Thymomas and thymic carcinomas are epithelial tumors of the thymus. The term, thymoma, is customarily used to describe neoplasms that show no overt atypia of the epithelial component. A thymic epithelial tumor that exhibits clear-cut cytologic atypia and histologic features no longer specific to the thymus is known as a thymic carcinoma (also known as type C thymoma).
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Invasive thymomas and thymic carcinomas are relatively rare tumors, which together represent about 0.2% to 1.5% of all malignancies. The overall incidence of thymoma is 0.15 cases per 100,000, based on data from the National Cancer Institute Surveillance, Epidemiology and End Results (SEER) Program. Thymic carcinomas are rare and have been reported to account for only 0.06% of all thymic neoplasms. In general, thymomas are indolent tumors with a tendency toward local recurrence rather than metastasis. Thymic carcinomas, however, are typically invasive, with a higher risk of relapse and death.[5,6]
Age at onset
Most patients with thymoma or thymic carcinoma are aged 40 through 60 years.
Unique disease features
The etiology of these types of tumors is not known. In about 50% of the patients, thymomas/thymic carcinomas are detected by chance with plain-film chest radiography.
Ninety percent of thymomas and thymic carcinomas occur in the anterior mediastinum. They are the most common malignancies of the anterior mediastinum.
World Health Organization pathologic classification of tumors of the thymus and stage correlate with prognosis. Although some thymoma histologic types are more likely to be invasive and clinically aggressive, treatment outcome and the likelihood of recurrence appear to correlate more closely with the invasive/metastasizing properties of the tumor cells.[1,10] Therefore, some thymomas that appear to be relatively benign by histologic criteria may behave very aggressively. Independent evaluations of both the tumor invasiveness (using staging criteria) and tumor histology should be combined to predict the clinical behavior of a thymoma.
Thymoma and thymic carcinoma should be differentiated from a number of nonepithelial thymic neoplasms, including the following:[1,11]