Rapid enlargement of a mediastinal mass: thymoma hemorrhage into a thymic cyst.

A 52-year-old man was admitted to Matsudo City Hospital because of an anterior mediastinal mass. One month prior, when he suddenly suffered chest pain, computed tomography (CT) showed a 4 x 3 cm mass in the chest. On admission, the symptom disappeared. However, chest CT revealed rapid enlargement of the mass to 10 x 6 cm. Based on the CT findings, we suspected a tumor. Therefore, we performed a thymothymectomy. The mass, measuring 12 x 8 x 6 cm, contained a solid tumor and a fluid-filled cyst. Pathology showed a thymoma and a multilocular thymic cyst. We speculated that hemorrhage from the thymoma spread into the multilocular thymic cyst and enlarged the mass.