Proctitis cystica profunda and radiation fibrosis in the rectum of the female wistar rat after X irradiation: A histopathological study

Part of the rectum of the female Wistar rat was X‐irradiated (1500 rad–3000 rad) without surgery. In the early phase (second week after irradiation) radiation damage of the rectum was haemorrhagic radiation proctitis and in the late phase a rectal obstruction. The dose‐effect relationship using rectal obstruction as the end point is a sigmoid curve with a LD50 of 2150 rad. Latency time decreases with increasing dose and is 150 days at the LD50. Histopathologic findings revealed excessive submucosal radiation fibrosis and numerous mucosal glands and cysts within the fibrotic submucosa. As the time after irradiation increased the extension of submucosal mucinous glands increased and sometimes the entire submucosa was replaced completely by mucosal glands and cysts. It is suggested that invasion of mucosal epithelium into the submucosa was allowed by manifold fragmentation of the muscularis mucosae due to lymphocysts having developed immediately after irradiation. The small arteries showed proliferation of the endothelium and adventitia, thickening and hyalinization of the wall, resulting in severe narrowing and even occlusion of the lumen.