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

Abstract

Proton beam therapy has been found to have enhanced biological effectiveness in targets that contain the boron isotope 11 B, with the alpha particles resulting from the p + 11 B → 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 + 11 B → 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 + 11 B → 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 11 B. Conclusions: Our findings do not support the theory that an alpha particlebased 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 11 B nuclei. However, it is necessary to reproduce the past experiments that indicated significant dose enhancement.

Estimating Particle Size for Therapeutic Application of Boron in Proton Therapy using the Finite Element Method

“…Mathematica [14] provided the environment used to program the 11 B(p, α) 8 Be and Auger simulations for varying boron particle sizes. These codes can be found on GitHub at [15].…”

“…The energy loss of the exiting alpha particles calculation was based on traversed distance from the particle's origin to the surface of the boron particle. The 11 B(p, α) 8 Be reactions were simulated at cell points, defined to be at the location of the proton beam's entrance and exit of each cell region on the x-axis as seen in Figure 8.…”

“…While these characteristics show promise in enhancing radiotherapy, it must be considered that the boron, in nanoparticle form, may absorb a significant amount of electron energy, effectively "self-shielding" the site from treatment. In addition to the Auger effect, boron particles may also undergo the 11 B(p, α) 8 Be reaction [8]. Previous studies show that proton boron fusion therapy(PBFT) enhances dose in targeted tissue [9,10].…”

“…These characteristics make this reaction attractive for enhancing proton therapy. [12] Figure 1: Energy spectrum of α-particle yield from the 11 B(p, α) 8 Be reaction using a proton beam energy of 660 keV with a detector at 150 • to the incident proton direction [12].…”

“…Figure6: Total cross section/stopping power ratio for the 11 B(p, α)8 Be reaction used as the integrand in Equation 5[16,17]. This was applied to calculations of the reaction fraction for combinations of incident and exiting proton energies.…”