70 A Rare Case of Localized Biphasic Pleural Mesothelioma with Distinct Morphologic and Immunophenotypic Features

Introduction/Objective Biphasic mesothelioma is a rare, aggressive tumor containing both epithelioid and sarcomatoid components. It often presents with a poor prognosis and can be challenging to diagnose. Morphological heterogeneity of biphasic mesothelioma has been well known for its diagnostic challenge and its broad differential diagnosis. Methods/Case Report We describe a rare case of a 54-year-old male with a history of smoking, decompensated cirrhosis, and interstitial lung disease who presented with a progressively enlarging anterior chest wall mass and unintentional weight loss. Imaging revealed a 3.8 x 2.5 x 1.8 cm isolated soft tissue lesion in the right anterior 4th intercostal space, along with suspicious metastatic lesions in the chest wall, thoracic lymph nodes, pleura, peritoneum, and bones. An incisional biopsy of the dominant chest wall mass revealed a tumor composed of both epithelioid and high-grade sarcomatoid components. The sarcomatoid areas showed pleomorphic features, including large, atypical cells with marked nuclear atypia, hyperchromasia, and numerous atypical or bizarre mitotic figures. The differential diagnosis included lymphoma, melanoma, epithelioid sarcoma, thymic tumors, poorly differentiated pulmonary carcinoma, neuroendocrine carcinoma, and metastatic carcinoma. A comprehensive panel of immunohistochemical stains was performed. Immunohistochemistry demonstrated that the epithelioid component was positive for pan-keratin, calretinin, HBME1, and D2-40, with loss of BAP1 expression. The sarcomatoid component showed a distinct immunophenotype, with loss of pan-keratin, WT1, calretinin, and D2-40, but positive expression for GATA3 and TLE1. SS18 rearrangement by FISH was negative. Additional markers including SOX10, LCA, synaptophysin, ERG, SMA, and desmin were negative. Results NA Conclusion This case highlights the diagnostic challenge of a rare case of localized biphasic malignant mesothelioma and its unique morphology and immunophenotypic profiles emphasizing the critical role of comprehensive immunohistochemical evaluation for accurate diagnosis and management.