Colorectal adenomas may be either protruding type or superficial type lesions. To delineate the clinicopathological characteristics of the latter, 153 superficial type adenomas (including the surrounding mucosa) obtained by endoscopic resection were studied morphologically. Superficial type adenomas were defined as flat or flat depressed adenomas with a height of ≤3 mm; histologically, the tubules proliferated horizontally without vertical overlap. The location of tubules in the mucosa was classified as: involvement of the surface layer only (m1), deeper invasion not reaching the muscularis mucosae (m2), or invasion to the muscularis mucosae (m3). The results of analysis indicated: 1) there was no relationship between atypia and size; 2) although macroscopic features (depression, etc.) were associated with the grade of atypia, a closer association was obtained for the location in the mucosa; 3) based on our classification system for tubule location, m2 and m3 adenomas had a significantly higher frequency of depressed type lesions than did m1 lesions; and 4) the height of superficial type adenomas was 295 to 413 μm. Height was lowest in the m3 group followed by, in ascending order, the m2 and m1 groups. These morphological and histological characteristics are expected to contribute to improved diagnosis of superficial type adenomas.
