A thymoma is a tumor which arises from the epithelial cells of the thymus. Despite the progressive involution of the thymus post-puberty, the peak incidence of thymomas is in adults, after the age of 40. Owing to the normal anatomic location of the thymus in the antero-superior mediastinum, thymomas account for 20-30% of masses in that situation, and, they have to be differentiated from other neoplasms which may be found in the anterior mediastinam, such as, lymphocytic lymphoma, Hodgkin's disease, Germinoma (seminoma), Thymic carcinoid tumor, oat cell carcinoma and parathyroid adenoma.

Embryologically, the thymus develops as an outgrowth of epithelial cells from the third pharyngeal pouches in association with the inferior parathyroid glands and migrates from the neck to the mediastinum. In embryonic life, blood borne stem cells from the yolk sac are seeded into the thymus which becomes the primary lymphoid organ. Within the thymus, the primitive stem cells differentiate into lymphocytes, which proliferate and differentiate into T-lymphocytes (thymus-dependent, thymocytes). Some of the T-lymphocytes are released into the blood stream and populate peripheral lymphoid organs. They constitute 60-70% of circulating lymphocytes.