Thymomas and thymic carcinomas are rare tumors of the outside surface of the thymus, a small gland under the breastbone. As part of the lymph system, the thymus produces white blood cells called lymphocytes, which travel through the body and fight infection.

Thymoma is a relatively slow-growing cancer and usually does not spread beyond the thymus. However, thymic carcinoma grows faster and typically spreads to other parts of the body. Because of this, thymic carcinoma is often more difficult to treat effectively. Patients who have myasthenia gravis are at increased risk of getting thymoma.