Solid Tumors of the Mediastinum in Adults
Introduction
The mediastinum is the tissue compartment located between the lungs, posterior to the sternum, anterior to the spine, and extending from the thoracic inlet to the diaphragm. A myriad of pathology can present in this region, both neoplastic and nonneoplastic. Mediastinal tumors may present with localizing symptoms secondary to tumors’ invasion or compression of surrounding structures (dyspnea, cough, dysphagia, and hoarseness) or systemic symptoms that typically result from release of excess hormones, antibodies, or cytokines such as in hypercalcemia from parathyroid adenoma or myasthenia gravis in thymoma. Regardless, the clinical manifestations are often nonspecific or vague, and imaging examination with either computed tomography (CT) or magnetic resonance (MR) is often required for determining size, location, composition, extension, and possible diagnosis of the mass.1 Traditionally, the differential diagnosis is derived from the tissue or structure from which the mass is arising (ie, lymph nodes, vascular structures, thymus, thyroid, vertebral column, esophagus, and trachea).
We review the normal mediastinal anatomy and discuss the role of CT and MR imaging (MRI) in the diagnosis and prognosis of the most common solid mediastinal tumors in the adult patient.
Section snippets
Anatomy
Several classification systems have been proposed over the years that divide the mediastinum into multiple compartments, although no physical boundaries between compartments exist that limit disease. Important differences exist in the way anatomist, surgeons, and radiologist divide the mediastinal compartments, which can significantly modify clinical significance and potentially affect patient care.2 Although most of mediastinal tumors are benign, masses in the anterior compartment are more
Thymic Lesions
Normally, over the age of 25 years (but in some patients as late as the age of 40), the thymus is not recognizable as a distinct soft tissue organ on imaging because of progressive fatty involution, with occasional wisps or islands of soft tissue. CT attenuation, size, and shape change, and in the same fashion, the MRI signal characteristics of the normal thymus change over time to reflect gradual fatty replacement, with greater generalized T1-hyperintensity and T2-hyperintensity and
Germ Cell Tumors
Of the extragonadal sites of occurrence for germ cell tumors (GCTs), the mediastinum is the most common, comprising approximately 10%-20% of primary mediastinal tumors in adults.
They most commonly occur in the anterior mediastinum, either within the thymus or adjacent to it.19, 20
Most of the patients affected with primary gem cell tumors of the mediastinum are men (99%) with only few cases reported in women. Extragonadal GCTs should always be considered when a bulky lobulated anterior
Parathyroid and Thyroid lesions
As the developmental association of the thymus is with other foregut-derived structures, other pathological epithelial lesions may arise in the mediastinum.
Lipoma
Lipomas are encapsulated slow-growing benign mesenchymal tumors that originate from adipose tissue. They can arise in any mediastinal compartment, but are most often found in the anterior mediastinum. They are commonly asymptomatic, but when large enough, mass effect on adjacent structure may produce cough, dyspnea, or vascular compression. In most cases, the homogeneous fat density allows a straightforward imaging diagnosis both on CT and MRI.29 The differential diagnosis includes mediastinal
Lymph Node Enlargement
On CT, lymph nodes are generally round, elliptical, or somewhat triangular discrete entities of soft tissue density and surrounded by mediastinal fat. There are significant variations in normal node size, but by convention, the upper limit of normal for mediastinal lymph nodes is 1 cm for a node measured along its shortest axis, and 1.5 cm for subcarinal nodes. The significance attributed to enlarged lymph nodes in diagnosis must be correlated to the patient’s clinical history, as enlarged lymph
Posterior Mediastinal Masses
Neurogenic tumors are the most common cause of a posterior mediastinal mass and constitute approximately 20% of adult mediastinal tumors. Conversely, of all intrathoracic neurogenic tumors, nearly 90% of them are located in the posterior mediastinum. Of these, the majority are benign across all age groups (78%), with slightly higher malignancy rate in children than in adults. These are generally grouped based on whether they arise from peripheral nerves, sympathetic ganglia, or parasympathetic
Conclusion
It is most important to base the differential diagnosis of a solid mediastinal tumor in an adult by localizing it to specific regions of the mediastinum, and noting key cross-sectional imaging features such as margins, borders, and enhancement patterns. For many lesions, CT alone cannot distinguish benign from malignant etiologies, but frequently can yield clues with respect to other manifestations in the thorax. Thymoma accounts for 15%-20% of primary mediastinal masses. Germ cell tumors
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