Overview

Thymoma is a type of cancer that begins in the thymus. The thymus is a small organ located under the sternum (breastbone) that makes white blood cells and is part of the lymph system.

The lymph system is made up of thin tubes that branch out to all parts of the body. Its job is to fight infection and disease. The lymph system carries lymph, a colorless fluid containing white blood cells called lymphocytes. Lymphocytes fight germs in the body. B-lymphocytes (also called B-cells) make antibodies to fight bacteria, and T-lymphocytes (also called T-cells) kill viruses and foreign cells and trigger the B-cells to make antibodies. The thymus is involved in the production and maturation of T-lymphocytes.

Groups of bean-shaped organs called lymph nodes are located throughout the body at different sites in the lymph system. Lymph nodes are found in clusters in the abdomen, groin, pelvis, underarms, and neck. In addition to the thymus, other parts of the lymph system include the spleen, which makes lymphocytes and filters blood, and the tonsils, located in the throat.

Cancer begins when cells become abnormal and begin to grow uncontrollably. These cells for a mass of tissue, called a tumor. The thymus contains two main types of cells: epithelial cells and lymphocytes. Thymic epithelial cells are the cells that line the thymus, and this is where thymoma and a specific type of thymoma known as thymic carcinoma start. If lymphocytes become malignant (cancerous), they can develop into a type of cancer called lymphoma; see the Cancer.Net Guide to Hodgkin Lymphoma and Cancer.Net Guide to Non-Hodgkin Lymphoma for more information. Rarely, another type of tumor called a carcinoid tumor can develop in the thymus.

Thymoma is generally a slow-growing tumor that does not usually spread outside of the thymus. Occasionally, it can spread to the pleura (lining of the lung). Less often, it can spread to other parts of the body.

Thymic carcinoma (see Staging) can also be localized to the thymus, but is more likely to spread to the pleura and other parts of the body. Thymic carcinoma can also be more difficult to treat.

In about 30% of people with thymoma, a condition called myasthenia gravis is also present. Myasthenia gravis comes from the Latin and Greek words meaning “grave muscular weakness.” It is an autoimmune disorder (a disorder caused by antibodies or T-cells that attack molecules, cells, or tissues of the organism producing them) that is characterized by fluctuating weakness of various muscle groups. Myasthenia gravis may affect any muscle that is under voluntary control, such as those that control eye movements, chewing, swallowing, coughing, and facial expression. Muscles that control breathing and movements of the arms and legs may also be affected.

In addition to myasthenia gravis, people with thymoma can also have other associated syndromes, which as a group are called “paraneoplastic syndromes”, including pure red cell aplasia (severe anemia [low red blood cell count]) or hypogammaglobulinemia (low levels of immunoglobulins [antibodies] in the blood).

Statistics

Thymoma is rare. Approximately 90% of tumors of the thymus are thymoma. The remaining 10% are thymic carcinoma, carcinoid tumors, or lymphomas. Approximately 500 to 700 people in the United States are diagnosed with thymoma each year. Thymoma usually occurs in people between the ages of 40 and 60. Survival rates range depending on several factors, including the stage and classification of thymoma (see Staging). The five-year relative survival rate for thymic carcinoids and carcinomas is 60% and 35% respectfully.

Cancer survival statistics should be interpreted with caution. These estimates are based on data from thousands of cases of this type of cancer in the United States, but the actual risk for a particular individual may differ. It is not possible to tell a person how long he or she will live with thymoma.

Source: American Cancer Society and the National Cancer Institute.

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