Epidemiologic characteristics of the flat adenoma of Muto. A prospective study

The flat adenoma is an endoscopically visible lesion that histologically consists of adenomatous change near the luminal surface of colonic tubules. We have described three families with hereditary colon cancer with later age of onset than familial adenomatous polyposis (FAP) and with multiple proximal colonic flat adenomas. These families have been linked to the FAP locus on chromosome 5. Our aim was to determine whether the flat adenoma is pathognomonic of this hereditary flat adenoma syndrome (HFAS) or merely an atypical or early tubular adenoma with occurrence in patients other than those from colon cancer-prone families.

We prospectively examined a population referred for colonoscopy within a one-year period. During colonoscopy, flat adenomas were specifically sought and all lesions were removed endoscopically and evaluated histologically. Members of known hereditary colon cancer families were excluded.

One hundred forty-eight patients underwent colonoscopy (64 men and 84 women). Median age was 61 years. Fifty-seven patients had 157 polyps. One hundred thirty-six polyps were reviewed histologically. Thirty-five (23.6 percent) of the referred patients had adenomas, of whom twelve patients had only flat adenomas while six had both flat and other adenomas (18 = 12 percent of 148). The associations between flat adenoma occurrence and various predictors (sex, race, prior colonic neoplasms, family history of cancer, synchronous adenomas) were similar to those seen with other adenomas. Flat adenomas were found in nearly equal proportions of patients under or over age 61 years (11 percent and 13 percent, respectively). Other adenomas were significantly more common in the older group (6 percent vs. 25 percent; P less than 0.002 by Fisher's exact test).

In a referral practice, the flat adenoma has the same prevalence and associated risk factors as other adenomas, except for younger age of onset. Our data suggest that the flat adenoma represents an early stage of adenoma development that is manifested in a subset of patients from the general population and that, as an isolated event, does not provide a marker for a hereditary colon cancer-prone syndrome.