Malignant Mesothelioma Extending from Peritoneum to Pleura: A Diagnostic Challenge and Case Report

Background: Sarcomatoid malignant peritoneal mesothelioma (SM-MPM) is an exceptionally rare and aggressive subtype of peritoneal mesothelioma, often presenting with nonspecific symptoms such as ascites. Due to its subtle clinical manifestations, diagnosis is frequently delayed, resulting in poor prognosis. Early and accurate identification is crucial for guiding appropriate therapeutic strategies. Case Presentation: We report the case of a 52-year-old woman with no history of smoking, alcohol use, or asbestos exposure, who previously underwent subtotal hysterectomy for uterine fibroids in 2017. Ten months prior to admission, she developed progressive abdominal distension, fatigue, and ascites, accompanied by elevated CA-125 levels. Initial symptomatic improvement was achieved with diuretics, but no definitive diagnosis was established. Eight months later, she was readmitted with recurrent ascites, chest tightness, and dyspnea. Imaging revealed right pleural effusion, pneumothorax, and peritoneal thickening. Pleuroscopic biopsy showed reactive mesothelial proliferation. One month into hospitalization, rapid re-accumulation of ascitic fluid prompted repeat imaging, which demonstrated irregular peritoneal thickening and nodularity. Laparoscopic peritoneal biopsy with immunohistochemistry confirmed the diagnosis of SM-MPM. The disease was staged as T3N0M1 based on the Peritoneal Cancer Index (PCI), indicating advanced disease with poor prognosis. The patient was referred to a specialized oncology center for further management. Conclusions: This case underscores the diagnostic challenges of SM-MPM, particularly in patients presenting with recurrent unexplained ascites and multiple serous effusions. It highlights the importance of maintaining a high index of suspicion and the pivotal role of laparoscopy, immunohistochemistry, and multidisciplinary collaboration in achieving timely diagnosis and optimizing patient outcomes.