Chapter 4
Mediastinum
Lymph Node Abnormalities and Masses
Lymph Node Groups
Mediastinal lymph nodes are generally classified by location, and most descriptive systems are based on a modification of Rouvière’s classification of lymph node groups.
Anterior Mediastinal Nodes
Internal mammary nodes are located in a retrosternal location near the internal mammary artery and veins (Fig. 4-1). They drain the anterior chest wall, anterior diaphragm, and medial breasts.
Paracardiac nodes (diaphragmatic, epiphrenic, and pericardial) surround the heart on the surface of the diaphragm and communicate with the lower internal mammary chain (Fig. 4-2). Like internal mammary nodes, they are most commonly enlarged in patients with lymphoma and metastatic carcinoma, particularly breast cancer.
Middle Mediastinal Nodes
Lung diseases (e.g., lung cancer, sarcoidosis, tuberculosis, and fungal infections) that secondarily involve lymph nodes typically involve middle mediastinal lymph nodes.
Pretracheal or paratracheal nodes occupy the pretracheal (or anterior paratracheal) space (Figs. 4-1, 4-3, and 4-4A). These nodes form the final pathway for lymphatic drainage from most of both lungs (except the left upper lobe). Because of this, they are commonly abnormal regardless of the location of the lung disease.
Aortopulmonary nodes are considered by Rouvière to be in the anterior mediastinal group, but because they serve the same function as right paratracheal nodes, they are illustrated in Figures 4-3C, 4-4B, and 4-4C. The left upper lobe is drained by this node group.
Subcarinal nodes are located in the subcarinal space, between the main bronchi (Fig. 4-4B to D), and drain the inferior hila and both lower lobes. They communicate in turn with the right paratracheal chain.
Peribronchial nodes surround the main bronchi on each side (Figs. 4-4B and C). They communicate with bronchopulmonary (hilar; Figs. 4-4C and D), subcarinal, and paratracheal nodes.
Posterior Mediastinal Nodes
Paraesophageal nodes lie posterior to the trachea or are associated with the esophagus, or both (Fig. 4-5). Subcarinal nodes are not included in this group.
Inferior pulmonary ligament nodes are located below the pulmonary hila, medial to the inferior pulmonary ligament. On computed tomography (CT), they are usually seen adjacent to the esophagus on the right and the descending aorta on the left. Below the hila, they are difficult to distinguish from paraesophageal nodes. Together with the paraesophageal nodes, they drain the medial lower lobes, esophagus, pericardium, and posterior diaphragm.
Paravertebral nodes lie lateral to the vertebral bodies, posterior to the aorta on the left (Fig. 4-5). They drain the posterior chest wall and pleura. They are most commonly involved, together with the retrocrural or retroperitoneal abdominal nodes, in patients with lymphoma or metastatic carcinoma.
Lymph Node Stations
In the 1970s, the American Joint Committee on Cancer (AJCC) and Union Internationale Contre le Cancer (UICC) introduced a numeric system for localization of intrathoracic lymph nodes for the purpose of lung cancer staging. Lymph nodes were described relative to regions in the mediastinum termed lymph node stations. The AJCC/UICC node-mapping system was modified in 1983 by the American Thoracic Society to more precisely define anatomic and CT criteria for each station, and the American Thoracic Society classification system has been in common usage since its development. In 1997, the AJCC/UICC published a further revision intended to be a compromise between the AJCC and American Thoracic Society classifications. Detailed knowledge of these lymph node stations is not necessary in clinical practice.
In 2009, the International Association for the Study of Lung Cancer (IASLC) introduced a simplified and more practical (and easily remembered) system for classifying lymph nodes, based on lung cancer survival statistics, in conjunction with a revision of the lung cancer staging system (Table 4-1). This classifies mediastinal nodes into four groups or zones known as (1) the upper zone (paratracheal and prevascular nodes), (2) the aortopulmonary zone (aortopulmonary window nodes), (3) the subcarinal zone (subcarinal nodes), and (4) the lower zone (paraesophageal and inferior pulmonary ligament nodes). Hilar lymph nodes and more peripheral peribronchial nodes represent two additional groups. Table 4-1 provides a comparison of IASLC zones and AJCC/UICC lymph node stations, and a diagrammatic representation of AJCC/UICC lymph node stations (Fig. 4-6) is provided for localization of node zones.
