Malignant mesothelioma arising from a benign mediastinal mesothelial cyst.

A 50-year-old man presented with a cervical lymph node metastasis of malignant tumor and a radiologically documented well-defined tumorous mass in the middle portion of the mediastinum. Under the presumptive diagnosis of an undifferentiated bronchogenic carcinoma, the patient underwent chemotherapy. However, the tumor did not respond to this treatment and showed further progression. The patient died four weeks after the lymph node biopsy. Autopsy revealed a well-defined cystic tumor in the mediastinum with infiltration of the pericardium. The tumor was not attached to the lungs. By histology and immunohistochemistry, a metastasizing malignant biphasic mesothelioma was diagnosed. The tumor had metastasized to intrapulmonary perivascular and peribronchiolar lymph vessels and hilar lymph nodes. Distant metastases were found in the right and left adrenal gland and in the brain. The strictly mediastinal localization and the cystic growth pattern of the tumor strongly suggest that this mesothelioma developed from a primary benign mesothelial cyst of the pericardium with secondary transformation into a malignant mesothelioma.