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.