Chronic radiation damage in the rectum of the rat after protracted fractionated irradiation.

In this experiment, a protracted fractionation regime (18 fractions/170 days, dose/fraction: 6,7,8 Gy) caused late rectal obstruction in rats independently of any clinically detectable acute damage. Rectal obstruction was due to an ulcer which developed secondary to an atrophic mucosa and capillary damage. In agreement with our earlier protracted fractionation schedules, the results were not fitted well by the linear-quadratic model. However, there are indications of relatively low alpha/beta values. Hydronephroses and hydroureters were the only competing risks to rectal obstruction and they occurred independently of it.