Abstract Introduction A 72-year-old lady presented with one-week history of palpitation and shortness of breath. She had pyrexia of unknown origin for 4 weeks associated with weight loss. Initial clinical examination revealed sinus tachycardia of 110 beats/min and saturations of 96% on 2L/min oxygen. Neck exam revealed a 2 x 3 cm firm mass at left supraclavicular fossa. Electrocardiogram showed sinus tachycardia at 106 beats/min. There was fixed T wave inversions over anterolateral chest leads and inferior limb leads. Chest radiograph showed enlarged cardiac silhouette and evidence of pulmonary congestion Procedure Transthoracic echocardiogram revealed a moderate pericardial effusion with an 8 x 4 cm pericardial mass compressing at the right atrioventricular groove (Figure B & C). Respiratory variation of Doppler mitral and tricuspid inflow velocities suggested cardiac tamponade (Figure A). Inferior vena cava was engorged with impaired inspiratory collapse. Left ventricular size and function were otherwise normal. Urgent pericardiocentesis was performed which yielded 1 litre of light blood stained fluid. Her had subjective improvement of symptoms. There was also resolution of sinus tachycardia and pulmonary congestion. PET/CT scan showed multiple enlarged hypermetabolic lymph nodes in multiple regions above and below the diaphragm, worrisome for malignant lymphoma. Cytological assessment of pericardial fluid showed scattered large lymphoid cells which are immunoreactive to B cell marker CD 20 (Figure E & F). Bone marrow exam showed normal marrow. First cycle of R-CEOP and intrathecal methotrexate was given. Follow-up echocardiogram in one week after chemotherapy showed complete resolution of pericardial effusion. Prior pericardial mass also showed marked reduction in size (Figure D). No tamponade physiology was present. She tolerated the chemotherapy well with complete resolution of palpitation and shortness of breath. She was discharged from hospital with outpatient follow up and continuation of chemotherapy course. Discussion Cardiac tumors are rare and secondary tumors remain the most common etiology. Most of these metastatic tumors arise from solid organs such as lung, breast, kidney, melanoma as well as lymphoma. Echocardiography is a valuable tool both in diagnosis and assessment of hemodynamic significance of cardiac masses. It also aids in diagnosis of associated condition such as pericardial effusion. In this case, the cardiac tamponade is both contributed by mechanical compression of cardiac mass coupled with moderate pericardial effusion. Successful pericardiocentesis achieved temporary normalization of tamponade physiology and aided in early histological diagnosis of malignant lymphoma with pericardial metastasis. Timely initiation of intensive systemic chemotherapy was the key to rapid tumor size reduction, complete resolution of tamponade physiology and control of recurrent pericardial effusion. Abstract 502 Figure. Echo & histology