[A case of thymic cyst differentiated from cardiac cyst with immunostaining].

This is a case of a 70-year-old woman with thymic cyst which was found by chance on admission for treatment of acute bronchial asthma and diagnosed by pathological examination. After remission of the asthma she was asymptomatic and no pathological physical findings were found except of cervical thyroid tumor of adenomatous goiter. Chest X-ray films revealed a large mediastinal mass with a sharp margin at the right cardiophrenic angle. Chest CT and MRI revealed that it occupied a space from upper anterior mediastinum to right cardiophrenic angle, and that it showed low density area without enhancement, suggesting a mediastinal cyst. A multilocular cyst, 15 cm in diameter, containing serous liquid was excised using thoracotomy. Pathological and immunohistochemical examination of the cyst revealed that its inner surface was lined with benign cylindrical epithelium and that thymic tissues existed in the walls of the cyst, resulting in a thymic cyst. The thymic cyst, of which reports have increased, should be considered in the differential diagnosis of anterior mediastinal cysts.