A neoplasm originating from thymic tissue, usually benign, and frequently encapsulated. Although it is occasionally invasive, metastases are extremely rare. It consists of any type of thymic epithelial cell as well as lymphocytes that are usually abundant and probably not neoplastic. Malignant lymphomas that involve the thymus, e.g., lymphosarcoma, Hodgkin's disease (previously termed granulomatous thymoma), should not be regarded as thymoma. (Stedman, 25th ed).