Abstract Body (Do not enter title and authors here): Background
Primary cardiac lymphoma (PCL) is an extremely rare malignancy, accounting for approximately 1–2% of primary cardiac tumors and about 0.5% of extranodal lymphomas. It primarily affects immunocompetent adults, with a median age of 55–65 years, and male predominance (65–85%).

Case Presentation A 44-year-old male with well-controlled HIV on Rilpivirine presented with a 2-week history of progressive shortness of breath, facial edema, and exertional peripheral cyanosis. His physical exam confirmed facial and upper extremities edema and jugular venous distension suggestive of superior vena cava syndrome. Contrast computed tomography (CT) of the chest identified a large 12 × 6.2 × 7.5 cm lytic rib lesion invading the pleura and chest wall, accompanied by left supraclavicular, axillary, mediastinal, and upper abdominal lymphadenopathy. CT also showed a large (8.3 × 5.8 × 6.0 cm) right atrial mass causing near-complete occlusion of the superior and inferior vena cava (Fig 1A). A transthoracic echocardiogram with contrast-enhanced imaging confirmed a large right atrial mass extending into the IVC (Fig 1B). Cardiac magnetic resonance imaging (MRI) further detailed an 8.3 × 6.5 × 6.4 cm mass nearly filling the right atrium, infiltrating the interatrial septum with mass effect on left atrium and encroaching on the right superior and inferior pulmonary veins (Fig 2A). Biopsy of the rib lesion confirmed high-grade B-cell lymphoma with 11q aberration. The patient was started on Rituximab combined with E-POCH chemotherapy on day 7 of admission. A follow-up PET-CT scan after the first cycle showed significant regression of the intracardiac mass (Fig 2B.)

Discussion
PCL is an uncommon extranodal lymphoma, most often presenting as diffuse large B-cell lymphoma (DLBCL). High-grade B-cell lymphoma with 11q aberration (HGBCL-11q) is a distinct entity characterized by a unique 11q gain/loss cytogenetic pattern, typically lacking MYC rearrangement and frequently showing Burkitt-like morphology. Standard treatment for primary cardiac DLBCL involves immunochemotherapy, most commonly R-CHOP unless contraindicated. Curative regimens yield 5-year survival rates of 70–80% in selected cohorts; however, prognosis worsens with advanced or extracardiac disease.