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

Abstract

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 6MeV 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 50cm 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.