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.

Accurate characterization of weak neutron fields by using a Bayesian approach

A Bayesian analysis of data derived from neutron spectrometric measurements provides the advantage of determining rigorously integral physical quantities characterizing the neutron field and their respective related uncertainties. The first and essential step in a Bayesian approach is the parameterization of the investigated neutron spectrum. The aim of this paper is to investigate the sensitivity of the Bayesian results, mainly the neutron dose H^*(10) required for radiation protection purposes and its correlated uncertainty, to the selected neutron spectrum parameterization.