Isolated primary pericardial sarcomatoid mesothelioma: a case report.

Primary pericardial mesothelioma is an exceedingly rare malignancy, often diagnosed late due to nonspecific clinical and radiologic findings. We report a case of isolated pericardial mesothelioma progressing from mesothelioma in situ to invasive sarcomatoid disease, presenting with recurrent effusions and constrictive pericarditis. A 58-year-old male with a remote history of Hodgkin's lymphoma (previously treated with chemotherapy and mediastinal radiotherapy) presented with progressive dyspnoea, orthopnoea, and recurrent pericardial and left pleural effusions. Initial diagnostic video-assisted thoracoscopic surgery (VATS) with mini-thoracotomy revealed densely adherent pericardium, intra-pericardial clots, and 1.5L of blood-stained pleural fluid. Histology demonstrated a sparse population of surfcatypical mesothelial proliferation with methylthioadenosine phosphorylase (MTAP) loss and retained BRCA1-associated protein 1 (BAP1). There was no invasive growth and therefore, the appearance for concerning for mesothelioma in situ, at a minimum. Cytology of the pleural fluid was non-malignant. Due to progressive constrictive physiology, a total pericardiectomy via median sternotomy was performed with femoro-femoral cardiopulmonary bypass. Intraoperatively, the pericardium was excised in the standard fashion for total pericardiectomy, and cardiopulmonary bypass was required due to dense epicardial adhesions. Final histopathology confirmed sarcomatoid mesothelioma with high-grade spindle cell morphology, brisk mitotic activity, necrosis, and MTAP loss. Immunohistochemistry was positive for WT1, calretinin, CK5/6, and D2-40. The patient was referred for oncology follow-up and underwent PET-CT staging for systemic therapy planning. This case highlights the importance of early diagnostic suspicion in patients with prior mediastinal irradiation who present with recurrent pericardial effusions or evolving constrictive physiology. While pericardiectomy remains the standard of care for constriction regardless of etiology, the diagnosis of mesothelioma-particularly sarcomatoid subtype-warrants timely intervention. Early recognition of pre-invasive histological changes may offer a critical opportunity for curative resection before progression to aggressive disease.