Boron neutron capture therapy (BNCT) for malignant melanoma with special reference to absorbed doses to the normal skin and tumor.

Twenty-two patients with malignant melanoma were treated with boron neutron capture therapy (BNCT) using¹⁰B- p-boronophenylalanine (BPA). The estimation of absorbed dose and optimization of treatment dose based on the pharmacokinetics of BPA in melanoma patients is described. The doses of γ-rays were measured using small TLDs of Mg₂SiO₄ (Tb) and thermal neutron fluence was measured using gold foil and wire. The total absorbed dose to the tissue from BNCT was obtained by summing the primary and capture γ-ray doses and the high LET radiation doses from¹⁰B(n,α)⁷Li and¹⁴N(n,p)¹⁴C reactions. The key point of the dose optimization is that the skin surrounding the tumour is always irradiated to 18 Gy−Eq, which is the maximum tolerable dose to the skin, regardless of the¹⁰B-concentration in the tumor. The neutron fluence was optimized as follows. (1) The¹⁰B concentration in the blood was measured 15−40 min after the start of neutron irradiation. (2) The¹⁰B-concentration in the skin was estimated by multiplying the blood¹⁰B value by a factor of 1.3. (3) The neutron fluence was calculated. Absorbed doses to the skin ranged from 15.7 to 37.1 Gy−Eq. Among the patients, 16 out of 22 patients exhibited tolerable skin damage. Although six patients showed skin damage that exceeded the tolerance level, three of them could be cured within a few months after BNCT and the remaining three developed severe skin damage requiring skin grafts. The absorbed doses to the tumor ranged from 15.7 to 68.5 Gy−Eq and the percentage of complete response was 73% (16/22). When BNCT is used in the treatment of malignant melanoma, based on the pharmacokinetics of BPA and radiobiological considerations, promising clinical results have been obtained, although many problems and issues remain to be solved. [ABSTRACT FROM AUTHOR]