TPD-based evaluation of near threshold mono-energetic proton energies for the 7Li(p,n)7Be production of neutrons for BNCT

Abstract

An evaluation of mono-energetic proton energies ranging from 1.885 MeV to 1.920 MeV was carried out to determine the viability of these near threshold energies in producing neutrons for BNCT via the 7Li(p,n)7Be reaction. Neutron fields generated at these proton energies were assessed using the treatable protocol depth (TPD) and the maximum TPD (TPDmax) as evaluation indices. The heavy charged particle (HCP) dose rate to tumour was likewise applied as a figure of merit in order to account for irradiation time and required proton current. Incident proton energies closer to the reaction threshold generated deeper TPDs compared to higher energy protons when no boron dose enhancers (BDE) were placed in the irradiation field. Introducing a BDE resulted in improved TPDs for high proton energies but their achievable TPDmax were comparatively lower than that obtained for lower proton energies. In terms of the HCP dose rate to tumour, higher proton energies generated neutron fields that yielded higher dose rates both at TPDmax and at fixed depths of comparison. This infers that higher currents are required to deliver the prescribed treatment dose to tumours for proton beams with energies closer to the 7Li(p,n)7Be reaction threshold and more achievable proton currents of around 10 mA or less for proton energies from 1.900 MeV and above. The dependence on incident proton energy of the TPD, TPDmax and the HCP dose rate to tumour with respect to the 10B concentration in tumour and healthy tissues were also clarified in this study. Increasing the 10B concentration in tumour while maintaining a constant T/N ratio resulted in deeper TPDmax where a greater change in TPDmax was obtained for proton energies closer to the 7Li(p,n)7Be reaction threshold. The HCP dose rates to tumour for all proton energies also went up, with the higher proton energies benefiting more from the increased 10B concentration.