The relative biological effectiveness of fast-neutron and x-radiation; survival and cataract studies of Swiss mice.

Abstract

The relative biological effectiveness (RBE) of fast-neutron and x-radiation is a concern of persons responsible for radiation safety measures, as well as those interested in the mechanisms of radiobiological action. The RBE of fast neutrons and x-rays has been found to vary when different criteria, different tissues, or different exposure patterns have been utilized in deriving the ratio (1, 2). In establishing an acceptable tolerance limit for neutron radiation, therefore, the most unfavorable effectiveness ratios must be considered. An earlier study by Evans (2) indicated that lens damage was one of the more critical biological effects of low-level neutron irradiation. Further study of the relative cataractogenic effectiveness of fast-neutron and x- or gamma radiation was then begun at this and other laboratories. Because the dosimetry of fast-neutron radiation had not been standardized, it seemed prudent also to determine a biological unit of exposure, the 30-day LD 50. This paper summarizes our observations on survival and cataract development after single exposures to several neutron sources as compared with observations after x-irradiation. Exposure Methods The animals utilized in all experiments were mice of an albino Swiss stock raised in our laboratory. At weaning time, the sexes were separated and put into stainless steel boxes, 8 mice of comparable age (no more than five days difference in date of birth) per box. The occupants of any one box were seldom all litter mates, thus tending to randomize the sampling. The sawdust in the boxes was changed three times per week; food and water were available at will. Mice from the stock colony were prepared for experimental use by clipping the toes to insure proper identification. The mice were irradiated at two and a half to three and a half months of age. The several boxes housing mice of the same age (± two days) were usually divided into two groups, one group for neutron irradiation and the other for x-irradiation. The several boxes in each group were randomly assigned to dose levels scattered over the entire range of exposures under investigation. The animals arbitrarily chosen as controls were not irradiated, but were otherwise transported, confined, and handled like the irradiated animals. In similar fashion, mice of different ages within the range of two and a half to three and a half months were allocated to the same or interspersed dose levels. With one exception (exposures to fission neutrons), experiments were repeated several times during a period of two to three years. The results of all repeated experiments have been combined for the final summary. X-radiation: The studies of x-radiation effects, which have extended over a period of seven years, were begun with the following exposure factors: 200 kv, 5 ma, 0.25 mm. Cu plus 1.0 mm.