Analysis of thyroid transcription factor-1 and cytokeratin 20 separates merkel cell carcinoma from small cell carcinoma of lung

Merkel cell carcinoma needs to be separated from small cell carcinoma metas tatic from visceral sires to skin. Pulmonary small cell carcinoma is the mo st common primary site Of small cell carcinoma, We evaluated the immunophen otypic characteristics of 21 Merkel cell carcinomas and 33 small cell carci nomas of lung using thyroid transcription factor-1 and cytokeratin 20. Thyr oid transcription factor-1 was 100% specific for the diagnosis of small cel l carcinoma of lung associated with a diagnostic sensitivity of 85%. Cytoke ratin 20 was present in 95% of Merkel cell carcinomas however, 33% of small cell carcinoma of lung were also positive. Both antibodies typically demon strate diffuse and intense staining of their respective tumor cells. We con clude that thyroid transcription factor-1 is a sensitive and specific marke r for small cell carcinomas of lung and that a combination of thyroid trans cription factor-1 and cytokeratin 20 is indicated to assist in the differen tiation of metastatic small cell carcinoma of lung from merkel cell carcino ma.