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 neg lected the thermal neutron self-shielding effects caused by the B-10 accumu lation in the brain and the tumor. The neglect of thermal neutron flux depr ession, therefore, results in an overestimation of the actual dose delivery . The relevant errors are expected to be mores pronounced when boronophenyl alanine is used in conjunction with an epithermal neutron beam. In this pap er, the boron self-shielding effects are calculated in terms of the thermal neutron flux depression across the brain and the dose delivered to the tum ors. The degree of boron self-shielding is indicated by the difference betw een the thermal neutron fluxes calculated with and without considering a B- 10 concentration as part of the head phantom composition. The baron self-sh ielding effect is found to increase with increasing B-10 concentrations and penetration depths from the skin. The calculated differences for B-10 conc entrations of 7.5-30 ppm are 2.3%-8.3% at 2.3 cm depth (depth of the maximu m brain dose) and 4.6%-17% at 7.3 cm depth (the center of the brain). The a dditional self-shielding effects by the B-10 concentration in a bulky tumor are investigated for a 3-cm-diam spherical tumor located either near the s urface (3.3 cm depth) or at the center of the brain (7.3 cm depth) along th e beam centerline. For 45 ppm of B-10 in the tumor and 15 ppm of B-10 in th e 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 ne glecting the baron self-shielding in transport calculations. (C) 1999 Ameri can Association of Physicists in Medicine. [S0094-2405(99)02311-1].