Merkel cell carcinoma can be distinguished from metastatic small cell carcinoma using antibodies to cytokeratin 20 and thyroid transcription factor 1.

AIM To investigate whether immunohistochemical staining for cytokeratin 20 (CK20) and thyroid transcription factor 1 (TTF-1) is useful in distinguishing Merkel cell carcinomas (MCCs) from metastatic small cell carcinomas (SCCs). METHODS Eleven cases of MCC and 10 of lung SCC were stained for CK20 and TTF-1. RESULTS Ten of 11 MCCs stained with the antibody to CK20. None was positive for TTF-1. No SCC stained with anti-CK20 and all stained strongly with anti-TTF-1. CONCLUSIONS The use of both anti-CK20 and anti-TTF-1 can reliably distinguish between MCC and metastatic SCC, thus avoiding the need for a detailed clinical investigation of patients with MCC in whom metastatic SCC must be excluded.