Background and aims. An attempt has been made to evaluate the clinicopathological characteristics of flat colorectal neoplastic lesions, and analyse the factors associated with the malignancy. Patients and methods. A total of 115 flat neoplastic lesions, ≥5 mm in size, diagnosed in 87 patients by colonoscopy, were investigated. Results. The rectum was the most common location. Almost half (49.6%) of the flat neoplasms were small (5-10 mm), 27.8% were 11-20 mm and the remainder (22.6%) larger than 20 mm. The surface was smooth in 55.7%, granular in 20.0% and nodular in 24.3%. Histologically, the flat lesions were tubular, tubulovillous and villous adenomas in 69.6%, 20.9% and 5.2%, respectively. Five lesions (4.3%) were composed of carcinomas without adenoma. High-grade dysplasia, intramucosal carcinoma and invasive carcinoma were diagnosed in 9.6%, 7.8% and 6.1% of all flat neoplasms, respectively. Univariate analysis demonstrated that the location, size, surface pattern and histologic type of the flat lesions were factors associated with malignancy. However, in multivariate analysis, the size of the flat lesions was the only significant risk factor for malignant transformation. Conclusions. Flat neoplastic lesions of the colorectum have a relatively high rate of malignancy, and size is the most important factor associated with malignancy.
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