A thymoma is an uncommon tumor or cancer of the thymus gland. The thymus is a gland located in the anterior mediastinum (the area between the lungs in the chest) that plays a critical role in the development of immune cells during childhood. The thymus gland enlarges during childhood, peaks in size at puberty (about 40 grams) and begins to shrink after an individual reaches puberty.

Normally, the thymus is made up of a combination of lymphoid cells (immune cells or lymphocytes) and lining cells (epithelial cells). Thymoma is a tumor that originates from the epithelial cells of the thymus. The term thymomic neoplasms is used to refer to tumors of the thymus, which consist of thymomas and thymic carcinomas. Thymic carcinomas are tumors of the thymus having cells with a markedly abnormal appearance under the microscope. Thymic carcinomas are more aggressive than thymomas (they are more malignant), but they account for only about 1% of thymic tumors.

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.

• Chest pain,
• shortness of breath, and
• cough are common symptoms when symptoms do occur.

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

• Fever
• Night sweats
• Weight loss

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 to cause symptoms.

The exact cause of thymomas is unknown. 

The treatment for thymoma is primarily surgical removal. The success of surgical treatment depends upon the particular characteristics of the tumor and its precise location; tumors that can be completely removed have a higher surgical cure rate. If all evidence of disease is unable to be removed and microscopic, or gross or macroscopic tumor remains after surgery, then radiation therapy and chemotherapy have been used as an adjunct to surgical resection. Chemotherapy is generally used for metastatic thymoma and thymoma that cannot be grossly surgically removed.

Thymomas are equally common in men and in 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 developing thymoma.