Primary cardiac synovial sarcoma 

Synovial sarcoma is a soft tissue malignant tumor that mostly affects children and young adults. Primary cardiac synovial sarcoma (CSS) is a very rare heart sarcoma that affects the pericardium or chambers, with a strong male predominance in the first decades of life.

Synovial sarcoma is rare and makes up less than 1% of all primary cardiac tumors. This is a very aggressive, and even with surgery and adjuvant chemoradiation, survival is usually less than nine months in this location. Only a few cases of primary cardiac synovial sarcoma have been documented in the literature. In a recent instance, a patient is still alive 22 months after main surgery, with a minor tumor recurrence but no severe symptoms at the Mayo Clinic, Rochester, Minnesota.
source: EACTS