[Cystic thymoma without solid mass lesions on diagnostic imaging--a case report].

A 68-year-old man was referred to our hospital because of an abnormal shadow on a chest X-ray film. A chest CT scan and MRI demonstrated a cystic mass without solid lesions on the anterior mediastinum. The CT scan also showed that it was a well-defined and homogenous mass without contrast enhancement effect. 67Ga and 201T1 scintigrams showed no uptake in the mass. Needle aspiration cytology was performed, and there was no evidence of neoplasms. A thymic cyst was diagnosed, and then an operation was performed. At the operation, the cystic tumor was removed along with thymic tissues by a median sternotomy. On the cut section of the resected specimen, the tumor was found to be a cyst with small solid nodules on the wall. Pathologic examination revealed that the tumor was a cystic thymoma. We consider that patients with cystic tumors on the anterior mediastinum should obtain histopathologic diagnosis because of the difficulty in excluding neoplasms such as thymoma.