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Background. Recent studies have noted that small flat carcinomas do exist in the human colon, but their clinicopathologic features have not been fully delineated. Methods. A clinicopathologic study of 62 flat colorectal carcinomas (FCC) and 80 polypoid colorectal carcinomas (PCC) was performed. FCC were defined for the study as colorectal adenocarcinomas confined to the mucosa or submucosa, macroscopically characterized as slightly elevated, often nearly flat, and sometimes with central shallow depressions. Results. In intramucosal and submucosal invasive carcinomas, the mean size of FCCs was significantly smaller than that of PCCs, (P < 0.001). When compared with PCCs, FCCs were found more often in the proximal colon, less frequently well differentiated, and had fewer adenomatous remnants. In submucosal invasive cases, FCCs showed more frequent deep invasion (67 versus 32%, P < 0.05) and lymphovascular permeation (41% versus 16%, P < 0.05) compared with PCCs. Conclusions. These results suggest that FCCs are characterized by frequent deep invasion with lymphovascular permeation even when small, and that they may be more prevalent away from the rectosigmoid area.

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