Differential expression of thyroid transcription factor 1 in small cell lung carcinoma and Merkel cell tumor

The distinction between metastatic small cell lung carcinoma (SCLC) and Merkel cell tumor is difficult by routine histology, prompting the search for specific markers that could separate these neoplasms. Thyroid transcription factor 1 (TFF-1) is a homeodomain containing transcription factor expressed in the normal airway epithelium. The expression of TTF-1 has also been shown in adenocarcinomas and small cell carcinomas of the lung. However, the utility of TTF-1 to differentiate between SCLC and Merkel cell tumor has not yet been investigated. In this study, paraffin sections of 36 SCLCs and 21 Merkel cell tumors were analyzed for the presence of immunoreactive TTF-1 and cytokeratin 20 (CK20), a marker previously demonstrated in Merkel cell tumors. Monoclonal TTF-1 and CK20 antibodies were used with a biotin-streptavidin detection system. Immunostaining for TTF-1 was observed in 97% of SCLCs and in no Merkel cell tumors. Immunoreactivity for CK20 was demonstrated in 76% of Merkel cell tumors and 3% of SCLCs. These data indicate that TTF-1 is a sensitive (97%) and specific (100%) marker for SCLCs and can be used to differentiate SCLCs from Merkel cell tumors.