Histopathological findings of radiation-induced colitis. A. Acute injury, characterized by ulceration, absence of viable crypts, diffuse infiltration by polymorphonuclear leucocytes, and prominent capillaries lined by plump endothelial cells (H + E × 400). B. Early regenerative changes, characterized by absence of ulceration, considerably less acute inflammation, infiltration by plasma cells and lymphocytes, presence of viable crypts with disarray, absence of cryptitis or acute epithelial damage (H+E × 400). C. Late regenerative changes, characterized by absence of acute inflammation, mild diffuse infiltration by plama cells and lymphocytes, architectural crypt distortion, with reduced crypts, crypt branching and shortening as well as moderate/severe fibrosis of the lamina propria (H + E × 400).