4498 Malignant potential of colorectal flat adenoma.

Abstract

Background: Colorectal flat adenoma, meaning that their growth above the muscularis mucosa is no more than two times the thickness of the normal mucosa or height is less than 1/2 or 1/4 of longest diameter of adenoma, has been the focus of attention because of its higher malignant potential compared to polypoid adenoma. Aim: We wanted to evaluate the clinicopathological characteristics of flat adenomas and to analyze the factors associated with malignant transformation of colorectal flat adenomas. Subjects and Methods: From Jan. 1991 to Jul. 1999, 115 flat adenomas larger than 5 mm were diagnosed in 87 patients (63 male and 24 female) by colonoscopy. Adenomas equal or smaller than 5 mm was excluded because of difficulty in differentiation of flat and sessile lesions. Of those, 24 and 5 patients had a history of colorectal polyp and cancer, respectively. Mean age of the patients was 61 years (range: 36-79). Half of the patients had synchronous adenoma and 12% had a synchronous carcinoma of the colorectum. Results: The rectum (32%) was the most common site of flat adenoma, followed by sigmoid (19%), transverse (18%) and ascending colon (15%). Half of the flat adenomas were small (6-10 mm), 28% were 11-20 mm and remainder (23%) were larger than 20 mm. The surface was smooth in 56%, granular in 20% and nodular in 24%. Histologically, 70%, 21% and 5% of flat adenomas were tubular, tubulovillous, and villous adenoma, respectively. Five lesions were composed of carcinoma without any adenomatous tissue. High-grade dysplasia, intramucosal carcinoma and invasive carcinoma were diagnosed in 10%, 8% and 6% of flat adenomas, respectively. Univariate analysis demonstrated that distal location, larger size, nodular surface pattern, and villous histologic type of flat adenomas were factors associated with malignant transformation in terms of intramucosal carcinoma or invasive carcinoma. However, the size of flat adenomas was revealed as the only significant risk factor for malignancy in multivariate analysis. Conclusion: Flat adenoma of colorectum is a real disease entity and it has a relatively high malignant potential. Therefore, colonoscopic recognition of flat adenoma may play an important role for primary and secondary prevention of colorectal carcinoma.