Objective. Several reports have suggested that flat colorectal adenomas might exhibit a higher potential for malignancy compared to polypoid adenomas. Although the clinical importance of the shape of polyps is stressed, the controversy surrounding the malignant potential of flat adenomas continues. The aim of this study was to compare the clinicopathologic characteristics, including degree of dysplasia and malignancy, between flat and polypoid adenomas 5 mm in size or larger. Material and methods. A total of 3263 polyps (254 flat adenomas and 3009 polypoid adenomas), ≥5 mm in size, diagnosed in 1883 patients by colonoscopy were analyzed. Results. Flat adenomas were found in 10% of patients, which represented 7.8% of all adenomas removed. The flat adenomas were larger in diameter than the polypoid adenomas (14.8±12.6 mm versus 8.6±5.0 mm, p <0.01), had a higher rate of villous components (18.5% versus 11.4%, p <0.01), a higher rate of high-grade dysplasia (9.4% versus 4.2%, p <0.01), and a higher rate of malignancy (10.2% versus 3.6%, p <0.01) than polypoid adenomas. However, there was no difference in the rate of high-grade dysplasia or carcinoma between flat and polypoid adenomas of equal size. It was shown by multivariate analysis that rectosigmoid location, larger size, and presence of a villous component were associated with a higher rate of malignancy, but not with flat morphology. Conclusions. Flat adenomas, which were of a relatively large size in this study, were not associated with a higher risk for high-grade dysplasia and carcinoma compared with polypoid adenomas.
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