Exercise: Mediastinal Masses and Compartments

EXERCISE 4-11. MEDIASTINAL MASSES AND COMPARTMENTS

4-17. The chest radiograph in Figure 4-52 shows

A.   an anterior mediastinal mass.

B.   a middle mediastinal mass.

C.   a posterior mediastinal mass.

D.   a superior mediastinal mass.

4-18. The chest radiograph in Figure 4-53 shows

A.   an anterior mediastinal mass.

B.   a middle mediastinal mass.

C.   a posterior mediastinal mass.

D.   a superior mediastinal mass.

4-19. The chest radiograph in Figure 4-54 shows

A.         an anterior mediastinal mass.

B.         a middle mediastinal mass.

C.         a posterior mediastinal mass.

D.         a superior mediastinal mass.

Radiologic Findings

4-17. A spherical mass 4 cm in diameter is present in the subcarinal region on the frontal radiograph (Figure 4-52 A), and superimposed on the hilar region on the lateral radiograph (Figure 4-52 B). CT (Figure 4-55) shows that the lesion is of fluid attenuation (greater attenuation than the subcutaneous fat, but less atten-uation than muscle). This mass is in the middle mediastinum. (B is the correct answer to Question 4-17.) In an asymptomatic individual, this most likely represents a congenital bronchogenic cyst. These masses can grow to sufficient size to cause symptoms such as dyspnea or dysphagia owing to compression of the trachea or esophagus. Bron-chogenic cysts may also occur within the lungs and are often surgically resected because of the likelihood of pulmonary infection. The differential diagnosis of a middle mediastinal mass can be seen in Table 4-10.

4-18. The frontal radiograph (Figure 4-53) shows a lobu-lated mass to the right of the lower thoracic vertebrae. Note that the right heart border remains visible, sug-gesting that this mass is either anterior or posterior to

Discussion

Two methods of dividing the mediastinum for radiographic purposes are in common use. The radiographic divisions are arbitrary and are intended to provide the most appro-priate differential diagnosis for abnormalities that occur in these locations. Neither of the divisions follows the divi-sions used by anatomists. In the older system, the medi-astinum is divided into three compartments. The anterior mediastinum is that portion of the mediastinum that is an-terior to the anterior margin of the trachea and along the posterior margin of the pericardium and inferior vena cava. The posterior mediastinum lies behind a plane that extends the length of the thorax behind a line drawn 1 cm posteri-orly to the anterior margin of the vertebral column. The middle mediastinum is the region between these two boundaries. This system has been superseded by a four-compartment model, which designates a superior mediasti-nal compartment as the space that lies above a planeextending from the sternomanubrial junction to the lower border of the fourth thoracic vertebra. The anterior medi-astinum is just caudad to the superior compartment and is anterior to a plane extending along the anterior aspect of the tracheal air column and along the anterior pericardium. Note that the heart shifts from the anterior to the middle mediastinum with the four-compartment system. The mid-dle mediastinum occupies the area from the anterior peri-cardium backward to a plane 1 cm posterior to the anterior margin of the vertebral column. The addition of the fourth compartment occurred when CT was developed and it be-came easier to identify structures in each compartment.

The differential diagnosis of lesions occurring in each compartment is in part dependent on the structures that exist there (see Table 4-10). Note that there are vascular structures and lymph nodes in each of the compartments. Therefore, abnormalities of the blood vessels (eg, aneurysms) and lymph node diseases (eg, lymphoma) would have to be included in the differential diagnosis of diseases occurring there. The differential diagnosis lists include the most com-mon disorders occurring in each region. The most com-mon mass to occur in the superior mediastinum is an enlarged substernal thyroid, which may become large enough to extend into the anterior or middle mediastinum (Figure 4-58 A,B).