The thymus, a small organ located beneath the breastbone, is part of the lymphatic system — a network of thin tubes that carry infection-fighting white blood cells called lymphocytes throughout the body. The thymus produces T-lymphocytes, which kill viruses, and prompts B-lymphocytes to make antibodies that fight bacteria. The thymus is most active during childhood, reaches its greatest size at puberty, and gradually shrinks in adulthood.
Types of Thymic Tumors
Tumors of the thymus gland are uncommon, and account for less than 1 percent of all cancers. Three main types of tumor can arise in the thymus.
About 90 percent of tumors that develop in the thymus are thymomas. These tumors begin in the epithelial cells that line the thymus. Many thymomas grow slowly and rarely spread outside of the thymus. However, some thymomas are more aggressive and can spread to neighboring organs in the chest or to the lining of the lung, called the pleura. Each year, fewer than 500 people — mainly between the ages of 40 and 60 — are diagnosed with this uncommon type of cancer in the United States.
Some people with thymoma also have an autoimmune disorder. Myasthenia gravis, which is found in as many as 30 percent of thymoma cases, is the autoimmune condition most often associated with thymoma. In myasthenia gravis, impulses from the nerves are not transmitted properly to the muscles, resulting in severe muscle weakness. The muscles that control eye and eyelid movement, facial expression, and swallowing are often affected.
An estimated 5 to 10 percent of people with thymoma also have a form of anemia known as red cell aplasia, or hypogammaglobulinemia, a deficiency of immune-fighting gamma globulins that increases susceptibility to infections. Diagnosing these conditions sometimes leads to the discovery of a thymoma.
Between 5 and 10 percent of thymic tumors are classified as thymic carcinoma. Like thymomas, thymic carcinoma also begins in the epithelial cells that line the thymus. However, when viewed with a microscope, these cells appear very different from normal thymus cells. Thymic carcinomas also grow more rapidly than thymomas, and are more likely to spread outside the thymus, making them more difficult to treat.
Carcinoid tumors are a type of neuroendocrine tumor — a slow-growing tumor that most often arises in the hormone-producing cells that line the organs of the digestive tract and lungs. Although carcinoids are the most common type of neuroendocrine tumor, carcinoids that arise in the thymus are quite rare. They occur more frequently in men than in women, and are more likely to spread or recur than thymomas.
Thymic carcinoids are often associated with multiple endocrine neoplasia type 1 (MEN-1) syndrome, a rare genetic disorder that can prompt the parathyroid glands, pituitary gland, and pancreas to produce too many hormones.
Many thymic tumors do not cause symptoms and are found during testing for other conditions. When patients do have symptoms, these can include:
- a persistent cough
- muscle weakness
- chest pressure or chest pain
- trouble breathing
Some thymic carcinoids produce too much ACTH, a substance that causes the adrenal glands to produce an excess amount of cortisol and other hormones. This can lead to symptoms such as:
- weight gain
- muscle weakness
- increased body and facial hair
- thin or darkened skin (Cushing's syndrome)