Insidious Mass Within a Sinus.

A 75-year-old patient was incidentally found to have an intracardiac mass by echocardiography. Subsequent cardiac magnetic resonance imaging and cardiac positron emission tomography confirmed a large and possibly malignant mass extending from the right atrium into the coronary sinus. The patient underwent an intracardiac echocardiography guided biopsy, which revealed diffuse B-cell lymphoma, and is currently undergoing rituximab, etoposide, vincristine, cyclophosphamide, and doxorubicin (R-EPOCH)-based chemotherapy.