TABLE 4-1
International Association for the Study of Lung Cancer (IASLC) Lymph Node Zones compared to AJCC/UICC Node Stations
| IASLC Nodal Groups | AJCC/UICC Description | AJCC/UICC Station |
| Mediastinal zone | ||
| Upper zone | Right upper paratracheal | 2R |
| Left upper paratracheal | 2L | |
| Prevascular | 3 | |
| Right lower paratracheal | 4R | |
| Left lower paratracheal | 4L | |
| Aortopulmonary zone | Subaortic | 5 |
| Paraaortic | 6 | |
| Subcarinal zone | Subcarinal | 7 |
| Lower zone | Paraesophageal | 8 |
| Pulmonary ligament | 9 | |
| Hilar/interlobar zone | Hilar | 10 |
| Interlobar | 11 | |
| Peripheral zone | Lobar | 12 |
| Segmental | 13 | |
| Subsegmental | 14 |
CT Appearance of Lymph Nodes
Lymph nodes are generally visible as discrete, round or elliptical in shape, and of soft-tissue attenuation, surrounded by mediastinal fat and distinguishable from vessels by their location. They often occur in clusters (Fig. 4-7). In some locations, nodes that contact vessels may be difficult to identify without contrast infusion. Normal lymph nodes may show a fatty hilum (Fig. 4-7).
Internal mammary nodes, paracardiac nodes, and paravertebral nodes are not usually seen on CT in healthy subjects, but in other areas of the mediastinum, normal nodes are often visible. The expected size of normal nodes varies with their location, and a few general rules apply. Subcarinal nodes can be quite large in healthy subjects. Pretracheal nodes are also commonly visible, but these nodes are typically smaller than normal subcarinal nodes. Nodes in the supra-aortic mediastinum are usually smaller than lower pretracheal nodes, and left paratracheal nodes are usually smaller than right paratracheal nodes.
Measurement of Lymph Node Size
The short axis or least diameter (i.e., the smallest node diameter seen in cross-section) is generally used when measuring the size of a lymph node. Measuring the short axis is better than measuring the long axis or greatest diameter because it more closely reflects the actual node diameter when nodes are obliquely oriented relative to the scan plane and shows less variation among healthy subjects. Different values for the upper limits of normal short-axis node diameter have been found for different mediastinal node groups (Table 4-2). However, except for the subcarinal regions, a short-axis node diameter of 1 cm or less is generally considered normal for clinical purposes. In the subcarinal region, 1.5 cm is usually considered to be the upper limit of normal.
Lymph Node Enlargement
Except in the subcarinal space, lymph nodes are considered to be enlarged if they have a short-axis diameter greater than 1 cm. In most cases, they are outlined by fat and are visible as discrete structures (Fig. 4-3). However, in the presence of inflammation or neoplastic infiltration, abnormal nodes can be matted together, giving the appearance of a single large mass or resulting in infiltration and replacement of mediastinal fat by soft-tissue opacity.
The significance given to the presence of an enlarged lymph node must be tempered by knowledge of the patient’s clinical situation. For example, if the patient is known to have lung cancer, then an enlarged lymph node has a 70% likelihood of tumor involvement. However, the same node in a patient without lung cancer is much less likely to be of clinical significance. In the absence of a known disease, an enlarged node must be regarded as likely to be hyperplastic or reactive. In addition, the larger a node is, the more likely it is to represent a significant abnormality. Mediastinal lymph nodes larger than 2 cm often have tumor involvement, although this may also be seen in patients with sarcoidosis or other granulomatous diseases.
Lymph Node Calcification
Calcification can be dense, involving the node in a homogeneous fashion, stippled, or egg-shell-like in appearance. The abnormal nodes are often enlarged but can also be of normal size. Multiple calcified lymph nodes are often visible, usually in contiguity.
Lymph node calcification usually indicates prior granulomatous disease, including tuberculosis, histoplasmosis and other fungal infections, and sarcoidosis (Fig. 4-8). The differential diagnosis also includes silicosis, coal workers’ pneumoconiosis, treated Hodgkin’s disease, metastatic neoplasm, typically mutinous adenocarcinoma, thyroid carcinoma, or metastatic osteogenic sarcoma. Egg-shell calcification is most often seen in patients with silicosis or coal workers’ pneumoconiosis, sarcoidosis, and tuberculosis.
Low-Attenuation or Necrotic Lymph Nodes
Enlarged lymph nodes may appear to be low in attenuation (Fig. 4-5), often with an enhancing rim if contrast has been injected. Typically, low-attenuation nodes reflect the presence of necrosis. They are commonly seen in patients with active tuberculosis, fungal infections, and neoplasms, such as metastatic carcinoma and lymphoma.
Lymph Node Enhancement
Normal lymph nodes may show some increase in attenuation after intravenous contrast infusion. Pathologic lymph nodes with an increased vascular supply may increase significantly in attenuation. The differential diagnosis of densely enhanced mediastinal nodes is limited and includes metastatic neoplasm (e.g., lung cancer, breast cancer, renal cell carcinoma, papillary thyroid carcinoma, sarcoma, and melanoma), Castleman’s disease (Fig. 4-15), angioimmunoblastic lymphadenopathy, infections such as tuberculosis, and sometimes sarcoidosis.
Differential Diagnosis of Mediastinal Lymph Node Enlargement
Lung Cancer
Approximately 35% of patients diagnosed with lung cancer have mediastinal node metastases (Fig. 4-9). Lung cancer most often involves the middle mediastinal node groups. Left upper lobe cancers typically metastasize to aortopulmonary window nodes, whereas tumors involving the lower lobes on either side tend to metastasize to the subcarinal and right paratracheal groups. Right upper lobe tumors typically involve paratracheal nodes.
