OBJECTIVETo study the clinicopathologic features and proliferative status of colorectal hyperplastic polyp (HP), sessile serrated adenoma (SSA) and traditional serrated adenoma (TSA).

METHODSOne hundred and four cases colorectal serrated lesions were collected from 2628 cases of colorectal polyps during the period from November, 2002 to December, 2007. The clinicopathologic features and expression of proliferation marker Ki-67 were studied.

RESULTSOn the basis of morphologic examination, 60 cases were classified as HP, 20 cases as TSA, 11 cases as SSA, 7 cases as mixed HP/SSA/TSA, and 6 cases as mixed serrated polyp/adenoma and tubular adenoma. Immunohistochemical study for Ki-67 showed that 40 cases (78%) of the 51 cases of HP were either mostly negative or rarely (<25% cells) positive. Most of the positive cells were located at crypt bases. Among the 15 cases of TSA, 11 of them revealed positive cryptal cells (25% to 50% or>50% positivity). Most of the positive cells were located in mid portion of crypts. The number and distribution of Ki-67 positive cells in SSA were similar to those in TSA but were significantly different from those in tubular adenoma and adenocarcinoma (chi2=34.601, P=0.000; chi2=63.077, P=0.000, respectively).

CONCLUSIONSHP, SSA and TSA have their morphologic characteristics, with some overlapping features noted. The distinction between SSA and HP can be difficult. Diagnosis of SSA relies mostly on architectural rather than cytologic features. The distinction between TSA and SSA depends mainly on the presence of dysplasia. Ectopic crypt formation is almost exclusively seen in TSA. The distribution and percentage of Ki-67-positive cells are also helpful in subtyping of various colorectal serrated lesions. In general, the proliferative index is lower in serrated adenoma (TSA or SSA) than in tubular adenoma.