Technical note: Monte Carlo study of the mechanism of proton–boron fusion therapy

Purpose: Proton beam therapy has been found to have enhanced biological effectiveness in targets that contain the boron isotope ¹¹B, with the alpha particles resulting from the p + ¹¹B → 3α reaction being hypothesized as the mechanism; in this study, we aimed to elucidate the causes of the enhanced biological effectiveness of proton–boron fusion therapy by performing a detailed Monte Carlo study of the p + ¹¹B → 3α reaction in a phantom geometry. Methods: We utilized the Geant4 toolkit to create Monte Carlo particle physics simulations. These simulations consisted of a proton beam with a range 30 mm, creating a Spread-Out Bragg Peak with a modulation width of 10 mm, directed into a water phantom containing a region of boron material. Energy deposition, particle energy, and particle fluence were scored along the path of the beam and grouped by particle species. The scoring was performed using a series of cylindrical volumes with a radius of 2.5 mm and depth of 0.1 mm, constructed such that the depth was parallel to the proton beam. Root was then used to perform the data analysis. Results: Our simulations showed that the dose delivered by alpha particles produced by p + ¹¹B → 3α was several orders of magnitude lower than the dose delivered directly by protons, even when the boron uptake region was comprised entirely of natural boron or pure ¹¹B. Conclusions: Our findings do not support the theory that an alpha particle-based mechanism is responsible for the enhanced biological effectiveness of proton–boron fusion therapy. We conclude that any enhanced biological effect seen in experimental studies was not caused by fusion reactions between protons and ¹¹B nuclei. However, it is necessary to reproduce the past experiments that indicated significant dose enhancement.