Biological support for the feasibility of Californium-Boron neutron capture therapy

Cultured hamster V79-S171 cells exhibited a decreased survival when 6.41 mM boric acid (93.6% boron-10) was present in the culture medium during exposure to Pt-Ir encapsulated Cf-252 brachytherapy sources totaling 351 mcg. The boron-10 concentration was 60 mcg/ml. Cf-252 emits neutrons with a mean energy of 2.3 MeV. Approximately 2/3 of the absorbed dose is from neutrons (RBE=5-6) with the remainder from 0-10 MeV gamma rays. The cells were irradiated in suspension over a 2h period in a phantom which centrally positioned the cells 1.6 cm from a cylindrical array of 24 Cf sources. There the thermal neutron flux was 5 x 10/sup 6/ n/cm/sup 2/sec, the neutron dose rate was 148 cGy/h, and the gamma dose rate was 68 cGy/h. Survival curves down to 0.01 survival were fitted to the linear-quadratic equation. The curve without boron fit an equation with an α/β ratio of 84. The curve with boron had a ratio of 121. This indicates an increased proportion of ''single-hit'' cell kills and is attributable to the /sup 10/B(n,α) Li reaction in which two energetic charged particles are produced, each with a range of about one cell diameter. It is concluded that boron-10 can be used to take advantage of the thermalized neutrons provided during Cf-252 brachytherapy