Cardiac Lymphoma Presenting as Superior Vena Cava Syndrome Mimicking Venous Thrombosis.

Primary cardiac lymphoma is an exceptionally rare extranodal non-Hodgkin lymphoma that often presents with non-specific cardiopulmonary symptoms, resulting in delayed diagnosis. Superior vena cava (SVC) syndrome as the initial manifestation is particularly uncommon and may mimic more prevalent conditions such as thrombosis, mediastinal malignancy, or right atrial myxoma. We report a case of a 37-year-old immunocompetent woman who presented with progressive breathlessness, facial puffiness, distended neck veins, and upper limb edema for two weeks. Initial Doppler evaluation suggested internal jugular venous thrombosis with distal obstruction, and anticoagulation therapy was initiated. However, persistent symptoms prompted further evaluation. Transthoracic echocardiography demonstrated a large homogeneous right atrial mass extending into the coronary sinus, while computed tomography revealed a non-enhancing mass occupying the right atrium with extension into the superior vena cava. Cardiac magnetic resonance imaging was inconclusive. Given persistent symptoms and uncertainty regarding the diagnosis, surgical exploration with biopsy was performed. Histopathological examination with immunohistochemistry confirmed diffuse large B-cell lymphoma with CD20 and CD30 positivity, establishing primary cardiac lymphoma. This case highlights the variable presentation of primary cardiac lymphoma and the limitations of imaging in differentiating malignant cardiac masses from thrombus. Although uncommon, primary cardiac lymphoma should be considered in patients with unexplained SVC syndrome, particularly when anticoagulation fails or imaging demonstrates right atrial involvement. Early tissue diagnosis is essential, as prompt initiation of chemotherapy may improve outcomes in this aggressive yet potentially treatable malignancy.