The superior mediastinum is bounded above by the thoracic inlet and below by a line from the sternomanubrial joint to the fourth thoracic vertebral body. The other mediastinal compartments lie inferior to this line. Abnormalities in the superior mediastinum include masses that extend down into the upper
mediastinum from the neck, such as thyroid goiters or cystic hygromas; mediastinal adenopathy; and vascular abnormalities, such as aneurysms.

Also referred to as the prevascular space, the anterior mediastinum is bounded above by the superior mediastinum, laterally by pleura, anteriorly by the sternum, and posteriorly by pericardium and great vessels. This compartment contains areolar tissue, lymph nodes, lymphatic vessels, thymus gland, thyroid gland, parathyroid glands, and internal mammary arteries and veins.

Masses occurring in the anterior mediastinum are listed in Table 6-1. In addition to the "4 Ts" (thymoma, thyroid mass, teratoma, and terrible lymphoma), which account for the great majority of lesions, there are many other less common causes of an anterior mediastinal mass. These include vascular tortuosity or aneurysm, cardiac tumors or prominent pericardiac fat (Fig. 6-2), cystic hygroma, bronchogenic cyst, pericardial cyst, hemangioma, lymphangioma, parathyroid adenoma (Fig. 6-3), various other mesenchymal tumors (e.g., fibroma or lipoma), sternal tumor, primary lung tumor invading the anterior mediastinum, Morgagni hernia (Fig. 6-4), abscess (Fig. 6-5), and mediastinal lipomatosis (Fig. 6-6).

An intrathoracic thyroid mass is usually a benign multi-nodular goiter that originates in the neck and extends downward into the mediastinum through the thoracic inlet. This continuity is an important diagnostic feature on chest radiography. Many thyroid masses displace or narrow the trachea (Fig. 6-7). Another useful sign of a thyroid mass is the relative high attenuation value of the thyroid tissue, at least 20 Hounsfield units above that seen in adjacent muscles on both precontrast and postcontrast computed tomographic (CT) images (2). CT scans can show cystic components. Calcification is common and usually caused by benign disease when it is dense, amorphous, and well defined with a nodular, curvilinear, or circular configuration. Distinguishing between benign and malignant thyroid masses on chest radiography or CT scanning is not possible unless the tumor has clearly spread beyond the
thyroid gland (indicating malignancy). Malignant thyroid masses can also have calcifications, generally with a configuration of fine dots grouped in a cloudlike formation (3,4,5). However, the patterns of benign and malignant calcifications serve as general guidelines, and malignant medullary thyroid carcinoma can contain well-defined, occasionally ring-shaped, dense calcifications that are similar in appearance to the calcifications seen with benign thyroid goiter. Radionuclide imaging is a very sensitive and specific method of determining the thyroid nature of an intrathoracic mass (Fig. 6-8), but CT provides more information about the mass.

Thymomas are tumors consisting of thymic epithelial cells and reactive lymphocytes, with noninvasive or invasive patterns of growth (6). The presence or absence of tumor spread beyond the capsule (usually determined surgically), rather than the histologic appearance within the thymus, determines whether a tumor is benign or malignant. Thymomas occur at an average age of 50 years and with equal frequency in men and women (6). They are associated with a variety of autoimmune diseases, most notably myasthenia gravis. Approximately 40% of patients with thymoma have myasthenia gravis, and the incidence of thymoma in patients with this disease is approximately 15% (7). Most thymomas arise in the upper anterior mediastinum, anterior to the ascending aorta above

the right ventricular outflow tract and main pulmonary artery (Fig. 6-9). They can extend into the adjacent middle or posterior mediastinum, and they can occur or extend into the lower third of the mediastinum, as low as the cardiophrenic angles. Punctate, curvilinear, or ringlike calcification is common in both benign and invasive thymomas (8). On a CT scan, thymomas are usually of homogeneous attenuation and show uniform enhancement, but rarely they can appear cystic with discrete nodular components (Figs. 6-10 and 6-11). In patients under age 40, diagnosing a small thymoma can be difficult because the normal gland is variable in size. A normal thymus, in contrast to a thymic mass, conforms to the shape of the adjacent great vessels on CT and magnetic resonance imaging (MRI). A mass gives rise to focal thymic enlargement, usually with its center away from the midline, whereas a normal gland is approximately symmetric and maintains a somewhat triangular shape on axial imaging. Invasive thymomas inhabit the mediastinal fat, spreading to the pericardium and pleura. Unless mediastinal invasion has occurred, distinguishing benign from invasive thymoma is not possible with CT scanning. Transpleural spread may manifest as so-called "drop metastases" at a site distant from the primary lesion (Fig. 6-12), and imaging of the entire pleural space and upper abdomen is therefore important. Extensive pleural involvement may mimic malignant mesothelioma. Other less common thymic masses include cyst (Fig. 6-13), abscess, thymolipoma, malignant lymphoma (most notably Hodgkin lymphoma), thymic carcinoid, germ cell tumors, and thymic carcinoma (Fig. 6-14).

