Minute adenomas of the depressed type in familial adenomatous polyposis of the colon. A pathway to ordinary polypoid adenomas

Background. It has been thought that colorectal adenomas are polypoid from their beginning. Although depressed adenomas have been described, they have been considered an unusual form and have not been studied systemically. Method. More than 5000 colorectal adenomas in 24 resected large intestines of patients with familial adenomatous polyposis were examined by dissecting microscopic study. These adenomas were classified by their surface appearance as depressed, flat, or polypoid type. The frequencies and sizes of the three types were determined. Results. In addition to ordinary polypoid adenomas, many adenomas of the depressed type were found. Of the 5287 adenomas detected, 635 (12.0%) were depressed, 967 (18.3%) were flat, and 3685 (69.7%) were polypoid. Depressed adenomas constituted approximately 15% of all adenomas smaller than 1.5 mm diameter. However, they were rare in those of 1.5–2.0 mm diameter and were not found among those of more than 2.0 mm in diameter. Conclusion. These findings suggest that some ordinary polypoid adenomas originate from minute depressed adenomas.