Primary cardiac monophasic synovial sarcoma mimicking cardiac fibroma: Diagnostic challenges. A case report and brief literature review

Primary cardiac synovial sarcoma (PCSS) is an exceptionally rare malignant mesenchymal tumor, accounting for a small minority of primary cardiac sarcomas and often requiring histopathologic and molecular confirmation because clinical and imaging findings are usually nonspecific. We report a 47-year-old female with progressive exertional dyspnea, recurrent syncope, ventricular ectopy, and evolving heart failure who was initially considered to have hypertrophic cardiomyopathy and later a septal cardiac fibroma. 
Imaging demonstrated a large interventricular septal mass extending into the left ventricular outflow tract, with fibrotic enhancement and limited metabolic activity, features favoring a benign fibrous tumor but not fully excluding sarcoma. Surgical exploration revealed infiltrative myocardial involvement, and pathology demonstrated a highly cellular spindle cell neoplasm with necrosis, brisk mitotic activity, diffuse TLE1 positivity, focal epithelial marker expression, high Ki-67, and SS18-SSX2 fusion, confirming primary monophasic cardiac synovial sarcoma. 
Despite gross resection with narrow margins and adjuvant ifosfamide-doxorubicin chemotherapy, the patient developed early local recurrence, pulmonary and hepatic metastases, refractory biventricular failure, and died 8.5 months after surgery. This case highlights the capacity of cardiac synovial sarcoma to mimic cardiac fibroma (CF) radiologically, the importance of maintaining malignant spindle cell neoplasia in the differential diagnosis of adult fibroma-like septal masses, and the prognostic significance of early molecular diagnosis, margin status, and intensive surveillance.