[Cystic thymoma; report of a case].

We present a case of a 33-year-old man with a noninvasive thymoma undergoing extensive cystic degeneration. The mediastinal tumor was asymptomatic and first noted on a routine chest radiograph. Chest computed tomography (CT) and magnetic resonance imaging (MRI) showed a 11 cm cystic mass with some solid portions in the anterior mediastinum. A cystic thymoma was suggested. The mass and remnants of the thymus were removed by video-assisted thoracoscopic surgery. On cut section, the tumor was predominantly cystic, with several solid nodules randomly attached to the cyst wall. The cystic space was filled with turbid brown fluid. Histopathological examination revealed a World Health Organization (WHO) type B3 thymoma with foci of hemorrhage, necrosis and cystic degeneration, and absence of an epithelial lining of the cyst.