Neutron dose from a 6-MV X-ray beam in radiotherapy

Although a 6-MV X-ray beam is employed clinically as a non-neutron-producing beam, no studies have reported how few neutrons are produced from a 6-MV beam. This study aimed to theoretically deduce the neutron dose from a 6-MV beam using Monte Carlo simulations for the notification of safety and risk in radiotherapy. Nuclei from a nuclear database with neutron separation energies below 6 MeV were surveyed, suggesting that the certain content of ²H in the human body may result in some contribution. Thus, Monte Carlo calculation considering ²H in a phantom was performed. The calculation suggested that the distribution of the neutron dose from a 6-MV beam consisted of two components: one had neutrons from ²H concentrated within an irradiation field, and the other had those due to other elements such as ¹⁸³W spreading from a gantry head to a treatment room. Although uncertainty owing to the normalization factor of the Monte Carlo calculations was a factor of three, the neutron doses at distances of 0 and 50 cm from an irradiation field were calculated as 27 and 1.5 nSv/MU, respectively, under intensity-modulated radiotherapy (IMRT) or volumetric modulated arc therapy (VMAT). The calculations suggest that neutrons produced by a 6-MV beam are approximately 70 and 20 times safer than those by a 10-MV beam in the case of IMRT/VMAT and total body irradiation, respectively. Thus, this study theoretically reported the approximate number of neutrons delivered by a 6-MV beam for the first time.