There are various types of polyps in the colon and rectum. Most colorectal polyps in the colon and rectum are either adenomas or hyperplastic polyps. In general, adenomas are considered to be premalignant. Adenoma-carcinoma sequence is an established theory of carcinogenesis in the large bowel in Western countries. However, small adenomas are ubiquitous phenomena, and it is questionable whether they are actually precancerous. Most small adenomas did not become larger in our colonoscopic follow-up study of small adenomas measuring 5 mm or less. It was considered safe to leave such small adenomas behind, particularly in a situation in which follow-up examination is periodically performed, such as in patients who have undergone colorectal cancer surgery before. In Japan, small flat carcinomas have been reported as an important precursor of advanced colorectal carcinoma. They are also found in Western population. The malignancy rate of small flat and depressed lesions was significantly higher than that of polypoid lesions. Although small flat carcinomas are less frequently seen than polypoid-type carcinomas in the early stages, they are considered to be an important precursor of advanced colorectal cancer.
Polyps of the colon and rectum are relatively common lesions. Most colorectal polyps are either adenomas or hyperplastic polyps.
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