Primary Mediastinal Synovial Sarcoma with Pericardial Involvement: A Case Report.

Introduction: Synovial sarcoma is a rare, malignant neoplasm accounting for approximately 5-10% of all soft tissue sarcomas. Synovial sarcomas generally occur in the extremities of adolescent or young adult (AYA)-aged patients; however, they have been reported to affect virtually every organ system. The mediastinum is an exceptionally rare location for synovial sarcoma and malignant pericardial effusions occurring secondary to synovial sarcoma can occur. Case Presentation: This case describes a 29-year-old male who presented with 3 weeks of dyspnea and leg swelling. An echocardiogram demonstrated a large pericardial effusion with early tamponade physiology. A subsequent pericardiocentesis returned a lymphocytic predominance with negative flow cytometry and cytology. Cross-sectional imaging with computed tomography (CT) scans of the chest, abdomen, and pelvis was negative for lymphadenopathy or detectable mass. He was discharged where repeat echocardiogram demonstrated recurrence of the effusion, but his symptoms had improved. Ultimately, he was readmitted with refractory dyspnea and edema where cardiac magnetic resonance imaging demonstrated a large mediastinal mass invading the pericardium. The biopsy demonstrated a high-grade spindle cell malignancy positive for a rearrangement of the SS18 gene, a finding diagnostic of synovial sarcoma. He passed away after one round of doxorubicin due to septic and obstructive shock (tamponade). Conclusion: Mediastinal synovial sarcomas are rare, aggressive, and highly morbid neoplasms. In some cases, they may present insidiously due to their sole presenting clinical manifestation being pericardial effusion. Particular care should be taken to rule out malignancy with MRI or PET-guided imaging in the AYA-aged population when idiopathic, recurrent pericardial effusion is found.