Multicystic Mesothelioma of the Omentum Presenting as an Incidental Finding

Multicystic mesotheliomas of peritoneal origin (also termed benign multicystic mesotheliomas) are uncommon and are exceptionally rare in the omentum. Lack of familiarity of this entity could lead to overinterpretation as a malignancy or present a challenge at frozen section. We describe a case of multicystic mesothelioma arising in the omentum found incidentally at laparotomy for endometriosis. Frozen section evaluation presented a challenge. Grossly, the mass in the omentum was multicystic and mucoid, raising the concern for a mucinous neoplasm. The cysts, however, were lined by flattened cells without atypia. At permanent section evaluation, calretinin positivity confirmed the mesothelial origin. Awareness of this entity is important, as it may be interpreted as a malignancy, particularly intraoperatively.