RBE values for colo-rectal injury after caesium 137 gamma-ray and neutron irradiation. I. Single doses.

Colo-rectal damage in mice has been assessed after caesium gamma irradiation and 3 MeV neutrons given as single doses. Several assays were used, including body weight changes, faecal deformity and lethality. Dose response curves have been constructed for each assay at different times after irradiation, ranging from 10 days to 16 months. The data have been analysed in terms of the time course of expression of damage and the RBE for neutrons. An initial loss of weight at 10-20 days was presumably related to epithelial denudation, but a dose-dependent weight reduction (compared with controls) persisted over the animal's life span. Mice died progressively after localised pelvic gamma irradiation; there was no sharp demarcation between an early and late phase of lethal injury. Death resulted from intestinal stricture or stenosis. The time course for lethality was qualitatively different after neutrons, with little progression of damage between 5 and 11 months. Faecal deformity was detectable as a higher proportion of small pellets when the rectum became constricted by fibrosis. No significant faecal deformity was observed before 6 months after which time dose response curves could be obtained. The RBE for early damage (assessed at 1-3 months) was 2.2-2.7. This fell to 1.7-1.9 for late damage (determined at 10-15 months) over the range of neutron doses of 7.5-12 Gy. The need for sublethal assays allowing for sequential evaluation of radiation damage within the same animal is stressed, as is the need to compare RBE values from early and late endpoints at equivalent neutron doses.