Flat serrated adenomas of the colorectal mucosa.

A total of 47 flat serrated neoplasias of the colorectal mucosa are presented: 44 were flat serrated adenomas and the remaining 3 flat serrated adenocarcinomas arising in flat serrated adenomas. These lesions were found among 600 flat mucosal lesions removed at colonoscopy during a 3-year period (1992 and 1994) at the Karolinska Hospital. Thirty-five of the 47 patients (74%) were males and the remaining 12 (26%), females. Depending upon the degree of cellular dysplasia within the epithelium, serrated adenomas were divided into those with low-grade dysplasia (LGD), when the dysplastic nuclei were present in the deeper half of the epithelium, and those with high-grade dysplasia (HGD), when the dysplastic nuclei were found even in the upper half of the epithelium. LGD was present in 37 (84.1%) of the 44 serrated adenomas and HGD in the remaining 7 (15.9%). Depending upon the topographic distribution of the dysplastic epithelium within the crypts, flat serrated adenomas were divided into type I, when the dysplastic epithelium was limited to the lower half of the serrated crypts, and type II, when the dysplastic epithelium was even present in the superficial half of the serrated crypts. Of the 44 serrated adenomas, 38 (86.1%) were type I and the remaining 6 (13.9%) type II. The dysplastic epithelium seemed to originate at the base of the crypts and to progress upwards, replacing the scalloped, serrated epithelium of the sides of the crypts. Invasive adenocarcinomas (i.e., with submucosal extension) were seen to arise from flat serrated adenomas with LGD type I (n = 2) or with HGD type II (n = 1). This preliminary survey suggests that flat serrated adenomas of the colorectal mucosa may be lesions with a propensity to evolve into invasive adenocarcinoma, irrespective of the degree of the epithelial dysplasia or of their extension along the crypts.