Immunohistochemical examination of 25 cases of Merkel cell carcinoma: a comparison with small cell carcinoma of the lung and oesophagus, and a review of the literature.

Merkel cell carcinomas (MCC) were compared to small cell carcinomas of the lung (SCCL) and oesophagus (SCCO). Most MCC were of the intermediate cell type while SCCL and SCCO were usually of the small cell type. Only MCC of trabecular type could be separated from SCCL and SCCO by means of histopathological examination alone. All MCC (25) stained with cytokeratin CAM 5.2, 20 of which in a "paranuclear globular" or combined "paranuclear globular"/diffuse pattern while 17 MCC stained with cytokeratin AE1/AE3. Cytokeratin CAM 5.2 reacted with 60 percent of the SCCL and 86 percent of the SCCO, and cytokeratin AE1/AE3 with 33 and 28 percent respectively. Neurofilament stained 17 MCC in a "paranuclear globular" pattern but none of the SCCL and SCCO. All MCC with a diffuse staining pattern for cytokeratin CAM 5.2 were negative for neurofilament. The results of this study and review of the literature indicate that in most instances Merkel cell carcinoma can be separated from other SCC, pulmonary as well as extrapulmonary, by means of histopathological and, above all, immunohistochemical examinations.