Imaging evaluation of pediatric mediastinal masses

The mediastinum is located in the central portion of the thorax, between the two pleural cavities, the diaphragm and the thoracic inlet [1]. The classification of Fraser et al [2] divides the mediastinum into the traditional anterior, middle, and posterior compartments based on the lateral chest radiograph. There are no fascial planes that separate these compartments, but this division categorizes diseases and masses to their location of origin. The anterior mediastinum is defined as the region posterior to the sternum and anterior to the heart and brachiocephalic vessels and from the thoracic inlet superiorly to the diaphragm inferiorly. It contains the thymus, fat, and lymph nodes. The middle mediastinal compartment is located posterior to the anterior mediastinum and anterior to the posterior mediastinum. This space contains the heart and pericardium, the ascending and transverse aorta, the brachiocephalic vessels, the vena cava, the main pulmonary artery and veins, the trachea, bronchi, and lymph nodes. The posterior mediastinal compartment is located posterior to the heart and trachea and extends posteriorly to the thoracic vertebral margin and includes the paravertebral gutters. It contains the descending thoracic aorta, esophagus, azygos veins, autonomic ganglia and nerves, thoracic duct, lymph nodes, and fat [3]. Mediastinal masses in children are a heterogeneous group of asymptomatic or potentially lifethreatening congenital, infectious, or neoplastic lesions that present complex diagnostic and therapeutic dilemmas. Most commonly they are discovered incidentally on chest radiographs. Large mediastinal masses can cause compression of adjacent mediastinal structures. Patients may have airway compression or cardiovascular compromise [4]. Mediastinal masses are usually assigned to a single mediastinal compartment to limit the differential diagnosis. Sometimes they cannot be localized to a single anatomic compartment. The epicenter of the mass and the direction of the mass effect on adjacent structures, such as trachea and great vessels, suggest the site of origin of the mass [5]. Because some normal structures are located within multiple mediastinal regions, a given tumor mass can arise in any compartment (Boxes 1–3). The epicenter of the mass, its effect on adjacent mediastinal organs, and its internal characteristics (calcification, fat, water, and so forth), based on conventional radiographic examination and cross-sectional imaging, can help in establishing a differential diagnosis and clinical planning [5].