Melanoma with signet-ring cells in a peritoneal effusion.

Melanoma may occasionally give rise to a malignant effusion and when this occurs it can present a challenging differential diagnosis. We present such a patient who developed a peritoneal effusion. Cytologic examination of the peritoneal fluid revealed individual malignant cells with prominent cytoplasmic vacuoles raising the possibility of a signet-ring cell adenocarcinoma. Immunohistochemical studies were indicative of a malignant melanoma. In this case we believe that the cytoplasmic vacuoles may be a degenerative phenomenon and as such may represent another mechanism for the development of a signet-ring morphology in malignant melanoma.