A thymic epithelial neoplasm characterized by the presence of expanded areas which resemble the normal thymic cortex. The neoplastic epithelial cells are small and scant and there is a dense T-lymphocytic component present. Areas of medullary differentiation with or without Hassall's corpuscles are also present. It may be associated with myasthenia gravis, pure red cell aplasia, and hypogammaglobulinemia. It has a low grade malignant potential. The majority of cases occur in the anterior mediastinum as Masaoka stage I tumors. A minority of the cases occur as stage II tumors.