Biological Effectiveness of High-energy Protons

Summary

Three biological test systems have been used to compare the effectiveness of 380 MeV protons and ⁶⁰Co γ-rays or 300 kVp x-rays in killing cells. Reduction in growth of the coleoptile and first leaf of barley seedlings after irradiation of dry seed gave a proton/γ-ray RBE of ≈ 1·3. Loss of viability of fern spores irradiated in the dormant state also indicated a greater effectiveness of protons than of γ-rays. Absence of melanocytes in hair follicles of mice irradiated at a previous telogen stage gave a proton/x-ray RBE of 1·4. The general conclusion is that 380 MeV protons are about 1·3 times as effective as x- or γ-rays in killing cells, and the implications for the permissible exposure of man to high-energy protons are discussed.