Maximum therapeutic depth in thermal neutron capture therapy.

The effective treatment depth in boron neutron capture therapy with thermal neutrons depends on the beam aperture, heavy water concentration, boron concentrations in the tumor, normal tissue and blood, tolerance dose to normal tissue and the capillary dose modification factor. In this study a 15 cm aperture is used and the tolerance dose to normal tissue is evaluated to be 15 Gy. Human pharmacokinetic data are evaluated for BSH and D, L-BPA, the two compounds currently used in thermal BNCT. Results show that the average measured tumor to blood boron ratios and standard deviations are 1.4 (0.4) for BSH and 4.3 (1.8) for subcutaneous melanoma. Experimental dose-depth phantom results for the Musashi Institute of Technology reactor are used with expected boron concentrations to calculate the maximum therapeutic depth in the brain for a thermal neutron beam. For BPA, the subcutaneous melanoma boron concentrations are assumed for intracranial metastases, and no allowance is made for possible enhanced uptake in the dopamine and noradrenaline tracts. Results for BSH are enhanced by inclusion of the capillary dose reduction factor. Calculations show that for expected boron tumor to blood ratios, the modified advantage depth is about 4 cm and the maximum therapeutic depth is about 1.5 cm for both BSH and BPA. Typical heavy water ratios of 15% increase the treatment depth by 0.5 cm, but this is offset by the use of smaller beam aperture in practice.