Malignant thymoma is a cancer of the thymus, a tiny organ that plays an important role in the immune system. This type of cancer tends to develop on the outside surface of the organ, rather than in its interior. In the early stages, thymoma can be treated effectively with surgery or radiation therapy. In the advanced stages, treatment is difficult, because the heart can become affected. This cancer usually is slow-growing, however, and it often is diagnosed early enough for treatment to be effective.
The thymus is located in the upper chest, just below the sternum. One of the most important processes in the immune system is called T cell selection, and it occurs in the thymus. This process results in the generation of T lymphocytes, which are capable of protecting the body from infection.
There are two types of cancer that can affect the thymus. One is called thymic cancer. This generally is a fast-growing cancer that often spreads to other parts of the body and is very difficult to treat. By contrast, malignant thymoma is a slow-growing type of cancer that often does not spread to other parts of the body. This type of tumor is much more easily treated than thymic cancer.
Thymoma tumors do not always cause symptoms. Often, this lack of symptoms means that the cancer is diagnosed during a routine examination or during a chest X-ray or other medical imaging scan that is performed for unrelated reasons. When symptoms do appear, they usually include chest pain and breathing difficulty or a recurrent cough.
This type of cancer is unusual in that it often is diagnosed, staged and treated as part of the same surgical procedure. The first part of the procedure is a biopsy, during which a small sample of tumor cells are removed and then tested for the presence of cancer. If thymoma or thymic cancer is diagnosed, the thymus is then removed. Other tissues such as nearby lymph nodes might also be removed, a procedure that is more likely when thymic cancer has been diagnosed. After surgery, it is common for a patient to undergo chemotherapy or radiation therapy, in order to kill any cancer cells remaining in the body.
People with certain autoimmune diseases have an increased risk of developing thymomas. Autoimmune diseases are those that cause the body’s immune system to attack its own tissues. The diseases that increase the risk of this type of cancer include myasthenia gravis, rheumatoid arthritis, thyroiditis, lupus erythematosus and Sjogren syndrome. The reason these diseases increase the risk of thymomas is not known, but it is clear that the immune system is an important part of the link.