A thymoma is typically a slow growing tumor. When all of the tumor cells are inside the capsule surrounding the thymus gland, this is called “encapsulated”. These types of thymoma are the easiest to cure by surgery.

Other types of thymoma may  require additional treatment and periodic follow-ups. These include tumors that are not encapsulated, where some of the tumor cells are outside of the thymus. In other, more serious cases, the thymoma can become invasive, interfering with other parts of the body or  growing  into the lungs and heart. 

People with thymoma often have an autoimmune disease. The most common is myasthenia gravis, a disease that weakens the muscles, but others include polymyositis, lupus erythematosus, rheumatoid arthritis, thyroiditis, Sjogren’s syndrome, and hypogammaglobulinemia.

Thymoma is rare, accounting for about 0.2% to 1.5% of all cancers. Approximately 90% of tumors of the thymus are thymoma. The remaining 10% are thymic carcinoma, carcinoid tumors, or lymphomas. Approximately 500 people in the United States are diagnosed with thymoma each year.