Poorly differentiated monophasic synovial sarcoma of the mediastinum.

Poorly differentiated synovial sarcoma is a diagnostically challenging neoplasm. Most commonly they occur in the soft tissue of the extremities and are rare in the mediastinum. They can be indistinguishable from other "round cell tumors" based on the morphology alone or at times by immunohistochemical studies. Here in, we report an extremely rare case of metastatic poorly differentiated monophasic synovial sarcoma of the mediastinum without a known primary in a 30-year-old man. The imaging studies on admission showed 10 × 9.5 cm anterior mediastinal mass with multiple nodules in the lung and pleura along with multiple enlarged mediastinal and axillary lymph nodes. Histopathologic and immunohistochemical analysis supported the diagnosis of poorly differentiated synovial sarcoma, which was further confirmed by molecular genetic analysis.