Anatomy.—Nominal anatomic compartments of the mediastinum include the anterior, middle, posterior, and (in some schemes) superior compartments. The anterior compartment is bounded anteriorly by the sternum and posteriorly by the anterior surface of the pericardium, the ascending aorta, and the brachiocephalic vessels. The middle compartment is bounded by the posterior margin of the anterior division and the anterior margin of the posterior division. The posterior compartment is bounded anteriorly by the posterior margins of the pericardium and great vessels and posteriorly by the thoracic vertebral bodies. In the four-compartment model, the superior compartment is defined as the compartment above the plane between the sternal angle to the T4-5 intervertebral disk or, more simply, above the aortic arch (84,85). Exact anatomic boundaries between the compartments do not exist, and there are no barriers (other than the pericardium) to prevent the spread of disease between compartments. Other classifications exist, but the three- and four-compartment models are the most commonly used. [Fleischner Society]