Effect of age on radiation-induced early changes of rat rectum. A histological time sequence

Background and purpose: Radiation treatment of the elderly (>75 years) is o ften modified due to an assumed decrease in normal tissue tolerance in this age group. Since more radiobiological data concerning normal tissue toxici ty as a function of age are needed, a histological study of age-related rad iation changes of the rectum was performed. Materials and Methods: The rectum of young and old female Wistar rats (12 a nd 78 weeks, respectively) was irradiated with single doses of 22 and 39 Gy . The held size was 1.5 x 2.0 cm. The animals were sacrificed at 1, 2, 4 an d 10 weeks after treatment. To evaluate radiation damage, 12 histological p arameters were scored in four areas of the rectum. A total radiation injury score was calculated. The number of proliferative epithelial cells was eva luated by 5-bromo-2'-deoxyuridine labeling. Results: Some age-related histological differences were observed; especiall y, the incidence of ulceration and vascular occlusion was higher in the old er group. In the low dose group of the older animals, 60% showed ulceration , which was 0% for the young low dose animals. Severe vascular changes occu rred early and were more extensive in older animals (4 weeks) than in the y ounger group (10 weeks). In the area adjacent to the treatment field, cell proliferation increased significantly in older rats at 1 week after 22 Gy, which did not occur in the young group. Conclusions: Discrete radiation-induced histological differences were obser ved between the rectum of young and old Wistar rats, especially in the deve lopment of ulceration and vascular changes. Although the survival of these Wistar rats in earlier studies was not affected by age, the impact of the o bserved histological differences for their importance in the long-term is c urrently being investigated. (C) 2001 Elsevier Science Ireland Ltd. All rig hts reserved.