Large cell neuroendocrine carcinoma arising in a sessile serrated adenoma: a novel observation.

A 68-year-old woman underwent polypectomy of 2 right-sided colonic polyps identified by screening colonoscopy. Histologic examination of both polyps showed features of sessile serrated adenoma. The larger polyp harbored an invasive tumor composed of large, high-grade cells arranged in nests and cords without tumoral mucin production. Immunohistochemistry demonstrated synaptophysin, cdx-2, cytokeratin 7, and cytokeratin 20 positivity. Both invasive carcinoma and sessile serrated adenoma showed a decreased expression level to focal negative expression of hMLH-1 by immunohistochemistry. Combined morphologic and immunohistochemical features favored large cell neuroendocrine carcinoma arising in a sessile serrated adenoma. Specific carcinoma subtypes and special histologic features (eg, tumor-infiltrating lymphocytes) have been previously reported in carcinomas arising from sessile serrated adenomas. Large cell neuroendocrine carcinoma has not yet been reported in association with sessile serrated adenomas, with this case suggesting a rare but potentially novel end point for the microsatellite instability pathway.