An epithelial neoplasm arising from the thymus. It may be associated with myasthenia gravis, pure red cell aplasia, and hypogammaglobulinemia. It includes thymoma type B1 which is a thymoma of low grade malignant potential, thymoma type B2 which is a thymoma of moderate malignancy, and thymoma type B3 which is also known as well differentiated thymic carcinoma.