Signet ring cell mesothelioma; A diagnostic challenge.

Signet ring cell mesothelioma is a rare variant of epithelioid mesothelioma with limited cases published. It has a male predilection and most commonly occurs on pleura; it can also arise in the peritoneal cavity. The signet ring cell morphology can pose a challenge leading to a potential diagnostic error. A variety of benign and malignant diseases, including reactive histiocytic hyperplasia, adenocarcinoma, melanoma, and lymphoma with signet ring cell morphology should be considered in the differential diagnosis. In signet ring cell mesothelioma work up, mucin stains are of limited value. Even though immunohistochemistry is routinely used in mesothelioma diagnosis, there is no sole specific mesothelial marker. Hence, a panel of mesothelial and epithelial markers are used; these should be interpreted with caution especially in this variant. Electron microscopy and genetic testing can be very helpful in distinguishing signet ring cell mesothelioma from its mimickers.