Signet-ring cell melanoma mimicking adenocarcinoma. A case report.

The fine needle aspiration findings in a lymph node involved by signet-ring cell melanoma, a very rare variant of malignant melanoma, are reported. The patient had a history of superficial spreading melanoma of the right foot, treated 10 years earlier by below-the-knee amputation. He presented with a right groin mass. Fine needle aspiration of the mass yielded poorly cohesive, large cells with eccentric nuclei and abundant, eosinophilic cytoplasm; many of them exhibited a signet-ring appearance. Melanin pigments were identified in a small proportion of tumor cells, and a diagnosis of metastatic melanoma was made. The subsequent lymph node excision revealed a metastatic tumor composed of polygonal and signet-ring cells that were positive for S-100 protein and HMB-45 but not cytokeratin. Nearly all reported cases of signet-ring cell melanoma occurred as metastatic or recurrent disease. It is important not to mistake signet-ring cell melanoma for adenocarcinoma in aspiration cytology, and a constellation of clinical features and of histochemical and immunohistochemical findings enables a correct diagnosis to be reached.