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 I (TTF-1) is useful in distinguishi ng Merkel cell carcinomas (MCCs) from metastatic small cell carcinomas (SCC s). Methods-Eleven cases of MCC and 10 of lung SCC were stained for CK20 and TT F-1. Results-Ten of I I MCCs stained with the antibody to CK20. None was positiv e for TTF-1. No SCC stained with anti-CK20 and all stained strongly with an ti-TTF-1. Conclusions-The use of both anti-CK20 and anti-TTF-1 can reliably distingui sh between MCC and metastatic SCC, thus avoiding the need for a detailed cl inical investigation of patients with MCC in whom metastatic SCC must be ex cluded.