Primary Cardiac Synovial Sarcoma of the Left Ventricular Outflow Tract Mimicking Cardiac Myxoma

Abstract Background: Primary cardiac synovial sarcoma (PCSS) is exceedingly rare and typically presents with imaging features characteristic of malignancy, including aggressive growth, poorly defined margins, and limited mobility. However, a subset of cases may closely mimic a myxoma with a benign imaging phenotype, thereby posing a significant preoperative diagnostic pitfall and compromising treatment planning. Case summary: A 34-year-old man was admitted with acute cerebral infarction. Echocardiography and coronary CTA demonstrated a solid, pedunculated subaortic mass attached to the left ventricular outflow tract, with marked mobility and imaging features highly consistent with a myxoma. Given the acute phase of cerebral infarction and high perioperative risk, tumor resection was deferred following multidisciplinary team discussion. At 4-month follow-up, the mass showed a mild interval increase in size, and open-heart surgery with mass excision was performed. Intraoperatively, the tumor exhibited a botryoid, semi-translucent appearance but was soft externally and firm internally on palpation. Histopathology and immunohistochemistry revealed positivity for TLE1 and SS18-SSX, and FISH confirmed SS18 gene rearrangement, establishing the diagnosis of PCSS. Adjuvant chemotherapy was administered postoperatively, with no evidence of residual tumor on follow-up imaging. Discussion: This case highlights that PCSS may completely mimic the growth pattern of a myxoma in its early phase, including a pedunculated attachment, marked mobility, and well-circumscribed borders—imaging features that readily lead to a benign misdiagnosis. For masses located in atypical locations with benign imaging features but complicated by systemic embolization, a high index of suspicion for malignancy should be maintained.