Radiation tolerance of the rat rectum to fractionated X-rays and pi-mesons.

A 2.5 cm segment of the rectum of female F344 rats was irradiated with up to 10 fractions of X-rays or pions. Transient signs of acute proctitis were followed by chronic rectal injury starting at about 2 months. Recto-vaginal fistulas were frequently observed in animals with severe chronic injury. Two patterns of chronic injury were observed, an early type consisting of deep ulcers and fistulas, and a late type characterized by vascular injury, fibrosis and mucosal cysts. In a 4-fraction X-ray experiment, the influence of a low-residue diet was compared with a regular diet, showing no significant differences in pathology or survival. Isoeffective doses were determined for the occurrence of severe rectal injury at 250 days. Fitting the data to an LQ-model yielded an alpha/beta ratio of 13 Gy for pions, and 6.5 Gy for X-rays. Increasing the overall treatment time from 9 days to one month gave a significant rise in isoeffective doses for chronic injury. This suggests that the more delayed types of injury did not develop independently from the acute mucosal changes. The RBE of pions for rectal injury at 250 days was 1.2 for single doses, increasing to about 1.4-1.5 at a dose per fraction of 4 Gy. These values are similar to those obtained in other acute and late responding tissues.