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Primary benign anterior and middle mediastinal tumors arise from mediastinal structures either as benign proliferative processes or as a consequence of inflammation. They may also occur as a result of the arrest of embryonic cells traversing this compartment with later tumor growth (germ cell tumors). The majority of benign mediastinal tumors are either cystic or solid; if solid they derive primarily from the thymus, or they extend downward from the superior mediastinum (thyroid substernal goiter), or to a lesser extent they derive from mediastinal mesenchymal tissue. Primary mesenchymal mediastinal tumors represent fewer than 10% of primary mediastinal tumors and have a higher prevalence for malignant potential in children.1

Primary mediastinal tumors are classified by their location in the following three compartments: anterior, middle, and posterior. Each compartment is defined by theoretical anatomic borders (Fig. 162-1), and each is specific for certain tumors. These compartments are described as follows.

Figure 162-1

Mediastinal tumors are divided for the convenience of diagnosis and surgical approach into three compartments: anterior, middle, and posterior.

Anterior mediastinum: This compartment is bounded anteriorly by the sternum and posteriorly by the pericardium, the aorta, and the brachiocephalic vessels. The thoracic inlet comprises the superior border, and the inferior border is demarcated by the diaphragm. Tumors in the superior aspect of this compartment derive largely from tissues native to or passing through this compartment or extending down from the neck. Tumors in the inferior aspect consist primarily of hernias that extend superiorly from the abdomen or pericardial fat pad extensions or pericardial or mesothelial cysts.

Middle mediastinum: This compartment is bounded by the anterior and posterior reflections of the pericardium, and it stretches from the thoracic inlet to the diaphragm. It is probably best described as the space that lies between the anterior and posterior mediastinum.

Posterior mediastinum: This compartment is defined anteriorly by the posterior trachea and the pericardium, and it extends to the vertebral column including the paraspinal areas with vertical dimensions from the apex of the thoracic cavity to the diaphragm.

This chapter focuses on benign anterior and middle mediastinal tumors and the common surgical methods used to deal with these tumors. It should be noted that histologic overlap may occur between benign and malignant tumors arising from similar tissues in the same compartment as noted in Table 162-1. In addition, it may be clinically and pathologically difficult to distinguish a benign from a malignant mediastinal mass (even if cystic) until the specimen has been removed and fully examined pathologically.


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