Cytokeratin 20-positive large cell neuroendocrine carcinoma of the colon.

Herein is presented a case of cytokeratin (CK) 20-positive large cell neuroendocrine carcinoma of the colon, in which the tumor was clinically at stage IV and located in the ascending colon. Pathological examination of the resected tumor revealed nested and solid proliferation of large undifferentiated cells with vesicular nucleus and prominent nucleoli. No areas showed differentiation toward adenocarcinoma or squamous cell carcinoma. Tumor cells were immunohistochemically positive for chromogranin A, synaptophysin, CD 56 (focal), and bore electron-dense granules. With these features, the tumor was diagnosed as a large cell neuroendocrine carcinoma of the colon. Liver metastasis and local recurrence progressed, and the patient died of the primary disease 7 months after operation. The autopsy confirmed this diagnosis without detectable tumors in the lungs. Interestingly, more than half of the tumor cells were positive for CK 20, while CK 7 was not expressed. Most neuroendocrine carcinomas do not express CK 20, with the exception of Merkel cell carcinomas, and most colorectal adenocarcinomas express CK 20. To the best of the authors' knowledge, the present case is the first CK 20-positive, CK 7-negative colorectal neuroendocrine carcinoma to be described, suggesting a link between colorectal neuroendocrine carcinoma and conventional adenocarcinoma.