Thymic epithelial tumors (TETs), according to the 3-compartment model, originate in the prevascular compartment, which is immediately posterior to the sternum and anterior to the pericardium and great vessels. The thymic gland comprises 2 types of cells: (1) epithelial cells, which can develop into thymoma and thymic carcinoma, and (2) lymphoid cells, which can develop into lymphoma. Thymic epithelial neoplasms, thymoma, thymic carcinoma, and thymic neuroendocrine tumors, although rare, are still the most common primary neoplasms of this compartment. These tumors typically present in the fifth and sixt decades of life with no gender prediliction. [1, 2, 3, 4, 5, 6]
Although thymic tumors are centered in the prevascular compartment, cases have been reported in the neck and other compartments. Tumors of thymic, lymphatic, or germ cell origin most commonly occur in this compartment, although aberrant parathyroid or thyroid tissue masses are sometimes found, along with vascular and mesenchymal tissue masses. These can present with venous obstruction, dysphagia, or stridor. Many conditions are associated with thymomas, with myasthenia gravis being the most common (between 30 and 50% of patients with thymoma have myasthenia gravis). Other conditions include pure red cell aplasia, hypogammaglobulinemia, systemic lupus erythematosus, rheumatoid arthritis, and nonthymic malignancies. [1, 2, 3, 4, 5, 6, 7]
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The most widely used classification system for thymic epithelial tumors is the World Health Organization (WHO) classification, which is based on the morphology of the epithelial cells and the ratio of the lymphocytes to epithelial cells. The WHO classification correlates well with the likelihood of invasiveness. The Masaoka-Koga classification describes the extent and spread of thymic epithelial malignancies, which is assessed at surgery. TNM staging systems for TETs are critically important for guiding treatment strategies. [1, 2, 3, 4, 5, 6, 7]
Imaging modalities
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Conventional radiology is the first imaging technique to perform when a prevascular compartment mass is suspected. A large anterior mediastinal mass is readily identified by chest radiography, as it typically appears as an extra soft tissue mass or opacity. Although the density of a soft tissue mass is similar to mediastinal structures, the silhouette sign, in which there is a loss of normal anatomic borders, helps make the diagnosis. Identifying a small mediastinal mass is more difficult. The presence of an anterior junction line, which represents the point of contact between the anterior lungs and their pleural surfaces anterior to the cardiovascular structures, can help exclude the presence of an anterior mediastinal mass; however, this line is seen in only 20% of normal chest radiographs. A lateral view is warranted and performed in most of cases at first presentation. Calcification, either morphous or flocculent, can sometimes be demonstrated.
Computed tomography (CT) scanning is routine and is very valuable in the diagnosis of thymic lesions that are usually of soft tissue attenuation and are mostly located between the sternum and the great vessels. A cystic component and calcification (10-50%) can be seen with these tumors. Calcification is more common in malignant thymic tumors but is not discriminatory as an isolated finding. Magnetic resonance imaging (MRI) may also be used. Routine chest CT and MRI can effectively identify thymoma, but neither is reliable to differentiate between thymic hyperplasia and normal thymus in patients with myasthenia gravis. Standard imaging for thymic tumors is IV contrast-enhanced CT of the thorax, which provides a complete exploration of the mediastinum and pleura from the apex to the costodiaphragmatic recesses. The CT images also help differentiate between the masses encroaching on the mediastinum from lung and other structures and the masses originating in the mediastinum. [8, 9, 1, 2, 10, 11, 12, 13, 14, 15]
The eighth edition of the TNM classification of malignant tumors includes an official staging system for thymic epithelial tumors recognized by the American Joint Committee on Cancer (AJCC) and the Union for International Cancer Control (UICC). Thymic epithelial tumors with clearly invasive or metastatic features seen on CT are associated with having higher AJCC/UICC pathologic TNM stage. However, features of lobulated contour and heterogeneous internal density are also associated with higher-stage disease. [9, 11]
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Ultrasonography has been used to differentiate between solid and cystic masses in the medistinum and to determine whether adjacent structures are affected. It may also be useful in assessing cardiac abnormalities due to mediastinal tumor.
(See the images below.)
About one half of all thymic tumors are malignant in individuals aged 20-40 years, and one third are malignant in persons younger than 20 years and those older than 40 years. (A malignant thymic tumor is seen in the image below.)
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