Primary colorectal small cell carcinoma: a clinicopathological and immunohistochemical study of 10 cases.

Colorectal small cell carcinoma (SmCC) is a rare tumor with an aggressive course. The aim of this study is to summarize our experience with this tumor and to highlight its immunohistochemical profile. Ten cases of colorectal SmCC were identified in our files and a panel of immunostains was performed. Follow up was available for the average of 3 years, during which 7 patients died and 3 were alive with disease. All cases were positive for LMWK, CK 19 and pancytokeratin but were negative for TTF-1 and CA 125. EGFR was positive in 7 cases. TTF-1 negative staining may be valuable in differentiating it from its pulmonary counterpart. CDX2, mCEA, CD56, synaptophysin, NSE and chromogranin can help differentiate it from non-endocrine poorly differentiated adenocarcinoma. The expression of EGFR in a subset of patients has not been reported earlier and has to be evaluated in larger series to assess its role in the planning of targeted biologic therapy.