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

Abstract

The intestine is particularly radiosensitive and can be a limiting tissue in wide field abdominal irradiation. In the radiotherapy of tumors of the pelvis much of the small intestine is avoided but often the large bowel has to be included in the treatment fields. Colo-rectal damage in mice was assessed after Ca .gamma. irradiation and 3 MeV neutrons given as single doses. Several assays were used, including body weight changes, fecal deformity and lethality. Dose response curves were constructed for each assay at different times after irradiation, ranging from 10 days to 16 mo. The data were analyzed in terms of the time course of expression of damage and the RBE [relative biological effectiveness] 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 localized pelvic 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 mo. Fecal deformity was detectable as a higher proportion of small pellets when the rectum became constricted by fibrosis. No significant fecal deformity was observed before 6 mo. after which time dose response curves could be obtained. The RBE for early damage (assessed at 1-3 mo.) was 2.2-2.7. This fell to 1.7-1.9 for late damage (determined at 10-15 mo.) over the range of neutron doses of 7.5-12 Gy [gray]. 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.