Non-Asbestos-Related Malignant Peritoneal Mesothelioma: A Diagnostic Challenge in a Case of Unexplained Ascites

Malignant peritoneal mesothelioma (MPM) is a rare and aggressive malignancy arising from mesothelial cells of the peritoneum. It is most frequently linked to asbestos exposure, although up to half of cases occur without such history. Because of its vague and non-specific presentation, diagnosis is often delayed; cytology is frequently non-diagnostic, and histological confirmation with immunohistochemistry is required. Prognosis remains poor, although advances in cytoreductive surgery and intraperitoneal chemotherapy have improved outcomes in selected patients. We describe a man in his late 60s with no history of asbestos exposure who presented with progressive abdominal distension over 18 months. He underwent repeated paracenteses with negative cytology and multiple imaging studies, initially attributed to an intraductal papillary mucinous neoplasm (IPMN) under surveillance. Subsequent computed tomography (CT) demonstrated extensive peritoneal disease with omental caking and splenic lesions. Omental biopsy confirmed epithelioid-type MPM with BRCA1-associated protein 1 (BAP1) loss on immunohistochemistry. Given advanced disease and poor performance status, the patient was deemed unsuitable for surgery or chemotherapy and was managed with best supportive palliative care. This case highlights the diagnostic challenges of MPM, particularly in the absence of asbestos exposure and in the presence of coexistent pathology, and underscores the importance of early histological evaluation in patients with unexplained recurrent ascites.