Background. Primary mediastinal paraganglioma is rare, accounting for only 2% of a large series of neural tumors of the thorax. Case. A 73-year-old Japanese man was hospitalized because of dyspnea, fever, general fatigue and cough. Chest X-ray and, CT and MRI of the chest revealed a tumor 14 cm in diameter which had invaded the pericardium, the inferior vena cava, the diaphragm, and the left atrium. The patient died of heart failure. At autopsy, the tumor was found to have originated from the

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... tside of the posterior atrium and to have invaded the mid-mediastinum without distant metastasis. Histologically, tumor cells were large-polymorphous and showed nuclear pleomorphism. Immunohistochemical staining showed that tumor cells were positive for synaptophysin and NSE, and electron microscopic examination revealed neurosecretory granules of the epinephrine type in the cytoplasm. Therefore, this tumor was diagnosed as cardiac malignant paraganglioma. Conclusion. An autopsy case of cardiac malignant paraganglioma is reported. Immunohistochemical staining and electron microscopy can be helpful particularly when light microscopic findings are atypical. (JJLC. 2004;44:241-244)