Synergy effect of alpha particles by using natural boron in proton therapy: Computational verification

The use of boron (11B) is recently being investigated to be applied in proton therapy as a proton boron fusion dose enhancement agent. Alpha particles are emitted from the p + 11B → 3α fusion (PBF) reaction analogous to the 10B(n,7Li)α capture (BNC) reaction. If a natural boron content (80% 11B and 20% 10B) is used in proton therapy, the contaminated neutrons, induced by the proton beam traversing a water medium, will react with 10B and the primary protons will react with 11B. Each reaction will emit alpha particles according to its reaction cross section. The dose due to these alpha particles, together with the primary proton beam, can induce tumor cell kill. The purpose of this study is to computationally investigate these synergy effects using the Monte Carlo simulation with the target region located in the water medium. A 79.9 MeV proton beam (proton density: 1.9 × 108 cm−2) with 4 monitor unit was used to irradiate the target consisting of water, 10B, 11B, and natural B. The variation of the dose, the location of the reaction, and the energy distribution of the alpha particles were calculated according to the target material. As a result, we confirmed contributions of both BNC and PBF reactions to emitting alpha particles from proton beam irradiation with natural boron. This synergy effect induced an additional 7.29% enhanced dose by 331 984 alpha particles. This enhanced dose can sufficiently reduce the number of treatment fractions in proton therapy.