When Synovial Sarcoma Mimics the Heart’s Most Common Tumor

A case report in AME Case Reports by Yun Xiao, Zhourui Liu, and Sha Li of the Department of Ultrasound at Guizhou Medical University, China, documents a rare case of synovial sarcoma arising directly in the pulmonary artery, a location so uncommon that it was initially mistaken for a benign cardiac tumor.

What the Case Involved

A 35-year-old man presented with a month of exertional chest pain. Echocardiography revealed a solid mass at the pulmonary valve with a broad base and unclear border, initially suspected to be myxoma, the most common and almost always benign cardiac tumor. But several findings didn’t fit: contrast imaging showed only mild perfusion within the mass, lower than typical for myxoma’s usual brisk enhancement, and MRI and PET/CT showed ill-defined borders and unusual metabolic activity, raising suspicion for malignancy instead.

Surgery Revealed a More Complex Picture

Surgeons found two separate tumors, not one, attached to the pulmonary valve and right ventricular outflow tract. Both were removed and the valves reconstructed. Pathology showed a spindle cell tumor, and FISH testing confirmed the SS18 gene rearrangement, the molecular signature that definitively diagnoses synovial sarcoma. The final diagnosis was biphasic synovial sarcoma with a Ki-67 index of about 60%, indicating an aggressive tumor. Despite complete surgical removal, imaging six months later showed local recurrence and lung metastases, and the patient began chemotherapy.

Lessons for Diagnosis and Care

This case illustrates how easily synovial sarcoma can be mistaken for myxoma when it appears in the heart, since both can look similar on standard imaging. The authors point to specific red flags that should raise suspicion for malignancy instead: a broad base rather than a thin stalk, poorly defined borders, and only mild contrast perfusion rather than myxoma’s typical brisk enhancement.

The case also reinforces that molecular testing, not imaging or histology alone, is what confirms a synovial sarcoma diagnosis. And the rapid recurrence seen here, despite a technically complete resection, supports the authors’ recommendation that tumors with a high Ki-67 index be considered for prompt adjuvant therapy rather than observation alone.

For more information, read the full case report here. For more information about the Synovial Sarcoma Foundation, please visit our website.