The thymus, a small organ beneath the breast bone, is part of the lymphatic system. It produces a type of white blood cell called a lymphocyte, which helps the body fight infections. The thymus is most active during childhood, reaches its greatest size at puberty, and then gradually shrinks in adulthood.
Tumors in this organ -- thymoma and thymic carcinoma -- are thought to develop in epithelial cells, the cells that line the thymus. About 90 percent of tumors of the thymus are thymomas. Many thymomas grow slowly and rarely spread outside of the thymus. However, some thymomas can be more aggressive and spread to neighboring organs in the chest or to the lining of the lung, which is called the pleura. It is an uncommon cancer -- diagnosed in nearly 500 people each year -- that is most often found in people between the ages of 40 and 60, but can be diagnosed in people of any age.
A small percentage of people with thymic tumors have thymic carcinoma. Thymic carcinoma is made up of cells that resemble those of cancers in other organs of the body, is more likely to spread outside the thymus, and may be more difficult to treat than thymoma.
Some people with thymoma also have an autoimmune disorder. The condition most often associated with thymoma is myasthenia gravis, which is found in as many as 30 percent of thymoma cases. In myasthenia gravis, impulses from the nerves are not transmitted properly to the muscles, resulting in muscle weakness. The muscles that control eye and eyelid movement, facial expression, and swallowing are often affected. Red cell aplasia, a form of anemia, and hypogammaglobulinemia, a type of immune deficiency that can make people more prone to specific infections, can also accompany thymoma. The diagnosis of these conditions sometimes leads to the discovery of a thymoma.
