Introduction
Thymoma, the most common neoplasm of the anterior mediastinum, originates within the epithelial cells of the thymus.
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 images below and Images 1-2). It reaches its maximum weight at puberty and undergoes involution thereafter.
Anatomy of the thymus, with emphasis on the blood supply and relation to recurrent laryngeal and phrenic nerves.
Lateral view: The thymic arteries are derived from the adjacent internal mammary arteries; the inferior thymic vein empties into the innominate vein. The thymus gland's surrounding vascular and neural structures may be invaded during the spread of a thymoma.
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.1 This patient achieved long-term remission; therefore, thymectomy became the definitive therapy for treatment of generalized MG.
Problem
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 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 syndrome of inappropriate antidiuretic hormone secretion.
Presentation
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 than adults to have symptoms. Several explanations for the prevalence of symptoms in children have been proposed, including the following: (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.
Four cases of patients who presented with severe chest pain secondary to infarction or hemorrhage of the tumor have been reported. Cases of invasion into the superior vena cava resulting in venous obstruction have also been reported.2 The clinician should be aware of these rare presentations of a thymoma.
Indications
Of patients with a thymoma, one third to one half are asymptomatic, and one third of patients present with local symptoms related to the tumor encroaching on surrounding structures. These patients may present with cough, chest pain, superior vena cava syndrome, dysphagia, and hoarseness if the recurrent laryngeal nerve is involved. One third of cases are found incidentally on radiographic examinations during a workup for MG.
Relevant Anatomy
The thymus gland is located behind the sternum in front of the great vessels and the pericardium. The gland can extend laterally to the phrenic nerves. The main blood supply is from the internal thoracic arteries; however, the thymus gland also is supplied with blood by the inferior thyroid and pericardiophrenic arteries. (See images below and Images 1-2.)
Anatomy of the thymus, with emphasis on the blood supply and relation to recurrent laryngeal and phrenic nerves.
Lateral view: The thymic arteries are derived from the adjacent internal mammary arteries; the inferior thymic vein empties into the innominate vein. The thymus gland's surrounding vascular and neural structures may be invaded during the spread of a thymoma.
Contraindications
If the thymoma invades both phrenic nerves, do not resect either nerve; only debulk the area.
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References
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Further Reading
Related eMedicine topics:
Myasthenia Gravis [Emergency Medicine]
Myasthenia Gravis [Neurology]
Myasthenia Gravis [Ophthalmology]
Surgery of the Thymus Gland
Thymoma [Pediatrics: General Medicine]
Thymic Tumors
Thymus, Lesions
Clinical guidelines:
The management of thymoma: guideline recommendations. Program in Evidence-based Care - State/Local Government Agency [Non-U.S.]. 2008 Sep 26. 41 pages. NGC:006749
Clinical trials:
Belinostat in Treating Patients With Recurrent or Metastatic Thymoma and Thymic Carcinoma That Cannot be Removed by Surgery
Efficacy of Octreotide Treatment in Patients With Primary Inoperable Thymoma
Paclitaxel and Cisplatin for Thymic Neoplasm
Preoperative Treatment of Patients With High Risk Thymoma
Keywords
thymoma, thymus, myasthenia gravis, thymus gland, myasthenia, thymectomy, thymic carcinoma, anterior mediastinal tumor, thymic tumor, thymus tumor, anterior mediastinal neoplasm, mediastinal tumor, mediastinal neoplasm, thymic neoplasm, thymus neoplasm
