The Relative Biological Effectiveness of Various Ionizing Radiations in Mammalian Systems

Abstract

The results of 39 experiments on the relative biological effectiveness (RBE) of 9 different ionizing radiations were reported. From 1 to 10 quantitative biological responses of rats and/or mice to radiation were utilized with each type of radiation. Each of the quantitative methods was described in detail. All values for RBE were expressed relative to radium gamma rays which served as the baseline radiation. The radiations were delivered in single short exposures and the biological responses were all of an acute type with latent periods of 30 days or less. Cobalt60 gamma rays were found to have an effectiveness identical to radium gamma rays. The 4-Mev gamma rays from neutron capture in graphite were generally less effective than radium gamma rays. The RBE of 250-kvp X-rays varied with the response measured. For the production of 30-day lethality and testicular atrophy, X-rays were slightly more effective than radium gamma rays. Tritium beta particles and 14-Mev neutrons showed RBE values of about 1.5. Three of the 4 RBE values for fission neutrons were approximately 2.0 and the fourth was 1.0. The RBE of the 0.6-Mev protons produced by thermal neutrons was measured in 10 different systems. The values ranged from 1.0-4.9, depending on the system, the majority falling between 1.0 and 2.5. Heavy particles from thermal neutron capture by boron10 (alpha particles and Li7 recoils) were only 1.3 times as effective as radium gamma rays for the production of 30-day lethality in mice, but were 3.5 times as effective in the production of testicular atrophy. Fission fragments from neutron capture by plutonium were less effective than radium gamma rays. It was tentatively concluded that RBE, in mammalian systems, increases with increasing lineal energy transfer (LET) of the radiation, reaches a maximum and then decreases with very high values for LET. Some of the implications of this conclusion were discussed.