Macroscopic and microscopic investigation of maximum effectiveness of proton-boron capture therapy using Monte Carlo simulation

Abstract

Proton-boron capture therapy (PBCT) has been proposed as a new method to enhance effectiveness of proton therapy. There is, however, no consensus on its potential effectiveness. Most theoretical studies have been done at a macroscopic level. At cellular dimensions, however, the dose ratio with PBCT can be strongly favourable due to proton-boron reactions. The goal of this study was to investigate the theoretical maximum dose enhancement with PBCT, and its radiation physics mechanisms, through both macroscopic and microscopic simulations. The Monte Carlo simulations involved two stages due to model limitations. In the macroscopic calculations, variations in Bragg-peak height and depth caused by a 10 mm thick boron region were examined for different proton energies (62, 80 and 100 MeV) using MCNPX. Percentage depth doses and energy spectra of protons, alpha particles, neutrons and photons were also computed. The second stage used Geant4 for dose calculation in cellular dimensions. Macroscopically, the energy deposition peak shifted towards the surface by about 3.1 mm and the Bragg peak dose increased by up to 53.1%, 53.4%, 53.9% for proton energies 62, 80 and 100 MeV, respectively, compared to the proton-only (no boron) case. Microscopically, the calculated total equivalent doses for these energies showed 5%, 23% and 50% enhancements, respectively. The results suggest increased effectiveness with PBCT due to production of short-range alpha-particles, especially at higher incident proton energies. Also, by adding boron, the mean proton energy is reduced thereby increasing the probability of local dose deposition. The theoretical maximum possible improvement in dose equivalent in the target region at the cellular scale was shown to slightly increase with beam energy, reaching about 54% at the highest studied energy. This combined approach has provided further insight into the potential of PBCT. Further studies, particularly at higher proton energies, are indicated.