Chapter 12 Recognizing Diseases of the Chest
TABLE 12-1 CHEST ABNORMALITIES DISCUSSED ELSEWHERE IN THIS TEXT
| Topic | Appears in |
|---|---|
| Atelectasis | Chapter 5 |
| Pleural effusion | Chapter 6 |
| Pneumonia | Chapter 7 |
| Pneumothorax, pneumomediastinum, and pneumopericardium | Chapter 8 |
| Cardiac and thoracic aortic abnormalities | Chapter 9 |
| Chest trauma | Chapter 17 |
Mediastinal Masses
The mediastinum is an area whose lateral margins are defined by the medial borders of each lung, whose anterior margin is the sternum and anterior chest wall, and whose posterior margin is the spine, usually including the paravertebral gutters.
The mediastinum can be arbitrarily subdivided into three compartments: the anterior, middle, and posterior compartments—and each contains its favorite set of diseases (Fig. 12-1).
The mediastinum can be arbitrarily subdivided into three compartments: anterior, middle, and posterior, with each containing its favorite set of diseases. The anterior mediastinum is the compartment that extends from the back of the sternum to the anterior border of the heart and great vessels (dotted black outline). The middle mediastinum is the compartment that extends from the anterior border of the heart and aorta to the posterior border of the heart and origins of the great vessels (solid white outline). The posterior mediastinum is the compartment that extends from the posterior border of the heart to the anterior border of the vertebral column (solid black outline). For practical purposes, however, it is considered to extend into the paravertebral gutters.
Pitfall: Since these compartments have no true anatomic boundaries, diseases from one compartment may extend into another compartment. When a mediastinal abnormality becomes extensive or a mediastinal mass becomes quite large, it is often impossible to determine which compartment was its site of origin.
Differentiating a mediastinal from a parenchymal lung mass on frontal and lateral chest radiographs:
Anterior Mediastinum
The anterior mediastinum is the compartment that extends from the back of the sternum to the anterior border of the heart and great vessels.
TABLE 12-2 ANTERIOR MEDIASTINAL MASSES (“3 Ts and an L”)
| Mass | What to Look For |
|---|---|
| Thyroid goiter | The only anterior mediastinal mass that routinely deviates the trachea |
| Lymphoma (lymphadenopathy) | Lobulated, polycyclic mass, frequently asymmetrical, that may occur in any compartment of the mediastinum |
| Thymoma | Look for a well-marginated mass that may be associated with myasthenia gravis |
| Teratoma | Well-marginated mass that may contain fat and calcium on CT scans |
Thyroid Masses
In everyday practice, enlarged substernal thyroid masses are the most frequently encountered anterior mediastinal mass. The vast majority of these masses are multinodular goiters, and the mass is called a substernal goiter or substernal thyroid or substernal thyroid goiter.
On occasion, the isthmus or lower pole of either lobe of the thyroid may enlarge but project downward into the upper thorax rather than anteriorly into the neck.
Substernal goiters characteristically displace the trachea either to the left or right above the level of the aortic arch, a tendency the other anterior mediastinal masses do not typically demonstrate. Classically, substernal goiters do not extend below the top of the aortic arch (Fig. 12-2).
Figure 12-2 Substernal thyroid mass.
The lower pole of the thyroid may enlarge but project downward into the upper thorax (white oval) rather than anteriorly into the neck. Classically, substernal thyroid goiters produce mediastinal masses that do not extend below the top of the aortic arch (solid white arrow). Substernal goiters characteristically displace the trachea (solid black arrow) either to the left or right above the aortic knob, a tendency the other anterior mediastinal masses do not typically demonstrate. Therefore, you should think of an enlarged substernal thyroid goiter whenever you see an anterior mediastinal mass that displaces the trachea.
Therefore, you should think of an enlarged substernal thyroid whenever you see an anterior mediastinal mass that displaces the trachea.
Radioisotope thyroid scans are the study of choice in confirming the diagnosis of a substernal thyroid because virtually all goiters will display some uptake of the radioactive tracer that can be imaged and recorded with a special camera.
On CT scans, substernal thyroid masses are contiguous with the thyroid gland, frequently contain calcification, and avidly take up intravenous contrast but with a mottled, inhomogeneous appearance (Fig. 12-3).
