Medically Reviewed on 3/7/2023

What is thymoma?

Thymoma is a rare tumor in the thymus gland.
Thymoma is a rare tumor in the thymus gland.

A thymoma is a rare type of tumor of the thymus gland.

  • The thymus is a gland located in the anterior mediastinum (the area between the two lungs and the sternum in the chest) that plays a critical role in the development of immune cells (lymphocytes) during childhood.
  • The thymus gland enlarges during childhood, peaks in size at puberty (about 40 grams), and then begins to shrink.

Normally, a combination of lymphoid cells (immune cells or lymphocytes) and lining cells (epithelial cells) makes up the thymus. Thymoma is a type of tumor that originates from the epithelial or lining cells of the thymus.

  • The term thymic neoplasms refer to tumors of the thymus, which consist of thymomas and thymic carcinomas.
  • The term thymoma refers to tumors of the thymus that grow slowly and usually do not spread beyond the thymus.

Thymic carcinomas are tumors of the thymus that grow aggressively and may metastasize to distant organs. Less than one person per 1.5 million people will develop a thymoma. This means about 400 people per year in the U.S. develop thymoma. Thymic carcinomas are very rare and makeup only 0.06% of all thymic tumors.

What are other types of medical conditions are associated with thymoma?

Several health conditions have been associated with thymoma. Medical conditions associated with cancers are paraneoplastic syndromes, and up to 50%-60% of patients with thymoma will have one of these related health conditions.

The most commonly associated condition with thymoma is myasthenia gravis, an autoimmune disease of the nerve-muscle junction that can manifest as weakness, fatigue, double vision, ptosis (drooping eyelids), and problems with swallowing.

Other associated conditions include other autoimmune diseases including pure red cell aplasia (underproduction of red blood cells in the bone marrow).

What causes thymoma?

The exact cause of thymomas is unknown. Thymomas are equally common in men and women and are most frequently seen in the fourth and fifth decades of life. There are no known risk factors that predispose a person to develop thymoma.

What are the symptoms of having a thymoma?

Up to 50% of thymomas are asymptomatic, meaning they do not produce any symptoms or signs. Physicians may diagnose a thymoma when they perform an imaging study for another reason. In other cases, the tumor may cause symptoms related to the size of the tumor and the pressure it exerts on adjacent organs.

Common symptom when they occur include:

The following symptoms and signs are less common but may occur:

Some cases may spread to the lining of the lungs or heart or even to tissues outside the chest. Less than 7% of cases spread outside the chest cavity. Thymic carcinomas are more aggressive types of tumors than thymomas and are more likely to spread both locally and distantly (metastasize) and cause symptoms.

How is a thymoma diagnosed?

If a thymoma is not causing symptoms, healthcare professionals may identify it incidentally, meaning it is found on an imaging test of the chest (for example, X-ray, computerized tomography, or CT scan) that is performed for another reason.

If symptoms are present, healthcare providers may carry out chest X-rays or other imaging studies, such as CT scans or magnetic resonance imaging (MRI) scans, or a combination of PET and CT scans, to identify the source of the symptoms.

While a mass in the anterior mediastinum can be seen in imaging studies, the definitive diagnosis can only be established when the mass is either removed surgically and examined by a pathologist or when a biopsy (surgical removal of a small portion of tissue for diagnostic purposes) is taken. Microscopic examination of the tumor biopsy tissue is necessary to confirm the diagnosis of thymoma or thymic carcinoma.

Healthcare professionals classify the appearance of the tissue itself under the microscope as type A, B, or C based on its characteristics. Type C thymoma is thymic carcinoma and is quite rare.

What are the stages of thymoma?

The stage of a tumor refers to the extent to which it has spread to other local organs and tissues or other parts of the body. There are two commonly used staging systems for thymoma, both of which classify the tumors as stage 1 through stage 4, depending upon the extent of spread and the degree of tissue invasion.

  • While there are some differences between the two methods used to stage thymomas, a stage 1 tumor represents an encapsulated tumor (surrounded by a capsular structure) that has not spread outside of the thymus gland.
  • Stage 4 represents the opposite extreme, in which invasion and spread to distant organs have occurred.
  • Locally recurrent thymoma refers to the situation in which a tumor returns to the same area after its surgical removal.


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What is the prognosis of thymoma?

The prognosis (outcome) for thymoma is dependent upon the stage of the tumor as well as the ability to remove the tumor by surgery. Thymic carcinomas tend to behave more aggressively and have a worse prognosis than thymomas. Thymomas tend to be slow-growing tumors, and the prognosis is good to excellent for those with stage 1 or stage 2 thymoma. It is hard to estimate exact survival statistics based on stage because of the low number of people diagnosed with this tumor.

In a German study of patients whose thymomas were completely removed by surgery, only 3% of the tumors recurred. Even 83% of patients with stage 3 thymoma were alive 10 years after diagnosis. The 10-year survival rate for stage 4 thymoma is approximately 47%. Overall, a majority of thymoma patients will live at least five years, while fewer than half of those with thymic carcinoma are expected to live that long.

Is thymoma cancer curable?

Surgery is the primary treatment for thymoma. The success of the surgery depends upon the particular characteristics of the tumor and its precise location; tumors have a higher surgical cure rate if physicians can remove them completely. If all evidence of disease is unable to be removed and microscopic, or the tumor remains after surgery, then radiation therapy, chemotherapy, and targeted therapy drugs have been used in addition to surgical resection.

Is it possible to prevent a thymoma?

Because the cause of thymoma is unknown and no risk factors have been identified, prevention of thymoma is not possible.

Medically Reviewed on 3/7/2023
Evans, Kendrix J. "Thymoma." Medscape. Dec. 27, 2016. <>.

Riedel, Richard F., and William R. Burfeind. "Thymoma: Benign Appearance, Malignant Potential." The Oncologist June 20, 2016. <>.