Irradiation characteristics of BNCT using near-threshold 7Li(p, n)7Be direct neutrons:: application to intra-operative BNCT for malignant brain tumours

A calculation method for the dosage of neutrons by near-threshold Li-7(p, n)Be-7 and gamma rays by Li-7(p, p'gamma)Li-7 was validated through experiments with variable distance between the Li target and the phantom, focusing on large angular dependence. The production of neutrons and gamma rays in the Li target was calculated by Lee's method and their transport in the phantom was calculated using the MCNP-4B code. The dosage in intra-operative boron neutron capture therapy (BNCT) using near-threshold Li-7(p, n)Be-7 direct neutrons was evaluated using the validated calculation method. The effectiveness of the usage of the direct neutrons was confirmed from the existence of the region satisfying the requirements of the protocol utilized in intra-operative BNCT for brain tumours in Japan. The boron-dose enhancer (BDE) introduced in this paper to increase the contribution of the B-10(n, alpha)Li-7 dose in the living body was effective. The void utilized to increase the dose in deep regions was also effective with BDE. For the investigation of 1.900 MeV proton beams, for example, it was found that intra-operative BNCT using near-threshold Li-7(p, n)Be-7 direct neutrons is feasible.