Figure 12-3 CT of a substernal thyroid goiter without and with contrast enhancement.
These are two images at the same level in a patient who was scanned both before (A) and then after intravenous contrast administration (B). A, On CT scans, substernal thyroid masses (solid white arrow) are contiguous with the thyroid gland, frequently contain calcification (dotted white arrow) and (B) avidly take up intravenous contrast but with a mottled, inhomogeneous appearance (solid white arrow). This mass is displacing the trachea (T) slightly to the left.
Lymphoma
Lymphadenopathy, whether from lymphoma, metastatic carcinoma, sarcoid, or tuberculosis, is the most common cause of a mediastinal mass overall.
Anterior mediastinal lymphadenopathy is most common in Hodgkin disease, especially the nodular sclerosing variety.
Hodgkin disease is a malignancy of the lymph nodes, more common in females, which most often presents with painless, enlarged lymph nodes in the neck.
Unlike teratomas and thymomas, which are presumed to expand outward from a single abnormal cell, lymphomatous masses are frequently composed of several contiguously enlarged lymph nodes. As such, lymphadenopathy frequently presents with a border that is lobulated or polycyclic in contour owing to the conglomeration of enlarged nodes that make up the mass.
On chest radiographs, this finding may help differentiate lymphadenopathy from other mediastinal masses.
Mediastinal lymphadenopathy in Hodgkin disease is usually bilateral and asymmetrical (Fig. 12-4). In addition, asymmetrical hilar adenopathy is associated with mediastinal adenopathy in many patients with Hodgkin disease.
Lymphoma will produce multiple, lobulated soft-tissue masses or one large soft-tissue mass from lymph node aggregation.
The mass is usually homogeneous in density on CT but may be heterogeneous when it achieves a sufficient size to undergo necrosis (areas of lower attenuation, i.e., blacker) or hemorrhage (areas of higher attenuation, i.e., whiter) (Fig. 12-5).
Some findings of lymphoma may mimic those of sarcoid since both produce thoracic adenopathy. Table 12-3 helps to differentiate them.
Figure 12-4 Mediastinal adenopathy from Hodgkin disease.
Lymphadenopathy frequently presents with a lobulated or polycyclic border due to the conglomeration of enlarged nodes that produce the mass (solid white arrows). This finding may help differentiate lymphadenopathy from other mediastinal masses. Mediastinal lymphadenopathy in Hodgkin disease is usually bilateral (dotted white arrows) and frequently asymmetric.
Figure 12-5 CT of anterior mediastinal adenopathy in Hodgkin disease.
On CT, lymphomas will produce multiple, lobulated soft-tissue masses or a large soft-tissue mass from lymph node aggregation (solid white arrows). The mass is usually homogeneous in density, as in this case, but may be heterogeneous when the nodes achieve a sufficient size to undergo necrosis (areas of low attenuation, i.e., blacker) or hemorrhage (areas of high attenuation, i.e., whiter). The superior vena cava (SVC) is compressed by the nodes while the ascending (A) and descending aorta (Ao) are typically less so.
TABLE 12-3 SARCOIDOSIS VS. LYMPHOMA
| Sarcoid | Lymphoma |
|---|---|
| Bilateral hilar and right paratracheal adenopathy classic combination | More often mediastinal adenopathy, associated with asymmetrical hilar enlargement |
| Bronchopulmonary nodes more peripheral | Hilar nodes more central |
| Pleural effusion in about 5% | Pleural effusion more common—in 30% |
| Anterior mediastinal adenopathy is uncommon | Anterior mediastinal adenopathy is common |
Thymic Masses
Normal thymic tissue can be visible on CT throughout life, although the gland begins to involute after age 20.
Thymomas are neoplasms of thymic epithelium and lymphocytes. They occur most often in middle-aged adults, generally at an older age than those with teratomas. Most thymomas are benign.
Thymomas are associated with myasthenia gravis about 35% of the time they are present. Conversely, about 15% of patients with clinical myasthenia gravis will be found to have a thymoma. The importance of identifying a thymoma in patients with myasthenia gravis lies in the favorable prognosis for patients with myasthenia after thymectomy.
