[Matured mediastinal teratoma with a giant cyst resembling pleural effusion on X-ray films].

A 23-year-old man complaining of right-sided chest pain was admitted to our hospital for further examination of an abnormal shadow on chest X-ray films. Initially, we suspected the abnormal shadow was that of a pleural effusion in the right thorax. Computed tomographic scans disclosed a tumor in the anterior mediastinum, adjacent to the abnormal shadow. After closer examination, particularly by fluoroscopy on catheterization, we recognized that the abnormal shadow was a giant cyst, and that it connected with a solid mass. There were no abnormal laboratory data on admission. An examination of fluid specimens from the giant cyst revealed high levels of the tumor markers SCC antigen, CEA, and CA 19-9. These findings together suggested that the solid mass with giant cyst might be a mediastinal teratoma. The tumor was completely resectable without serious complications. The giant cyst contained 1,200 ml of fluid, and occupied half the volume of the right thoracic cavity. Pathological examination disclosed that the resected mediastinal tumor was a matured teratoma. Although matured teratoma are often composed of cysts, those that radiologically resemble pleural effusion, as in this case, were considered uncommon.