In meicine (oncology), thymoma is a tumor originating from the thymus. It consists of any type of thymic epithelial cell as well as lymphocytes that are usually abundant and probably not neoplastic. Thymoma usually is benign, and frequently encapsulated; when occasionally malignant, then it is invasive: metastasis is extremely rare. Malignant lymphomas that involve the thymus, e.g., lymphosarcoma, Hodgkin's disease (termed "granulomatous thymoma" in the past), should not be regarded as thymoma. Thymoma is a rare disease, best known for its enigmatic association with the neuromuscular disorder myasthenia gravis.