei

The use of carbon-ion radiotherapy (CIRT) is gradually increasing. Owing to the generation of high-energy secondary neutrons during CIRT, its use presents new challenges in radiation protection. Thus, secondary neutron dose distributions must be explored and evaluated under clinical scenarios based on different treatment configurations. However, neutron dose and energy spectrum measurements are often difficult. This can be primarily attributed to the inherent limitations of most neutron detectors, such as their unsuitability for spectral measurements and inaccurate responses to neutrons with energies above 20 MeV. Numerical calculation methods based on probabilistic statistical theory are fast and convenient for neutron dose evaluation. In this study, external secondary neutron doses at the heavy ion medical machine in Wuwei, which is equipped with a passive beam delivery system, were calculated using the Monte Carlo method. The dependence of neutron doses on various treatment parameters (incident carbon-ion beam energy, spatial location, field size, and spread-out Bragg peak (SOBP) width) was investigated. Furthermore, the feasibility of applying an analytical model to predict the ambient dose equivalent was verified. For the combination involving an energy of 400 MeV/u and SOBP width of 6 cm, the ambient dose equivalent per therapeutic dose (H/D) at the isocenter was 79.87 mSv/Gy. The H/D value decreased rapidly with increasing spatial distance and slightly with increasing aperture size and SOBP width. The H/D values derived from the Monte Carlo simulations were in good agreement with the results reported in the literature. The analytical model could be used to quickly predict the H/D value along the incidence direction of the beam with an error of less than 20%. Thus, our study contributes to the understanding of the relationship between neutron radiation and treatment configuration parameters, which establishes a basis for predicting non-therapeutic radiation doses in CIRT.