A 77-year-old man who had had undergone mitral valve replacement 5 years previously was admitted to our hospital because of chest discomfort. A small mass near the left side of the heart had been detected 2 years earlier by chest roentgenography. The mass had gradually increased in size. The patient had been receiving anticoagulant therapy with warfarin since the previous operation. He had taken an average dose of 10 mg of warfarin daily for 2 years. The international normalized ratio (INR) range was kept at 2.0-2.5. He had no history of pulmonary tuberculosis or chest trauma. Laboratory studies revealed chronic hypochromic anemia, with a hemoglobin level of 11.9 g/dL and hematocrit level of 35.5%. There was no apparent increase in the levels of tumor markers, including carcinoembryonic antigen (CEA), carbohydrate antigen 19-9 (CA19-9), neuron-specific enolase (NSE), squamous cell carcinoma-related antigen (SCC), and interleukin-2 receptor.
Chest roentgenography revealed an enlargement of the left side of the heart.
Contrast-enhanced computed tomography (CT) demonstrated a huge mass adhered to the left atrial appendage and covered with pericardium. The mass was not uniform; it lacked calcification and showed non-homogeneous enhancement. The mass measured 9 x 6 x 4 cm (Figure 1a, 1b). 
On magnetic resonance imaging (MRI), a T1-weighted image (T1WI) demonstrated a well-defined mass of slightly high intensity (Figure 2a). A T2-weighted image (T2WI) demonstrated a mixture of high- and low-intensity areas (Figure 2b). A low-intensity septum and a peripheral rim were observed in the mass on both T1WI and T2WI. 
FDG-PET images revealed uptake in the peripheral rim of the mass (Figure 3a, 3b). The maximum standardized uptake value (SUV) of this lesion was 3.50. Other signs of abnormal uptake suggesting a malignant lesion were not observed. The patient fasted for 6 h before receiving an intravenous injection of 18 F-FDG (5 MBq/kg). FDG PET/CT scans were obtained using Biograph 16 (Siemens Medical Solutions; Knoxville, TN, USA) scanners, with a 700-mm field of view (FOV) and a slice thickness of 3.27 mm. The CT was acquired to correct PET transmission using the following parameters: 140 kV and 120-240 mAs to produce 128 x 128 matrix images. The patient was scanned in the arms-down position, from head to thigh. Shallow breathing was advised to avoid motion artifacts and minimize misregistration of CT and PET images. Intravenous contrast material was not administered for CT scanning. After the CT scan, the PET data were acquired, and acquisition time was 3 min per bed position. CT images were reconstructed using the conventional filtered back-projection method. Axial full width at half-maximum at 1 cm from the center of the FOV was 6.3 mm. 
The findings were interpreted as being suggestive of hematoma associated with a malignant lesion. We could not rule out a pericardial or mediastinal malignant tumor with bleeding.
After obtaining informed consent, we performed surgical resection and found an elastic, hard mass covered with pericardium. The mass was widely adhered to the left atrial appendage, which was carefully detached. Partial pericardiectomy and complete removal of the mass were successful. The location of the mass was distinct from the anastomosis site in the left circumflex artery, and the source of bleeding seemed to be the branch of the left coronary artery. Rapid pathological examination identified a hematoma without neoplastic changes. Macroscopic observations showed that the resected mass consisted of a dark red, partially organized hematoma containing a small amount of liquid with a fibrous membrane (Figure 4a). The result of bacterial culture was negative. Pathological examination showed a hematoma surrounded by hyaline fibrous tissues, and the center of the hematoma consisted of fresh and old hemorrhages. Focal infiltration of hemosiderin-laden macrophages was observed in the outer zone of the peripheral wall. No malignant change was observed (Figure 4b). This confirmed a diagnosis of chronic expanding intrapericardial hematoma. From the histological viewpoint, the low-intensity septum and a peripheral rim on both T1WI and T2WI corresponded with the pseudocapsule of hyaline fibrous tissue. The mixture of high- and low-intensity areas on T2WI corresponded with fresh and old hemorrhages. The area of focal infiltration of hemosiderin-laden macrophages was consistent with high FDG uptake in the peripheral area of the mass. 
The patient had an uneventful postoperative recovery, without complications. His chest discomfort was alleviated. Approximately 2 years after the operation, there is no sign of recurrence.