A80-4-32 Uncommon Space for a Rare Tumor: Posterior Mediastinal Synovial Sarcoma Causing Cardiac Compression

Abstract Introduction Synovial sarcoma is a rare malignancy of mesenchymal origin, accounting for 5-10% of all soft tissue sarcomas. It carries a poor prognosis with poor median survival in advanced disease. Case Presentation A 29-year-old female who immigrated to the US from Niger in 2023 and with a history of right lower extremity amputation also in 2023 (no records available). She presented to the hospital with complaints of dyspnea, productive cough with non-bloody sputum. She denied fever, chills, weight loss, fatigue, hemoptysis, or chest pain. On admission, she was found to have two significant posterior mediastinal masses with right sided mass measuring 8.4 x 6 x 11.4 cm and the left mass measuring 7.4 x 7.8 x 13.4 cm (Figure 1), associated with bilateral effusions. She underwent right side diagnostic and therapeutic thoracentesis which drained 600cc of sanguineous fluid. Fluid cytology was negative for malignancy however patient underwent IR biopsy with FNA and core biopsy taken of left posterior mediastinal mass. Core biopsy resulted as poorly-differentiated synovial sarcoma. The patient was then discharged home and started on treatment with Adriamycin, Ifosfamide, and Mesna. She completed two cycles of therapy with interval imaging not yet completed. Discussion Synovial sarcoma is an uncommon mesenchymal malignancy that accounts for about 5-10% of all soft-tissue sarcomas, with an incidence of ∼900 new cases annually in the US. Our case is challenging as the patient underwent a right lower extremity amputation for an unknown reason, so a presumption was made that this may be primary pulmonary and mediastinal involvement. Primary pulmonary synovial sarcoma constitutes 0.5% of primary lung malignancies and 1% of soft tissue sarcomas with posterior mediastinal or pericardial involvement, which is even rarer. There are only a few case reports of pericardial invasion as the initial manifestation. Clinical symptoms are often nonspecific and radiologically mimic other conditions. The patient’s pulmonary/mediastinal SS with cardiac compression can complicate treatment as complete surgical resection is not possible, and the mainstay of therapy is systemic chemotherapy. Despite therapy, prognosis remains grim with a median survival of advanced disease estimated at ∼15 months and a 5-year survival 10% in metastatic disease. Our case represents the importance of consideration for synovial sarcoma despite its rarity when presented with a large mediastinal mass, and other etiologies seem unlikely. This abstract is funded by: None