Development of an online neutron beam monitoring method for accelerator-based BNCT using an 8-MeV proton-beryllium...

Boron neutron capture therapy (BNCT) has emerged as a promising cell-selective clinical treatment for recurrent glioblastoma as well as head and neck cancers. In recent years, BNCT has been implemented in several hospitals using accelerator-based neutron sources. However, direct online measurement of the incident neutron beam intensity during clinical BNCT remains a major challenge. Neutron dosimetry is typically inferred from the accumulated electric charge of the incident proton beam delivered to the beryllium neutron-production target.

To improve the reliability of neutron beam control, it is essential to implement an online neutron-monitoring system that complements proton beam current measurements. Such a system enables precise real-time regulation of the neutron dose delivered to patients. In this study, we developed a new online neutron-beam monitoring technique capable of measuring the neutron beam intensity in real time.

Reliable monitoring of neutron intensity at the patient position was achieved by detecting fast neutrons. The fast-neutron detector consisted of a silicon diode coupled with a high-density polyethylene neutron converter. The spatial distribution of neutrons was evaluated using Monte Carlo simulations to determine the optimal placement of the neutron beam monitor. The proposed method was validated through gold-activation analysis, cell irradiation experiments, and accelerator tuning.

Online neutron-beam monitoring was successfully achieved by detecting fast neutrons at the end of the vacuum beam duct, where neutrons propagate directly without traversing radiation shielding. The neutron counting rate increased immediately at the onset of proton-beam irradiation and decreased upon its termination, exhibiting periodic fluctuations throughout the irradiation period. These fluctuations did not significantly affect the integrated neutron counts, as they were averaged over a typical BNCT irradiation time of approximately 30 min. The fast-neutron detector produced a mean counting rate of 124.6 counts per mC of incident proton-beam charge, with a relative standard deviation of 1.2%, and yielded a neutron-calibration factor of $4.9\times {10}^6{\mathrm{cm}}^{-2}$ per monitor unit. The neutron counting rates remained consistent even when different irradiation phantoms were placed at the patient position.

The online neutron-beam monitoring technique using a fast-neutron detector demonstrated high reproducibility, as the neutron counting rates remained stable regardless of the irradiation phantom placed downstream.