On CT scans, thymomas classically present as a smooth or lobulated mass that arises near the junction of the heart and great vessels, which, like a teratoma, may contain calcification (Fig. 12-6).
Figure 12-6 Thymoma, chest radiograph, and CT scan.
Thymomas are neoplasms of thymic epithelium and lymphocytes that occur most often in middle-aged adults, generally at an older age than those with teratomas. A, The chest radiograph shows a smoothly marginated anterior mediastinal mass (solid black arrow). B, Contrast-enhanced CT scan confirms the anterior mediastinal location and homogeneous density of the mass (solid white arrow). The patient had myasthenia gravis and improved following resection of the thymoma. (A = ascending aorta; Ao = descending aorta; PA = main pulmonary artery; SVC = superior vena cava.)
Teratoma
Teratomas are germinal tumors that typically contain all three germ layers (ectoderm, mesoderm, and endoderm). Most teratomas are benign and occur earlier in life than thymomas. Usually asymptomatic and discovered serendipitously, about 30% of mediastinal teratomas are malignant and have a poor prognosis.
The most common variety of teratoma is cystic; it produces a well-marginated mass near the origin of the great vessels that characteristically contains fat, cartilage, and possibly bone on CT examination (Fig. 12-7).
Figure 12-7 Mediastinal teratoma.
Teratomas are germinal tumors that typically contain all three germ layers. They tend to be discovered at a younger age than thymomas. The most common variety of teratoma is cystic, as in this case (solid white arrows). As shown here, they usually produce a well-marginated mass near the origin of the great vessels. On CT, they characteristically contain fat (solid black arrow), cartilage, and sometimes bone (dotted white arrow).
Middle Mediastinum
The middle mediastinum is the compartment that extends from the anterior border of the heart and aorta to the posterior border of the heart and contains the heart, the origins of the great vessels, trachea, and main bronchi along with lymph nodes (see Fig. 12-1).
Lymphadenopathy produces the most common mass in this compartment. While Hodgkin disease is the most likely cause of mediastinal adenopathy, other malignancies and several benign diseases can produce such findings.
Figure 12-8 Middle mediastinal lymphadenopathy.
While lymphoma is the most likely cause of adenopathy in the middle mediastinum, other malignancies, such as small cell lung carcinoma and metastatic disease, as well as several benign diseases, can produce these findings. This patient has a mediastinal mass demonstrated on both the frontal (A) (solid white arrows) and lateral (B) views (solid black arrow). The mass is pushing the trachea forward (dotted white arrow) on the lateral view. The biopsied lymph nodes in this patient demonstrated small cell carcinoma of the lung.
Posterior Mediastinum
The posterior mediastinum is the compartment that extends from the posterior border of the heart to the anterior border of the vertebral column. For practical purposes, however, it is considered to extend to either side of the spine into the paravertebral gutters (see Fig. 12-1).
Neurogenic Tumors
Although neurogenic tumors produce the largest percentage of posterior mediastinal masses, none of these lesions is particularly common. Neurogenic tumors include such entities as neurofibroma, schwannoma (neurilemmoma), ganglioneuroma, and neuroblastoma.
Nerve sheath tumors (Schwannoma or neurilemmoma) are the most common and are almost always benign. They usually affect persons 20-50 years of age. These slow-growing tumors may produce no symptoms until late in their course.
Neoplasms that arise from nerve elements other than the sheath, such as ganglioneuromas and neuroblastoma, are usually malignant.
Neurogenic tumors will produce a soft tissue mass, usually sharply marginated, in the paravertebral gutter (Fig. 12-9). Both benign and malignant tumors may erode ribs (Fig. 12-10A). They may enlarge the neural foramina producing dumbbell–shaped lesions that arise from the spinal canal but project through the neural foramen into the mediastinum (Fig. 12-10B).
Neurofibromas can occur as an isolated tumor arising from the Schwann cell of the nerve sheath or as part of a syndrome called neurofibromatosis. As part of the latter, they are a component of a neurocutaneous bone dysplasia that can cause numerous abnormalities, including subcutaneous nodules, erosion of adjacent bone (rib notching), scalloping of the posterior aspect of the vertebral bodies (Fig. 12-11), absence of the sphenoid wings, pseudarthroses, and sharp-angled kyphoscoliosis at the thoracolumbar junction.
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