Boron self-shielding effects on dose delivery of neutron capture therapy using epithermal beam and boronophenylalanine

Previous dosimetry studies for boron neutron capture therapy have often neglected the thermal neutron self-shielding effects caused by the {}^{10}\mathrm{B} accumulation in the brain and the tumor. The neglect of thermal neutron flux depression, therefore, results in an overestimation of the actual dose delivery. The relevant errors are expected to be more pronounced when boronophenylalanine is used in conjunction with an epithermal neutron beam. In this paper, the boron self-shielding effects are calculated in terms of the thermal neutron flux depression across the brain and the dose delivered to the tumors. The degree of boron self-shielding is indicated by the difference between the thermal neutron fluxes calculated with and without considering a {}^{10}\mathrm{B} concentration as part of the head phantom composition. The boron self-shielding effect is found to increase with increasing {}^{10}\mathrm{B} concentrations and penetration depths from the skin. The calculated differences for {}^{10}\mathrm{B} concentrations of 7.5–30 ppm are 2.3%–8.3% at 2.3 cm depth (depth of the maximum brain dose) and 4.6%–17% at 7.3 cm depth (the center of the brain). The additional self-shielding effects by the {}^{10}\mathrm{B} concentration in a bulky tumor are investigated for a 3-cm-diam spherical tumor located either near the surface (3.3 cm depth) or at the center of the brain (7.3 cm depth) along the beam centerline. For 45 ppm of {}^{10}\mathrm{B} in the tumor and 15 ppm of {}^{10}\mathrm{B} in the brain, the dose delivered to the tumors is approximately 10% lower at 3.3 cm depth and 20% lower at the center of the brain, compared to the dose neglecting the boron self-shielding in transport calculations.