The compartmental division of the mediastinum has important implications in the diagnosis of mediastinal masses that have a predilection for specific compartments. The mediastinum is anatomically defined by the pleural cavities bilaterally, the thoracic inlet superiorly, and the diaphragm inferiorly. The anteroposterior mediastinal limits are the posterior surface of the sternum anteriorly and the anterior surface of the vertebral bodies posteriorly. The paravertebral (costovertebral) regions bilaterally should be included in any discussion of mediastinal masses. Practically, it is easiest to divide the mediastinum into three compartments: anterior, middle (visceral), and posterior which includes the paravertebral sulci. The anterior mediastinum contains the thymus gland, internal mammary vessels, lymph nodes, connective tissue, and fat. The middle mediastinum contains the pericardium, heart and great vessels, trachea, proximal bronchi, esophagus, phrenic and vagus nerves, and lymph nodes. The posterior mediastinum contains the autonomic ganglia and nerves, proximal portions of the intercostal vessels, lymph nodes, and fat. Further in-depth mediastinal anatomic details are discussed in Chapter 162.

Davis and colleagues⁹ reported on 400 consecutive patients with primary mediastinal masses and found 54% located in the anterior mediastinum, 20% in the middle, and 26% in the posterior. Takeda and colleagues³⁴ reported similar mediastinal mass distributions for adults. In the series by Davis and colleagues,⁹ 59%, 29%, and 16% of the anterior, middle, and posterior mediastinal masses, respectively, were malignant.

Primary mediastinal tumors and cysts are uncommon. Summaries of selected major reports in the American, European, and Japanese literature—shown in Tables 174-1 and 174-2—give some insight into their incidence. Those reports from 1956 to 2003 total 3,735 adults and children. Not included in that patient number are other reports dedicated to certain specific types of mediastinal tumors.

Perhaps a more helpful measure of the incidence of primary mediastinal tumors and cysts is the average number of cases seen annually in large institutions. Wychulis and colleagues⁴⁰ from the Mayo Clinic reported on 1,064 surgical patients seen over the 40-year period from 1929 through 1968 (approximately 27 patients per year). Davis and colleagues⁹ reported approximately seven patients per year from the Duke University Medical Center over a 55-year period. Cohen and associates⁷ from the Walter Reed Army Medical Center reported approximately five patients per year from 1944 to 1989. A review of the New Mexico Tumor Registry³⁶ from 1973 to 1995 revealed approximately 10 primary mediastinal malignancies per year. Teixeria and Bibas³⁵ reported eight patients per year at the Hospital des Servidores de Estrado in Brazil during the 10-year period from 1975 through 1985. More recently, Takeda and associates³⁴ reported an experience of 16 cases per year from 1951 to 2000 at the Toneyama
National Hospital. Thus it is evident that these masses are rarely encountered by the average thoracic surgeon. Nonetheless, familiarity with the clinical features and location of the various masses is essential.

Primary mediastinal masses are a heterogeneous mixture of neoplastic, congenital, and inflammatory conditions.³ Of surgically resected mediastinal masses, benign cysts, neurogenic tumors, and thymomas account for almost 20% each, while lymphoma and teratoma account for an additional 10% each. The remaining 20% of resected masses include granulomas, intrathoracic goiters, mesenchymal tumors, and primary carcinoma.⁴⁰ In a collection of 214 mediastinal cysts from five authors, Oldham found that 41% were bronchogenic, 35% pericardial, 10% enteric, and 14% nonspecific.²⁷

Characteristically, each variety of mediastinal cyst or tumor has a predilection for a specific compartment, as shown in Table 174-3. Nonetheless, migration or growth from one compartment into an adjoining compartment is not uncommon. Additionally, masses from specific tissue may originate in more than one space. This is especially true for lymphatic tumors, which may originate in both the anterior and middle compartments and even rarely in the posterior compartment. Neurogenic tumors most commonly occur in one of the paravertebral sulci but can also arise from the phrenic or vagus nerves, located in the middle compartment. In children, the paravertebral sulci are the most common location of primary mediastinal tumors, with 52% of their mediastinal masses being found here.³⁴ Tumors of mesenchymal origin (hemangiomas, lipomas, lymphangiomas, and their malignant counterparts) may occur in any of the mediastinal compartments. Table 174-4 outlines the relative frequency of primary anterior mediastinal tumors in 702 adults. Finally, many lesions arising outside the mediastinum may project into the various compartments and masquerade as primary mediastinal masses on a chest radiograph (see Chapter 175).

The incidence and types of primary mediastinal tumors and cysts varies with patient age. The collected series highlighted in Table 174-1 show that in infants and children, the primary mediastinal masses in order of decreasing frequency are neurogenic tumors, germ cell tumors, enterogenous (foregut) cysts, lymphomas, angiomas and lymphangiomas, thymic tumors, stem cell tumors,
and pleuropericardial cysts. In the collected series totaling 3,017 mostly adult patients (Table 174-2), the masses in decreasing order of frequency were thymomas and thymic cysts, neurogenic tumors, germ cell tumors, lymphomas, enterogenous cysts, and pleuropericardial cysts. Mullen and Richardson²⁴ found that thymomas constituted 47% of all mediastinal tumors in adults in the anterior compartment (Table 174-4). Thus, from the collective series reviewed, mediastinal masses of thymic origin appear to be the most common primary mediastinal masses in adults. It should, however, be noted that in a review of mediastinal tumors in the files of the Walter Reed Army Medical Center and Walter Reed Tumor Registry, while thymic lesions were overall most common, lymphomas constituted the largest number of adult mediastinal tumors in the last 19 years of this 45-year review.⁷