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; it reaches its maximum weight at
puberty and undergoes involution thereafter.
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 thymus 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. The
anatomy of the thymus is shown in the images below.
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