A thymic epithelial neoplasm characterized by the presence of neoplastic large, polygonal epithelial cells with large vesicular nuclei and prominent nucleoli. The neoplastic cells are arranged around perivascular spaces and along septa. Immature T-lymphocytes are also present. It may be associated with myasthenia gravis, pure red cell aplasia, and hypogammaglobulinemia. It is a tumor of moderate malignancy. The majority of cases occur in the anterior mediastinum as Masaoka stage I, stage II, or stage III tumors. Metastatic, stage IV tumors occur less frequently.