What is the best proton energy for accelerator‐based BNCT using the reaction?

Abstract

With a growing interest in the use of accelerator-based epithermal neutron sources for BNCT programs, in particular those based upon the 7Li(p,n)7Be reaction, there is a need to address the question of "what is the best proton energy to use?" This paper considers this question by using radiation transport calculations to investigate a range of proton energies from 2.15 to 3.5 MeV and a range of moderator sizes. This study has moved away completely from the use of empty therapy beam parameters and instead defines the beam quality and optimizes the moderator design using widely accepted in-phantom treatment planning figures of merit. It is concluded that up to a proton energy of about 2.8 MeV there is no observed variation in the achievable therapy beam quality, but a price is paid in terms of treatment time for not choosing the upper limit of this range. For higher proton energies, the beam quality falls, but with no improvement in treatment time for optimum configurations.

A method for fast evaluation of neutron spectra for BNCT based on in‐phantom figure‐of‐merit calculation

“…Further, the RBE values are generally obtained for reactor spectra and with some exceptions there is a lack of measurements for accelerator-based sources as pointed out in Ref. 12. If the neutron spectrum is stable throughout the brain, an average RBE might be acceptable, even if RBE is a function of energy.…”

“…Thus a huge amount of data should be generated. Consequently to take angular distribution into account and other effects not considered in this model, such as neutronic coupling between phantom and moderator/filter assembly 12 and photons coming from the source and the filter, a single final run must be done. Of course, in this final run the phantom should be included within the geometry and dose distribution should be directly obtained.…”

A 50 kW Liquid-Lithium Target for BNCT and Material-Science Applications

“…A high thermal neutron cross section and minimal residual activity are also important criteria in the selection of the target nucleus. The availability of 10 B, a stable isotope of boron, following the rich medical literature developed on the toxicology of boron and a large cross section of 3840 barns of the 10 B(n,) 7 Li reaction for thermal neutrons have made this nuclide appropriate in the modality called Boron Neutron Capture Therapy (BNCT). On absorption of a thermal neutron by 10 B, the excited 11 B nucleus promptly (~ 10-12 s) decays to two charged particles ( and 7 Li) accompanied with high probability by a 478 keV -ray.…”

“…The availability of 10 B, a stable isotope of boron, following the rich medical literature developed on the toxicology of boron and a large cross section of 3840 barns of the 10 B(n,) 7 Li reaction for thermal neutrons have made this nuclide appropriate in the modality called Boron Neutron Capture Therapy (BNCT). On absorption of a thermal neutron by 10 B, the excited 11 B nucleus promptly (~ 10-12 s) decays to two charged particles ( and 7 Li) accompanied with high probability by a 478 keV -ray. The emitted charged particles  and 7 Li sharing a total energy of 2.31 MeV (1.47 MeV and 0.84 MeV, respectively) are highly-ionizing particles with short range in organic matter (~ 9 m and ~5 m, respectively) that are similar to the size of a single cell.…”

“…Worldwide efforts to design an accelerator-based neutron converter have focused on the use of lithium through the reaction 7 Li(p,n) 7 Be [7][8][9][10][11][12][13] at proton energies of 1.9-2.8 MeV, which produces average neutrons energies in the range 25 -500 keV. However, a reliable conventional lithium target working under beam power levels (~ tens kW), as considered for therapy purpose in the energy range above, proves very difficult to build because of the mechanical, chemical and thermal properties of lithium (principally its low melting point at 181°C).…”