M. Sakashita et al., Flat-elevated and depressed, subtypes of flat early colorectal cancers, should be distinguished by their pathological features, INT J COL R, 15(5-6), 2000, pp. 275-281
Flat-type colorectal tumors have are being detected with increasing frequen
cy. It has become clear that these flat lesions contain two subtypes; flat-
elevated and depressed lesions. However, their clinicopathological features
and roles in colorectal carcinogenesis remain obscure. We classified color
ectal adenomas and submucosal invasive cancers into three types: polypoid,
flat-elevated, and depressed types. A clinicopathological study of 2505 col
orectal tumors (2407 adenomas, 98 submucosal invasive cancers) was then per
formed. Furthermore, 64 tumors (25 adenomas with high-grade dysplasia, 39 s
ubmucosal invasive cancers) from which DNA was extracted were examined for
K-ras gene mutation. The percentages of each configuration in the resected
materials were 62.0%, 36.4%, and 1.64b of the polypoid, flat-elevated, and
depressed types, respectively. The rate of submucosal invasive cancer in th
e depressed type was always high regardless of size. In the polypoid and fl
at-elevated types, lesions of larger size showed higher rates of invasion.
Analysis of submucosal invasive cancers revealed no adenomatous components
in any of the depressed-type lesions; in the polypoid and flat-elevated typ
es the frequencies of cancer with adenomatous components were 83.6% and 77.
8%, respectively. The flat-elevated type was more frequently located (77.8%
) in the proximal colon than the other types (polypoid type 16.4%, depresse
d type 25.0%). The incidence of K-ras gene mutation was 47.2%, 18.2%, and 0
% in the polypoid, flat-elevated, and depressed types, respectively. These
findings suggest that the flat-elevated and depressed types are similar in
that they are both morphologically flat and have infrequent incidences of K
-ras gene mutation, but these two lesions differ in their pathological feat
ures. Especially, depressed type lesions have a tendency to invade the subm
ucosal layer even when they are small. Therefore one should always be aware
of this type of lesion during colonoscopic examination.