Thymoma is a type of cancer that begins in the thymus. Located under the breastbone, also called the sternum, the thymus is a small organ that is part of the body’s immune system.

About the lymphatic system

The lymphatic system is made up of thin vessels that branch out to all parts of the body. These vessels carry lymph. Lymph is a colorless fluid that carries away waste and transports a type of white blood cell called lymphocytes. Lymphocytes fight germs throughout the body. B-lymphocytes, or B cells, make antibodies to fight bacteria. T-lymphocytes, or T cells, destroy foreign cells and trigger the B cells to make antibodies.

Early in life, the thymus is involved in the development and maturation of white blood cells, in particular T-lymphocytes. T-lymphocytes eventually travel to lymph nodes and around the body. In adulthood, the thymus is quite small and often replaced by fat.

Within the lymphatic system, there are several other structures. Lymph nodes are tiny, bean-shaped organs that filter the lymph fluid. Lymph nodes are found in clusters in the neck, underarms, abdomen, pelvis, and groin. When bacteria get into the lymph nodes, they swell. Other lymphatic organs include the spleen, which makes lymphocytes and filters blood, and the tonsils, located in the throat.

About thymoma and thymic carcinoma

Cancer begins when healthy cells change and grow uncontrollably, forming a mass called a tumor. A tumor can be cancerous or benign. A cancerous tumor is malignant, meaning it can grow and spread to other parts of the body. A benign tumor means the tumor can grow but will not spread.

The thymus contains two main types of cells:

  • Epithelial cells. Thymic epithelial cells are the cells that line the thymus. This is where thymoma and thymic carcinoma start.

  • Lymphocytes. If lymphocytes become cancerous, they can develop into lymphoma. Learn more about Hodgkin lymphoma and non-Hodgkin lymphoma.

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 lining of the lung, called the pleura. Less often, it can spread to other parts of the body.

Thymic carcinoma (see Stages) also starts in the thymus, but it is more likely to spread to the lining of the lungs and other parts of the body. Thymic carcinoma can also be more difficult to treat.

About 30% of people with thymoma also have a condition called myasthenia gravis. Myasthenia gravis is an autoimmune disorder caused by antibodies or T-cells that attack molecules, cells, or tissues of the person producing them. The main symptom of myasthenia gravis is fluctuating weakness in various muscles. It 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 more severe cases. Myasthenia gravis may appear before thymoma is diagnosed or it may develop during or after treatment.

In addition to myasthenia gravis, people with thymoma may have other groups of syndromes, called paraneoplastic syndromes. These may include severe low red blood cell count or anemia, called pure red cell aplasia, or low levels of antibodies known as immunoglobulins in the blood, called hypogammaglobulinemia.