Monte Carlo study toward the development of a radiation field to simulate secondary neutrons produced in carbon-ion radiotherapy

This study investigates the feasibility of developing a neutron field to simulate secondary neutrons produced in carbon ion radiotherapy (CIRT) with a passive beam using a monoenergetic carbon beam, a neutron-producing target, and neutron-shaping filters via Monte Carlo calculations. The general-purpose particle and heavy-ion transport code system (PHITS) was employed throughout this investigation. After characterizing the targeted neutron field, a neutron-producing target and neutron-shaping filters were selected. A dataset of the neutron energy spectra was compiled using the selected target and filters. Then, we determined the best combination of target and filters and the best angle to the beam axis to reproduce the neutron energy spectrum in the targeted neutron field. The root mean square percentage error and the mean absolute percentage error were used as figures of merit to quantify the agreement with neutron energy spectra in the CIRT treatment room. We found that a monoenergetic carbon beam can sufficiently reproduce the neutron energy spectra in a CIRT treatment room with a passive beam using an iron neutron-producing target doubling as a sharping filter and/or a polyethylene filter and selecting the angle to the beam axis. Therefore, the development of a neutron field to simulate secondary neutrons produced in CIRT with a passive beam is feasible in a non-clinical facility. This simulated neutron field can be useful for not only biological experiments but also physics experiments such as the development and calibration of neutron dosimeters