Lymph node Merkel cell carcinoma with no evidence of cutaneous tumor--report of two cases.

We report two cases of Merkel cell carcinoma within inguinal and axillary lymph nodes, respectively, showing no clinico-pathologic evidence of a primary (extranodal) tumor; one of our patients was alive with no evidence of disease five years and ten months after the surgical excision of the neoplasm with no postoperative chemotherapy. The diagnosis of nodal Merkel cell carcinoma needs to be supported by a careful immunohistochemical study: in fact, cytokeratin- and neurofilament-positive paranuclear "dots", as well as epithelial antigens and neuroendocrine markers may be variably expressed in tumor cells, thus requiring the application of a complete antibody panel. In the presence of a nodal Merkel cell tumor, an exhaustive clinico-radiologic search for a primary tumor must be carried out. After the exclusion of any reasonable starting point of the neoplasm, a provisional diagnosis of "primary" nodal Merkel cell carcinoma may be acceptable; since a primary extracutaneous tumor is expected to follow a less aggressive course than a metastatic one, follow-up data may provide indications as to the truly extracutaneous origin of Merkel cell carcinoma.