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- 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 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.