Abstract
Synovial sarcoma is a rare and aggressive mesenchymal neoplasm characterised by the presence of the SS18-SSX fusion oncogene, resulting from the chromosomal translocation t(X;18)(p11.2;q11.2). Although these tumours typically arise in the extremities, they have also been documented in atypical locations such as the pericardium, underscoring their versatile and aggressive nature. This case involves a 46-year-old male who presented with a 2-month history of neck and precordial chest pain, ultimately diagnosed with a biphasic synovial sarcoma of the pericardium. Initial imaging studies, including magnetic resonance imaging and transthoracic echocardiogram, revealed a large encapsulated intrapericardial mass with hemorrhagic and thrombotic components, severe pericardial effusion and biventricular dysfunction. Histopathological examination confirmed the diagnosis, with immunohistochemistry findings positive for CKAE1/AE3, TLE-1, EMA, BCL-2 and CD99, along with a proliferation index of 40%. The chemotherapy regimen of ifosfamide, mesna and doxorubicin proved effective for this condition, leading to a significant reduction in tumour size and metabolic activity. However, due to disease recurrence and the presence of a KDM5A-positive marker, second-line therapy with trabectedin and pazopanib became necessary.