Chapter 82: Primary Benign and Malignant Neoplasms of the Mediastinum

INTRODUCTION

Primary lesions of the mediastinum are less common than lesions that secondarily involve the mediastinum. Overall, the majority of masses discovered in the mediastinum will be found to be metastases from a primary lung cancer. Neoplasms that arise primarily in the mediastinum, however, are often encountered in the clinic and are represented by a variety of lesions. This chapter first reviews the anatomy of the mediastinum and then focuses on the benign and malignant neoplasms that arise within each of the anatomic regions of the mediastinum. Covered elsewhere in this textbook are nonneoplastic disorders of the mediastinum that include pneumomediastinum, acute mediastinitis, chronic mediastinitis, and other miscellaneous disorders (Chapter 80); as well as congenital lesions of the mediastinum—which are nearly always benign—such as bronchogenic, enterogenous, neurogenic, thymic, pericardial, and thoracic duct cysts (Chapter 81).

ANATOMY OF THE MEDIASTINUM

The mediastinum constitutes an anatomic space located between the thoracic inlet superiorly, the diaphragm inferiorly, and the pleural cavities on the left and right sides. This central anatomic location houses or borders vital structures of almost every major organ system, including the heart and great vessels of the circulatory system, the esophagus of the digestive system, major airways of the respiratory system, the thymus of the immune system, and the phrenic and vagus nerves of the neurologic system. Further, various endocrine organs may extend into it, distant malignancies may metastasize to it, and infectious processes can manifest themselves within it.

The mediastinum is compartmentalized based upon the borders of anatomic structures as seen on a lateral chest radiograph (Fig. 82-1). We believe that the most anatomically appropriate and clinically useful model of the mediastinum is the three-compartment model, which divides the mediastinum into anterior, middle, and posterior compartments.¹ A new classification scheme developed by the International Thymic Malignancy Interest Group divides the mediastinum into similar compartments (prevascular, visceral, and paravertebral) based on computed tomography (CT) scans.² The anterior mediastinum extends from the thoracic inlet superiorly to the diaphragm inferiorly, and from the posterior table of the sternum anteriorly to the anterior pericardium and the great vessels posteriorly. It contains most importantly the thymus, but also fat, lymphatic tissues, and the internal mammary arteries and veins. The middle mediastinum is bounded anteriorly by the pericardium and posteriorly by the pericardium and posterior wall of the trachea, extending only as high as the pericardial reflection. It contains the heart, pericardium, superior and inferior vena cavae, ascending and transverse aorta, trachea and mainstem bronchi, and lymphatic tissues. The posterior mediastinum extends from the thoracic inlet to the diaphragm and lies posterior to the posterior pericardium and airway. It contains the descending aorta, thoracic duct, esophagus, vagus nerves, and lymph nodes, as well as structures emerging from the spinal canal such as intercostal nerves.