The mediastinum is defined as the space between the lungs. It is bordered by the sternum anteriorly, the thoracic inlet superiorly, the diaphragm inferiorly, and the ribs (Fig. 156-1). Mediastinal masses arise from structures that normally reside in the mediastinum, as well as those that migrate through it during development. The mediastinum is compartmentalized into four major spaces based on anatomic landmarks according to the Shields classification. The superior mediastinum extends from the thoracic inlet to an imaginary line between the angle of Louis and the fourth thoracic vertebral body. The anterior mediastinum spans from the back of the sternum to the front of the ascending aorta and pericardium. The posterior mediastinum is located between the posterior pericardium and the spine; this includes the costovertebral sulci. The middle mediastinum lies between the anterior and posterior mediastinal compartments. These divisions are not precise and become less defined as lesions invade or displace adjacent organs, leading to distorted anatomy. Nevertheless, they provide a framework to classify and understand mediastinal diseases. With knowledge of the patient’s age, location of the lesion, and presence or absence of sentinel signs and symptoms, a reasonable differential diagnosis often can be made.
The anterior, middle, and posterior mediastinal compartments.
It should be noted that the International Thymic Malignancy Interest Group (ITMIG) has proposed a classification of the mediastinal compartments to standardize the terminology used by both clinicians and radiologists. Thus, the ITMIG classification is based on cross-sectional imaging (multidetector CT landmarks) and uses three compartments instead of four: prevascular (which combines the superior and anterior zones), visceral (middle), and paravertebral (posterior).1
Tumors and cysts occur in the mediastinum across all ages and consist of both benign and malignant entities (see Chapters 162, 163, and 164). The location of the most frequent mediastinal masses differs by age. In children, the most common lesion is the neurogenic tumor in the posterior mediastinum, which accounts for about half of all mediastinal masses in the pediatric population. By contrast, the most frequent lesion in adults is thymoma in the anterior mediastinum. Posterior mediastinal lesions are less common in adults, whereas thymic lesions are rare in children. The trend is otherwise similar in adults and children, with lymphoma and germ cell tumor as the next most common mediastinal tumors, in order.2–6
Most mediastinal masses are asymptomatic, but many can be associated with specific symptoms and signs. Symptoms depend on the size of the lesion, whether it is benign or malignant, and the presence or absence of infection. It is generally agreed that malignant lesions are more likely to be symptomatic than benign lesions.6,7 Approximately 25% of all mediastinal tumors are malignant in both adults and children. Roughly two-thirds of children are symptomatic at presentation, whereas ...