Sessile serrated adenomas of the large bowel. Clinicopathologic and immunohistochemical study including comparison with common hyperplastic polyps and adenomas.

Sessile serrated adenoma (SSA) is a newly characterized type of the large bowel adenoma. It arises in hyperplastic polyp (HP) and represents a precursor lesion of colorectal carcinoma with microsatellite instability. SSAs differ from common HPs by abnormal proliferation of the crypt epithelium and by nuclear atypia. We examined 15 SSAs from 15 patients. The age range was 25-80 years (average 60 years). Six patients were females and 9 were males. For comparison, we examined 10 conventional tubular adenomas and 10 common HPs with vesicular cells. The sites of SSAs were as follows: 8 in rectum, 4 in rectosigmoid colon, 1 in transverse colon, 1 next to mucinous carcinoma of ascending colon, 1 in anastomosis after resection of the transverse colon adenocarcinoma. The diameter of the lesions ranged from 5 to 12 mm. Histologically, SSAs showed asymmetrical proliferation of the epithelium, irregular shape of the crypts with their branching and some crypt dilatations especially in the basal parts of the crypts. Cellular atypia (dysplasia) was usually low. In 5 cases the nuclei were focally stratified and localized in the lower part of the cells. High-grade dysplasia was found only in SSA adjacent to mucinous adenocarcinoma. Immunohistochemically, SSAs showed secretion of gastrointestinal mucin expressing MUC2 and MUC5A. Both MUC2 and MUC5A were also positive in mucinous carcinoma. In previous studies these expressions were considered specific for serrated type of carcinogenesis. However, our study found positivity of MUC2 and MUC5A also in conventional adenomas. Expression of p53 in SSAs was minimal. SSAs have malignant potential comparable with conventional adenomas and for this reason they must be distinguished from HPs.