In-situ and invasive carcinoma of the colon in patients with ulcerative colitis.

Four cases are described in which ulcerative colitis was complicated by carcinoma and in which the extent of the accompanying in-situ carcinoma (precancer) was determined. The extent of the in-situ carcinoma was very variable and in all cases the rectum was spared in whole or part. Search for in-situ carcinoma in rectal biopsies will fail to indicate the development of malignancy in some patients with ulcerative colitis. Alternative methods of cancer detection in this group are discussed.