A teratoma is a neoplasm derived from more than one embryonic germ layer. Other germ cell tumors include benign dermoid cyst, malignant seminoma (the most common germ cell tumor), teratocarcinoma, embryonal carcinoma, endodermal sinus tumor (yolk sac tumor), choriocarcinoma, and mixtures of these types. Germ cell tumors in the mediastinum arise from primitive rest cells and generally are not metastatic from gonadal tumors. Some malignant germ cell tumors secrete beta-human chorionic gonadotropin and alpha-fetoprotein, which can be used to diagnose and monitor progression of disease.

Benign teratomas are found in patients of all ages but are most common in adolescents and young adults (Fig. 6-15). They usually produce a well-defined, rounded, or lobulated mass in the anterior mediastinum. They grow slowly, although rapid increase in size may occur as a result of hemorrhage, producing imaging features suggestive of a malignant mass.



Calcification, ossification, teeth, or fat may be visible on a chest radiograph and on CT scans. CT scans may show cystic components and/or a fat–fluid level. A cyst wall with curvilinear calcification is often present. Unequivocal fat within the mass confirms the diagnosis of teratoma, but the absence of fat or calcium does not exclude a teratoma (Fig. 6-16).

Imaging features of malignant germ cell tumors are similar to those of benign teratoma except that fat density is not noted and calcification is rare. The malignant tumors grow rapidly, and metastases may be seen in the lungs, bones, or pleura. The adjacent mediastinal fat planes may be obliterated. The tumors may be of homogeneous attenuation or show areas of contrast enhancement interspersed with rounded areas of decreased attenuation from necrosis and hemorrhage. Rarely, coarse tumor calcification may be seen (9).

Lymphoma often extends beyond the anterior mediastinal compartment, involving a variety of lymph node chains, and is discussed along with middle mediastinal masses (see following). When lymphoma is isolated to the anterior mediastinum, the CT appearance can be similar to that of thymoma and germ cell neoplasms.

Also referred to as the vascular space, the middle mediastinum is bounded in front by the anterior mediastinum and posteriorly by the posterior mediastinum. The middle mediastinum contains the heart and pericardium, ascending and transverse aorta, superior vena cava and azygos vein that empties into it, phrenic nerves, upper vagus nerves, trachea and its bifurcation, main bronchi, pulmonary artery and its two branches, pulmonary veins, and adjacent lymph nodes. The esophagus is variably described as a middle or posterior mediastinal structure. The main categories of abnormalities occurring in the middle mediastinum include adenopathy, aneurysm/vascular abnormalities, and abnormalities of development (Table 6-2). Less common middle mediastinal abnormalities include giant lymph node hyperplasia (Castleman disease; Fig. 6-17); neural tumor (involving the vagus or phrenic nerve); abscess; fibrosing mediastinitis; hiatal hernia (Fig. 6-18); primary tumors of the trachea or esophagus (namely, leiomyoma, leiomyosarcoma, or carcinoma; Figs. 6-19 and 6-20); and hematoma.

Also referred to as the postvascular space, the posterior mediastinum lies behind the heart and pericardium and contains the thoracic descending aorta, esophagus, thoracic duct, azygos and hemiazygos veins, lymph nodes, sympathetic chains, and inferior vagus nerves. Neural tumors are the most common tumors to develop in the posterior mediastinum, but a variety of uncommon abnormalities can occur in this compartment, as listed in Table 6-3 (Figs. 6-35, 6-36, 6-37, 6-38).

Neural tumors can be differentiated into nerve sheath tumors and ganglion cell tumors. Nerve sheath tumors comprise schwannomas, neurofibromas, and their malignant counterparts. The schwannoma is the most common intrathoracic




nerve sheath tumor (Fig. 6-39). Both schwannomas and neurofibromas are derived from Schwann cells and occur most commonly in patients in their 30s and 40s. Almost all intrathoracic nerve sheath tumors arise from either the intercostal or sympathetic nerves.

The ganglion cell tumors include neuroblastoma (malignant), ganglioneuroma (benign), and ganglioneuroblastoma (intermediate between benign and malignant). The adrenal gland is the most common primary site for these tumors, with the mediastinum being the second most common site. Ganglioneuroma can occur between the ages of 1 and 50, while neuroblastoma and ganglioneuroblastoma occur during childhood, generally under the age of 20 (19).

Nerve sheath and ganglion cell tumors are seen as well-defined masses with a smooth or lobulated outline on imaging studies. Some are very large and can occupy most of a hemithorax. Calcification can be seen in all types. In neuroblastoma, the calcification is usually finely stippled, whereas in ganglioneuroblastoma and ganglioneuroma it is denser and coarser. The bone adjacent to the tumor shows a scalloped edge, with preservation but thickening of the bony cortex. The ribs can be thinned and splayed apart, and the intervertebral foramina can appear widened. On CT scans, many tumors have mixed attenuation, including low-attenuation regions, that enhance on images taken after administration of intravascular contrast material.

Paragangliomas are tumors of the paraganglionic cells and can be benign or malignant chemodectomas or pheochromocytomas. Mediastinal paragangliomas are rare, comprising only 2% of the large series of neural tumors of the thorax (19). Paragangliomas occur in the area of the aortic arch and are classified as aortic body tumors. They form rounded soft tissue masses that are extremely vascular and enhance brightly on CT scans after administration of intravenous contrast material.