Novel approach to long-term monitoring of accelerator-based boron neutron capture therapy

Background

Boron neutron capture therapy (BNCT) was conducted in a hospital using an accelerator-based neutron source. The neutron beam intensity at the patient position was evaluated offline using a gold-based neutron activation method. During BNCT neutron beam irradiation on patients, the neutron intensity was controlled in real time by measuring the proton beam current irradiated on a lithium neutron target. The neutron intensity at NCCH decreased owing to the degradation of the lithium neutron target during neutron irradiation. The reduction in the neutron beam intensity could not be monitored via proton beam measurement due to the dependence of neutron production on the neutron target condition.

Purpose

The duration of BNCT neutron irradiation should be controlled by monitoring the neutron beam intensity with a real-time neutron detector for reliable neutron irradiation on patients. The measurement accuracy of the online neutron beam monitor was experimentally obtained by comparing the gold radioactivity measured at the patient position. Radiation-induced damage was observed from the variation in the pulse height distributions of multichannel analyzer during long-term neutron exposure.

Methods

Neutron beams were measured during neutron beam irradiation at the BNCT facility of Edogawa hospital in Japan using a neutron beam monitor comprising a 0.07-$\mu m$ LiF layer and 40-$\mu m$ back-illuminated thin Si pin diode. The proton beam was continuously irradiated until a cumulative total beam charge of approximately 3 kC was achieved. The online neutron beam monitor counting rates on the neutron target unit and gold saturation activities at the patient position were simultaneously measured through the entire duration of proton beam irradiation.

Results

The experimental results demonstrated the long-term operation of the online neutron beam monitor positioned on the neutron target unit during the entire duration of the neutron target lifespan without significant performance deterioration. A good synchronization was observed in a correlation distribution measured using the online neutron beam monitor and the gold neutron activation method. A conversion coefficient of 1.199 ${\mathrm{Bq}}^{-1}$ g with a standard deviation of 2.5% was evaluated. The neutron beam intensity irradiating on patients within an acceptable level of $\pm$5% as per the International Commission on Radiation Units and Measurements was evaluated from the online neutron counting rate at the 95% confidence level. The channel numbers of the triton peak and alpha particle edge decreased linearly owing to displacement damage and total ionizing dose effects induced mainly by thermal neutrons and photons.

Conclusions

Neutron doses can be accurately administered by complementing proton beam current measurements with the online neutron beam monitor. The online neutron beam monitoring technique allows monitoring fluctuations in the neutron beam intensity and tracking the degradation of the lithium target through the neutron target lifespan. Using a calibrated online neutron beam monitor, a prescribed dose can be administered in a manner similar to that in x-ray therapy, and the duration of neutron beam irradiation on the patient can be controlled in real time.