The light and electron microscopic features of early and late phase radiation-induced proctitis.

The light and electron microscopic features of rectal biopsies from 10 symptomatic patients treated with irradiation for pelvic malignancies are detailed. They are divided into two groups. Group I: biopsies taken during or shortly after the course of irradiation (six patients). Group II: biopsies taken 4 months or more after course completion (four patients). The distinguishing light microscopic features in the first group are epithelial meganucleosis, lack of mitotic activity, and patchy fibroblastic proliferation in the lamina propria. The blood vessels appear normal. In the second group, there are severe vascular changes characterized by narrowing of the arterioles by subintimal fibrosis, telangiectasia of capillaries and post-capillary venules, endothelial degeneration, and platelet thrombi formation. These vascular changes are always associated with severe fibrosis of the lamina propria and crypt distortion. The ultrastructural and light microscopic findings indicate that the cellular epithelial reaction and fibroblastic proliferation antedate the vascular injury, and the latter has no role in the acute phase reaction.