A Biphasic Tumor of the Mediastinum with Features of Synovial Sarcoma

Four cases of a biphasic mediastinal tumor histologically identical to synovial sarcoma of soft tissue were observed. The tumors presented as solitary mediastinal masses. Although the tumors were frequently adherent to adjacent pleura or pericardium, none appeared to be arising from a mesothelial surface. All cases were composed of an intimate admixture of keratin-positive epithelial cells and vimentin-positive spindle cells with areas of transition, hyalinization, and calcification. Follow-up was available on three patients, who died of their disease 10 months, 14 months, and 4 years after diagnosis, respectively. Although synovial sarcoma usually occurs in deep soft tissues near joints in the extremities, it has been reported in locations removed from synovial, tendon sheath, and bursal structures. This report adds a previously unrecognized location for synovial sarcoma, the mediastinum. Occurrence in this location further supports an origin from pluripotential mesenchyme as opposed to synovium. The differential diagnosis includes mesothelioma, thymoma, germ cell tumors, malignant peripheral nerve sheath tumor with glandular differentiation, and metastatic carcinoma.