A thymoma is a tumor of the epithelial cells of the thymus gland. They are usually benign, but can have more serious complications such as myasthenia gravis (30-45% of cases), pure red cell aplasia, Addison's disease, Cushing's disease, anemia, arthritis, sarcoidosis, scleraderma and lupus. However, up to half of persons with a thymoma are completely asymptomatic and the thymoma is often found during scans for other conditions. They appear in equal numbers in both men and women, and the typical age of a diagnosed patient is 30-40, with both older and younger patients being at lesser risk.
Thymomas show up readily on a CT Scan, which is used to determine it's size and general shape. A biopsy of the mass is performed for confirmation.
Most thymomas are large and distinct and surgery is a good first choice for treatment. However, if the thymoma is large and invasive, chemotherapy is usually tried first.
Thymoma at Wikipedia