Thymoma

Definition

A thymoma is a tumor of the thymus gland.

Description

The thymus gland is located near the heart and between the lungs. It is a lymphoid tissue and is part of the immune system. The thymus is very active in the first fifteen years of life. During this time it grows in size. It then becomes less active and starts to shrink.

Thymomas are classified by the type of tissue involved in the tumor, such as epithelial or endothelial tissue. Some forms of thymoma, such as those of epithelial tissue, do not tend to spread to other parts of the body. Other forms of thymoma will spread, usually by traveling through the lymph system. Thymomas from lymphoid tissue tend to spread widely throughout the body, causing a non-Hodgkin's lymphoma. Another form of thymoma, granulomatous thymoma, results in a disease that mimics Hodgkin's lymphoma. Thymic teratoma is benign in women, but malignant in men. Thymic teratoma is a tumor in the thymus, but is composed of tissue from other parts of the body.

Causes & symptoms

Typically, when a thymoma is present, part of the thymus will enlarge, sometimes causing coughing, shortness of breath, or chest pain. Thymomas are more common in people who have received radiation treatment to the head, neck, or chest regions. The disease myasthenia gravis, an autoimmune disease, is associated with the development of thymoma. (About 30% of thymomas occur in people with myasthenia gravis; about 15% of people with myasthenia gravis develop thymomas.) Thymomas may interfere with the function of the immune system, creating an increased susceptibility to infection. Thymomas can spread by means of the lymph system and blood circulation.

Diagnosis

Thymomas are often first identified by a chest x ray, in many cases, before they even produce symptoms. Computed tomography scans (CT scans) can help doctors better see the extent of the tumor. Other tests to evaluate the extent of the thymoma include a barium x ray of the esophagus, an examination of the bronchial tubes in the lungs (bronchoscopy), and an examination of the movement of blood through veins near the thymus gland (venous angiography). Thymomas must be distinguished from noncancerous nodules and inflammation of the thymus. A biopsy, or small sample of tissue, may be taken from the thymus growth and examined through a light microscope to detect cancerous cells. Usually, examination of biopsy material is the best method to distinguish thymomas from noncancerous nodules.

Treatment

Most thymomas can be cured by removal of the tumor. This sometimes requires removal of a major portion, if not all, of the thymus. Regional lymph nodes are sometimes removed at the same time, in case the cancer has started to spread. After surgery, radiation therapy may be used to kill any cancer cells that may remain.

In persons in their late teens and older, removal of the thymus does not present difficulties in responding to future infections. Their immune systems should be able to function without the thymus. Children whose thymus has been removed before the age of 15 may suffer immunodeficiency diseases because the thymus plays a major role in the development of the cellular branch of the immune system.

Prognosis

Thymomas can usually be treated successfully. If the cancer is identified, and removed or treated before it extends into the lymphatic system, the cure rates are very good. The exception is anaplastic thymoma, which appears mainly in elderly women and grows very rapidly. Approximately 80 % of patients with anaplastic thymoma die within one year.

Key Terms

Malignant

Threatening to life. Having a tendency to spread to other sites.

Further Reading

For Your Information

Books

• Berkow, Robert, ed. Merck Manual of Medical Information. Whitehouse Station, NJ: Merck Research Laboratories, 1997.
• Sabiston, D.C. and H.K. Lyrly. Essentials of Surgery. Philadelphia, PA: W.B. Saunders Company, 1994.
• Way, L.W. Current Surgical Diagnosis and Treatment. Norwalk, CT: Appleton and Lange, 1994.

Gale Encyclopedia of Medicine. Gale Research, 1999.