The RBE of the leakage radiation from the Hiletron neutron therapy unit

The RBE of the leakage radiation from the Hiletron 14.7 MeV neutron therapy unit was measured using three sensitive biological systems in mice. These systems comprise type A spermatogonia and bone marrow stem cells, (affected insignificantly by dose rate) and pigment abnormalities in hair follicles, (affected markedly by dose rate). For mice irradiated at 10 cm depth in a water phantom, leakage radiation up to 40 cm from the beam axis was virtually as effective as the primary beam for the latter two biological systems, and for spermatogonia in mice when irradiated in air. The total dose rate was about 0.2 cGy (rad) per minute (3% of the primary beam), and the γ-ray component was about 70%. This equal effectiveness of the total dose for all three systems was considered fortuitous, and implied high RBE values for equal effect with the small neutron component at far distances. The evidence suggested either a positive interaction of neutron and γ-ray components in killing bone marrow stem cells when the neutron component was less than 40% of the total dose, or an increased efficiency of neutrons when delivered at very low dose rates. The components were additive in killing spermatogonia. (author)