Thymic tumors are rare occurrences in children. These masses account for 18% of mediastinal tumors in adults but only 3% in children. Tumors involving the thymus include cysts and hamartomas, thymomas and thymic adenomas, thymolipomas, germ cell tumors, and carcinoids, as well as leukemia and lymphoma. Thymomas are primary neoplasms of the thymic epithelium. Fewer than 10% of thymomas occur in children and adolescents.

Although thymomas are found in 8% to 15% of adult patients with myasthenia gravis, only two cases of thymoma in children with this autoimmune disease have been reported, suggesting that the neoplasm plays a lesser role in the pathophysiology of myasthenia gravis in younger patients. Red cell aplasia and hypogammaglobulinemia also are associated with thymoma.

Thymomas are slow-growing tumors that often are diagnosed as incidental findings on routine chest roentgenography. Approximately 50% of patients will present with symptoms including cough, dysphagia, dyspnea, and vague chest pain. Computed tomographic (CT) scans are useful in defining the characteristics of the mass and to assess the involvement of contiguous mediastinal structures. The center of the mass usually is where the normal thymus would be located, below the innominate vein and apposing the posterior surface of the sternum. These masses normally are of soft tissue density but may contain calcifications. Vascular invasion, encasement, and pleural deposits are associated with malignancy.

The differential diagnosis of anterior mediastinal masses includes lymphoma and germ cell tumors. The typical appearance of lymphoma on CT is an ill-defined lobulated mass, often with associated regional or distant lymphadenopathy. Benign germ cell tumors normally have well-defined margins and may contain cystic, calcified, and adipose tissue. Nonseminomatous germ cell tumors tend to have large, diffuse appearance on CT imaging, with areas of necrosis and hemorrhage. The latter also will have diagnostic elevations of alpha-fetoprotein and beta-human chorionic gonadotropin.