An 81-year-old female patient was referred to the cardiology department because of cardiomegaly on chest radiography () and non-contrast computed tomography (CT) (). She had a 12-year history of dermatomyositis and interstitial pneumonia that were controlled with cyclosporine (125 mg/day). She reported shortness of breath on exertion. Her physical examination results were normal, except for fine crackles in the lower lung fields. Electrocardiography showed sinus rhythm with various abnormalities including negative T-waves in leads I, II, III, aVL, aVF, and V2–6 (). Transthoracic echocardiography and transoesophageal echocardiography revealed a heterogeneous hypoechoic tumour attached to the epicardial free wall of the left atrium and ventricle with a diameter of 83 × 56 mm ( and ). No pericardial effusion or haemodynamic impairment was observed. Colour Doppler imaging revealed feeding vessels within the mass (). A laboratory analysis revealed elevated levels of soluble interleukin-2 receptor (1034 U/mL; reference values, 122–496 U/mL), β2 microglobulin (3.3 mg/L; 0.6–1.6 mg/L), N-terminal pro-brain natriuretic peptide (209 pg/mL; 0–125 pg/mL), and sialylated carbohydrate antigen KL-6 (573 U/mL; 0–499 U/mL). Cancer antigen 19–9 (58.0 U/mL; 0–36.9 U/mL), cancer antigen 125 (45.7 U/mL; 0–34.9 U/mL), squamous cell carcinoma antigen (1.5 ng/mL; 0–1.2 ng/mL), and cytokeratin 19 fragment (3.6 ng/mL; 0–2.0 ng/mL) levels were slightly elevated. Electrolyte levels and complete blood counts were normal, and serological screening tests for human immunodeficiency virus, hepatitis B virus, and hepatitis C virus showed negative results. Whole-body contrast-enhanced CT revealed no early contrast enhancement, whereas an inhomogeneous contrast enhancement was observed in the delayed phase ( and ). No lymph node enlargement or other tumour lesions were observed. Coronary CT angiography showed feeding vessels from the left circumflex artery and the posterolateral branch of the right coronary artery (). 18F-fluorodeoxyglucose positron emission tomography (18F-FDG-PET) showed increased uptake of the mass, with a maximum standardized uptake value of 12.8, consistent with a malignant rather than benign mass ( and ). No other lesions with abnormal uptake were observed, indicating metastasis. Bone marrow tests did not reveal tumour invasion. Although malignant lymphoma or lymphoproliferative disorders were suspected from above examinations, a pathological examination was required for a definitive diagnosis. Considering the tumour location, a needle biopsy with total endoscopic anterolateral mini-thoracotomy was performed. Under general anaesthesia, a double-lumen endotracheal tube was inserted for right single-lung ventilation. In a supine position with 30° elevation of the left side, a 5-cm skin incision was made in the fourth intercostal space at the left anterior axillary line. An 11-mm camera port was inserted through the fourth intercostal space in posterior side of the skin incision, and a 5-mm additional working trocar port was inserted through the sixth intercostal region at the anterior axillary line. Meanwhile, bilateral femoral veins were exposed for venovenous extracorporeal membrane oxygenation (VV-ECMO) in case hypoxaemia occurred, because her spirogram demonstrated reduced respiratory function due to interstitial pneumonia. The pericardium was opened longitudinally, and the tumour was exposed. It was soft and had irregular surface with relatively clear boundary (). Pericardial effusion was a little and serous. The tumour was punctured with a biopsy needle twice, and adequate tissue specimens were obtained. Although mild hypoxaemia occurred before the needle biopsy, bilateral lung ventilation was used temporarily and she tolerated without the use of VV-ECMO. A 19-Fr chest drain was inserted through the trocar port site, the pericardium and the wound were closed, and the procedure was completed. Histopathological examination of the mass showed moderate-to-large malignant lymphoid cells with high nuclear-cytoplasmic ratios on haematoxylin and eosin staining ( and ). Immunohistochemistry results were as follows: CD20+, CD3-, CD5-, CD10-, Bcl6-, MUM1+, and EBER+ ( and ); these results were consistent with a diffuse large B-cell lymphoma (DLBCL) of non-germinal centre phenotype. The patient did not have B symptoms and the international prognostic index was 2 (age, 81 years; Performance Status, 2). She received systemic chemotherapy with a combination of rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisolone (R-mini-CHOP), and 1-month follow-up transthoracic echocardiography and 18F-FDG-PET revealed tumour regression ( and ). After completing six cycles of chemotherapy, the patient achieved complete remission and is still alive more than one and a half years after the initial diagnosis.