ACR Codes: 673.3154
Thymomas are tumors consisting of an admixture of thymic epithelial cells and reactive lymphocytes, with both circumscribed and invasive patterns of growth. Cystic change is common and in some cases the majority of the tumor is cystic. Foci of calcification, hemorrhage and necrosis may also be seen. It is the presence or absence of spread beyond the capsule rather than the histological appearance within the thymus that determines whether a tumor is labeled benign or malignant by the pathologist. For this reason the term “invasive thymoma” is used to describe any tumor that has spread beyond the capsule. 15-40% of thymomas turn out to be invasive.
The average age at diagnosis is approximately 50, or earlier in those who present with myasthenia gravis, with virtually equal frequency in men and women. Thymomas are unusual below the age of 20.
Most patients with noninvasive thymoma have no clinical symptoms due to the tumor itself. When symptoms occur, they are due to local compression causing chest pain, cough, dyspnea, dysphagia, hoarse voice, or superior vena cava syndrome.
Approximately 40% of patients with thymoma have myasthenia gravis, and the incidence of thymoma in patients with myasthenia gravis is 15%. Thymectomy, regardless of whether or not the thymus harbors a thymoma, improves the myasthenia in some patients. Thymectomy for relieving myasthenia gravis is less effective in patients with thymoma than it is for patients without thymoma. Thymomas should be removed surgically, preferably when still confined to the thymus.
A hematologic cytopenia, notably pure red cell aplasia or aplastic anemia, is seen in 21-50% of patients and thymoma is associated with hypogammaglobulinemia in up to 10% of patients. There is a variety of conditions associated with thymoma.
Most thymomas arise in the upper anterior mediastinum. They are usually found anterior to the ascending aorta, above the right ventricular outflow tract and the main pulmonary artery, and project into one or other hemithorax. They are usually spherical or have a lobulated border. Punctate, curvilinear or ringlike calcification is frequent in both benign and invasive thymomas. At CT, they are usually of homogeneous density and enhance uniformly, but they may appear cystic with discrete nodular components.
Before age 40, and particularly before 30, diagnosing a small thymoma can be difficult because the normal gland is variable in size and in myasthenia gravis the associated hyperplasia may lead to a bulky gland. Invasive thymomas invade the mediastinal fat and eventually spread to the pericardium and pleura. Until mediastinal invasion has occurred, it is not possible to distinguish benign from invasive thymoma even with CT. Adjacent mediastinal fat planes are preserved when tumors are confined to the thymus, whereas they may be obliterated by invasive tumors. Preservation of adjacent fat planes does not exclude capsular invasion, but when these fat planes are preserved, extensive invasive disease is unlikely. Pleural deposits “drop metastasis” may be so extensive that they mimic malignant pleural mesothelioma. In some patients there is no visible mediastinal tumor.
Reference(s): Imaging of Diseases of the Chest, 3rd ed., Armstrong, P. et al, 2000.
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