Differentiation between merkel cell carcinoma and malignant melanoma: An immunohistochemical study.

Although Merkel cell carcinoma (MCC) exhibits specific clinical and histologic features, differentiation from other cutaneous neoplasms, such as lymphoma, metastatic oat cell carcinoma and malignant melanoma (MM), may sometimes be difficult. The aim of our study was to immunohistochemically differentiate MCC from MM. Paraffin sections from 6 cases of primary MCC and 6 cases of primary MM were investigated. For immunostaining, the APAAP method was used. Neuron-specific enolase was positive in all cases of MCC, as well as in 2 cases of MM. Marked positivity for cytokeratins 18, 20 and chromogranin A was observed in the MCC group, whereas a complete absence of expression of these three markers was noted in the MM group. Immunostaining with HMB45 and NKI/C3 was positive in all cases of MM and negative in all cases of MCC. S-100 protein was positive in all but 1 case of MM. In contrast, only 1 case of MCC reacted with S-100 protein. Our results underline the role of immunohistochemistry in the diagnosis and differential diagnosis of MCC. In particular, the combination of neuron-specific enolase, cytokeratins 18, 20 and chromogranin A positivity for MCC and HMB45, NKI/C3 and S-100 protein positivity for MM is of great value in the distinction between these two cutaneous neoplasms.