Tumors of the thymus are rare, accounting for 0.2% to 1.5% of all malignancies. Thymomas account for 15% to 20% of all mediastinal tumors and are the most common anterior mediastinal tumor in adults, with a relatively equal sex distribution.1–4 Thymoma in children is extremely rare. Most tumors are indolent, slow-growing, and relatively asymptomatic. Symptoms, if present, include chest pain, cough, pneumonia, phrenic nerve paralysis, or superior vena cava syndrome. Thymomas grow through local extension, and metastases, when present, include pleural, pericardial, or diaphragm invasion. Late metastatic sites include the lung, liver, and distant visceral sites. The association of thymoma with myasthenia gravis (MG) is well documented, and seen in 25% to 40% of patients with thymoma. A spectrum of other autoimmune disorders of neuromuscular, hematologic, dermatologic, endocrine, renal, or hepatic origin has been described.5
Thymic carcinomas are virtually never associated with autoimmune disorders.6 Patients with thymoma have a well-documented risk of second, unrelated malignancies with an incidence as high as 22%. Second malignancies include carcinoma of the stomach, liver, lung, uterine cervix, and head and neck and soft tissue tumors.7 The risk of second malignancies in patients without MG may be higher than in those with MG, suggesting inherent immune differences between tumor surveillance and autoimmunity.8 Thymoma is not unique to man, but also has been described in a dog, cats, dairy goats, small laboratory rodents, and a cow.9,10
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