The thymus is a lymphoid organ located in the anterior mediastinum. In early life, the thymus is responsible for the development and maturation of cell-mediated immunological functions.
The thymus is a lymphoid organ located in the anterior mediastinum. In early life, the thymus is responsible for the development and maturation of cell-mediated immunological 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 lymphocytes. The thymus gland is located behind the sternum in front of the great vessels (see
Image 1 and
Image 2). The thymus gland reaches its maximum weight at puberty and undergoes involution thereafter. Thymoma, the most common neoplasm of the anterior mediastinum, originates within the epithelial cells of the thymus.
History of the Procedure: A relationship between myasthenia gravis (MG) and thymomas was determined incidentally in 1939 when Blalock and coworkers reported the first excision of a thymic cyst in a 19-year-old girl with MG. This patient achieved long-term remission; therefore, thymectomy became the definitive therapy for treatment of generalized MG.
Problem: No clear histiologic 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 of a person with an invasive thymoma is 12.5%, and it is 47% for a person with a noninvasive thymoma. Death usually occurs from cardiac tamponade or other cardiorespiratory complications.
Frequency: Thymoma, the most common neoplasm of the anterior mediastinum, accounts for 20-25% of all mediastinal tumors and 50% of anterior mediastinal masses.
Etiology: The etiology of thymomas has not been elucidated; however, it has been associated with various systemic syndromes. As many as 30-40% of patients who have a thymoma experience symptoms suggestive of MG. An additional 5% of patients who have a thymoma have other systemic syndromes, including red cell aplasia, dermatomyositis, systemic lupus erythematous, Cushing syndrome, and the syndrome of inappropriate antidiuretic hormone secretion.
Clinical: Peak incidence of thymoma occurs in the fourth to fifth decade of life; mean age of patients is 52 years. No sexual predilection exists. Although development of a thymoma in childhood is rare, children are more likely to have symptoms than adults are. Several explanations for the prevalence of symptoms in children have been proposed, including (1) children are more likely to have malignancy, (2) lesions are more likely to cause symptoms by compression or invasion in the smaller thoracic cavity of a child, and (3) the most common location for mediastinal tumors in children is near the trachea, resulting in respiratory symptoms.
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