[True thymic hyperplasia which was difficult to distinguish from malignancy in adult; report of a case].

We report a case of 45-year-old man with true thymic hyperplasia. Three years earlier he had undergone operation for carcinoma of the floor of mouth. He had no symptoms but had been pointed out an anterior mediastinal mass on chest computed tomography (CT). Chest CT revealed a well defined solid mass in front of the ascending aorta. The mass showed sail sign. The size of this mass did not increase on a follow-up chest CT. The signal intensity of this mass was slightly inhomogeneous on chest magnetic resonance imaging (MRI). No invasion of the surround tissues was observed. Since the possibility of thymoma or other malignancy, extended thymectomy under median sternotomy was performed. His postoperative course was uneventful. Histopathological examination revealed true thymic hyperplasia.