X-Ray


Between 25% to 50% of thymomas are undetectable on PA chest radiographs [6,12]. CT scan is the study of choice in the evaluation of patients with myesthenia gravis or a suspected thymoma [12]. CT has a sensitivity of 91% and a specificity of 97% for the diagnosis of a thymic mass [12]. MR adds little additional information. Note: The use of iodinated contrast agents in patients with myasthenia gravis has been reported to cause increased weakness. Although not contraindicated, intravenous contrast agents should not be used routinely in the assessment of these patients. [1] 
On CT, thymomas appear as a lobulated, well-defined, soft tissue density mass with rounded margins in the anterior mediastinum and growth is usually to one side. Mild, homogeneous contrast enhancement is characteristic. Calcification is common (20% of cases on plain film). Cystic areas (due to hemorrhage or necrotic degeneration) are also found in 20-30% and these are typically larger lesions. A clue to the diagnosis is that a thymoma will be surrounded by fat, while the normal thymus is diffusely infiltrated with fat. Partial obliteration of the mediastinal fat planes about the lesion does not confirm the diagnosis invasion (finding may be seen in about 50% of patients with non-invasive lesions) [2]. However, an indistinct border between a thymoma and the adjacent lung or the presence of a pleural effusion or pleural nodularity suggest invasion [20]. Spread to periaortic nodes in the upper abdomen has been reported in up to 31.5% of patients with invasive thymomas. 
On MR, thymomas commonly appear as homogeneous or heterogenenous masses with low signal on T1 and high signal on T2 [20].Differentiation from non-neoplastic thymic enlargement can be performed with chemical shift MR imaging [21]. This technique allows identification of the normal fatty infiltration of the thymus that will manifest with homogeneous decreased signal on the out-of-phase images [21].
Thallium imaging has also been used to evaluate patients [12]. The normal thymus demonstrates no increased thallium concentration, whereas lymphoid follicular hyperplasia shows moderate thallium uptake on delayed images, and thymoma shows moderate to strong thallium uptake on both early and delayed images [12].

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