Epidemiology

Thymomas are rare tumors, with an annual incidence of 0.13 to 0.15 cases per 100,000 person-years, based on data from the National Cancer Institute Surveillance, Epidemiology and End Results Program.[1]​ Men and women are affected equally.[8][11][12][13]​ Thymomas have been reported in infants and also in older adults. The mean age at diagnosis is about 50 years, with a peak broad range between 35 and 70 years. The prevalence of myasthenia gravis in patients with thymoma is about 40%; in comparison, 10% to 15% of patients with myasthenia gravis have an associated thymoma.[8][9]​ These patients tend to present at a slightly younger age.

Thymic carcinomas occur less frequently than thymomas, with an incidence of 0.2 to 0.5 per million.[2] The age distribution is broad, ranging from early childhood to adulthood, with a mean age at diagnosis of 50 to 60 years.[2] These tumors are slightly more common in men than in women (male-to-female ratio is 1.5:1.0). Most patients present with advanced-stage thymic carcinoma. Myasthenia gravis and other parathymic syndromes are only rarely associated with thymic carcinoma.

The relative prevalence of the WHO histologic types of thymic tumor are: A: 8%; AB: 26%; B1: 6%; B2: 19%; B3: 10%; C: 13%.[3][6]​ Most patients present with early-stage tumors that are either fully encapsulated or invade only the capsule (Masaoka-Koga stage I: 49%; stage II: 22%). About one third present with advanced thymomas that invade other organs (Masaoka-Koga stage III: 19%; stage IVA [pleural or pericardial dissemination]: 7%; stage IVB [hematogenous or lymphatic dissemination]: 3%).[3][8]

Neuroendocrine tumors of the thymus (NETT) are an extremely rare histologic subtype, with only about 200 cases reported.[13][14] Again, all age groups may be affected. The male-to-female ratio is 3:1. About 30% of patients with thymic carcinoid tumors have associated Cushing syndrome. Myasthenia gravis and other parathymic syndromes are not associated with carcinoid tumors. The carcinoid syndrome has only rarely been reported in relation to thymic carcinoid tumors.

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