Thymomas and thymic carcinomas are rare tumors that form in cells on the outside surface of the thymus. The thymus is a small organ that lies in the upper chest under the breastbone. It is part of the lymph system and makes certain types of white blood cells that help the body fight infection.

The tumor cells in a thymoma look like the normal cells of the thymus, grow slowly, and rarely spread beyond the thymus.

The tumor cells in a thymic carcinoma look very different from the normal cells of the thymus. They grow more quickly and have usually spread to other parts of the body when the cancer is found. Thymic carcinoma is harder to treat than thymoma.

People with thymoma often also have autoimmune disorders such as myasthenia gravis and rheumatoid arthritis.

Thymoma and thymic carcinoma may not cause early signs or symptoms. The cancer may be found during a chest x-ray or CT scan that is done for another