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. © Wiley‐Liss, Inc.