What is thymoma?

Updated: Dec 03, 2018
  • Author: Kendrix J Evans, MD, MS; Chief Editor: John Geibel, MD, MSc, DSc, AGAF  more...
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Thymoma originates within the epithelial cells of the thymus, a lymphoid organ located in the anterior mediastinum. This organ is located behind the sternum in front of the great vessels; it reaches its maximum weight at puberty and undergoes involution thereafter.

In early life, the thymus is responsible for the development and maturation of cell-mediated immunologic functions. The thymus is composed predominantly of epithelial cells and lymphocytes. Precursor cells migrate to the thymus and differentiate into lymphocytes. Most of these lymphocytes are destroyed, with the remainder of these cells migrating to tissues to become T cells.

A relation between myasthenia gravis (MG) and thymomas was determined incidentally in 1939, when Blalock et al reported the first excision of a thymic cyst in a 19-year-old girl with MG. [1] This patient achieved long-term remission; therefore, thymectomy became the definitive therapy for treatment of generalized MG.

No clear histologic distinction between benign and malignant thymomas exists. The propensity of a thymoma to be malignant is determined by the invasiveness of the thymoma. Malignant thymomas can invade the vasculature, lymphatics, and adjacent structures within the mediastinum. The 15-year survival rate is 12.5% for a person with an invasive thymoma and 47% for a person with a noninvasive thymoma. Death usually occurs from cardiac tamponade or other cardiorespiratory complications.

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