Thymoma is the most common cause of an anterior mediastinal mass in adults, accounting for ~40% of all mediastinal masses. The other major causes of anterior mediastinal masses are lymphomas, germ cell tumors, and substernal thyroid tumors. Carcinoid tumors, lipomas, and thymic cysts also may produce radiographic masses. After combination chemotherapy for another malignancy, teenagers and young adults may develop a rebound thymic hyperplasia in the first few months after treatment. Granulomatous inflammatory diseases (tuberculosis, sarcoidosis) can produce thymic enlargement. Thymomas are most common in the fifth and sixth decades, are uncommon in children, and are distributed evenly between men and women.
About 40–50% of patients are asymptomatic; masses are detected incidentally on routine chest radiographs. When symptomatic, patients may have cough, chest pain, dyspnea, fever, wheezing, fatigue, weight loss, night sweats, or anorexia. Occasionally, thymomas may obstruct the superior vena cava. About 40% of patients with thymoma have another systemic autoimmune illness related to the thymoma. About 30% of patients with thymoma have myasthenia gravis, 5–8% have pure red cell aplasia, and ~5% have hypogammaglobulinemia. Thymoma with hypogammaglobulinemia also is called Good's syndrome. Among patients with myasthenia gravis, ~10–15% have a thymoma. Thymoma more rarely may be associated with polymyositis, systemic lupus erythematosus, thyroiditis, Sjögren's syndrome, ulcerative colitis, pernicious anemia, Addison's disease, scleroderma, and panhypopituitarism. In one series, 70% of patients with thymoma were found to have another systemic illness.