Lung Cancer Staging
In patients with non–small-cell bronchogenic carcinoma, although the genetics, cell type, and histologic characteristics of the tumor affect prognosis, the anatomic extent of the tumor (tumor stage) is usually most important in determining the therapeutic approach. The most widely used anatomic staging classification is the TNM classification revised in 2009.
TABLE 4-3
TNM Classification of Lung Cancer (TNM 7, 2009)
| T (primary tumor) | |
| T0 | No evidence of a primary tumor |
| T1 | A tumor that is: |
| a. 3 cm or less in greatest diameter | |
| T1a ≤ 2 cm | |
| T1b > 2 cm but ≤ 3 cm | |
| b. Surrounded by lung or visceral pleura | |
| c. Without invasion proximal to a lobar bronchus (i.e., not involving main bronchus) | |
| T2 | A tumor with any of the following features: |
| a. Larger than 3 cm and ≤ 7 cm in greatest diameter | |
| T2a > 3 cm but ≤ 5 cm | |
| T2b > 5 cm but ≤ 7 cm | |
| b. Invades the visceral pleura | |
| c. Involves a main bronchus ≥ 2 cm distal to the carina | |
| d. Associated with atelectasis or obstructive pneumonia, extending to the hilum, but involving less than the entire lung | |
| T3 | A tumor with any of the following features: |
| a. Larger than 7 cm (T3>7) | |
| b. Associated with additional tumor nodule(s) in the same lobe (T3Satell) | |
| c. Invades chest wall, diaphragm, phrenic nerve, mediastinal pleura, or parietal pericardium (T3Inv) | |
| d. Involves the main bronchus < 2 cm distal to the carina, without involvement of the carina (T3Centr) | |
| e. Associated with atelectasis or obstructive pneumonia of an entire lung (T3Centr) | |
| T4 | A tumor of any size with any of the following features: |
| a. Invasion of the heart, great vessels, trachea, carina, recurrent laryngeal nerve, esophagus, or vertebral body (T4Inv) | |
| b. Additional tumor nodule(s) in another ipsilateral lobe (T4Ipsi Nod) | |
| N (regional lymph nodes) | |
| N0 | No regional lymph node metastases |
| N1 | Metastases to ipsilateral peribronchial, perihilar, or intrapulmonary nodes, including direct extension |
| N2 | Metastases to ipsilateral mediastinal nodes or subcarinal nodes |
| N3 | Metastases to contralateral hilar or mediastinal lymph nodes, or scalene or supraclavicular lymph nodes |
| M (distant metastases) | |
| M0 | Metastases absent |
| M1 | Metastases present |
| M1a | Intrathoracic metastases, with either |
| a. Tumor nodules in the contralateral lung (M1aContr Nod) | |
| b. Tumor with pleural nodules or malignant pleural effusion (M1aPl Dissem) | |
| (pleural effusion not obviously associated with metastases has no effect on stage) | |
| M1b | Distant metastases |
Modified from Rami-Porta R, Crowley JJ, Goldstraw P: The revised TNM staging system for lung cancer. Ann Thorac Cardiovasc Surg 15:4–9, 2009, and Detterbeck FC, Boffa DJ, Tanoue LT: The new lung cancer staging system. Chest 136:260–271, 2009.
This classification is based on consideration of the following characteristics: (1) the size, location, and extent of the primary tumor (T); (2) the presence or absence of lymph node metastases (N); and (3) the presence or absence of distant metastases (M) (Tables 4-3 and 4-4). Based on this classification, excellent correlations are found between tumor stage and survival after treatment.
For radiologic purposes, precise classification of tumor stage is not usually necessary. However, differentiation of potentially resectable stages (stages I to IIIa) and stages usually considered unresectable (stages IIIb to IV) is important (Table 4-4). The criteria for resectability are generally accepted but are not absolute; different surgeons may have different criteria for what is resectable. In particular, the resectability of some stage IIIa tumors is controversial; stage IIIa tumors may be treated with chemotherapy or radiation prior to resection.
TABLE 4-4
Lung Cancer Stage Based on TNM Classification (TNM 7, 2009)
| Stage Groups | T | N | M |
| Ia | T1a,b | N0 | M0 |
| Ib | T2a | N0 | M0 |
| IIa | T1a,b | N1 | M0 |
| T2a | N1 | M0 | |
| T2b | N0 | M0 | |
| IIb | T2b | N1 | M0 |
| T3 | N0 | M0 | |
| IIIa | T1–3 | N2 | M0 |
| T3 | N1 | M0 | |
| T4 | N0,1 | M0 | |
| IIIb | Any T | N3 | M0 |
| T4 | N2 | M0 | |
| IV | Any T | Any N | M1a,b |
Modified from Rami-Porta R, Crowley JJ, Goldstraw P: The revised TNM staging system for lung cancer. Ann Thorac Cardiovasc Surg 15:4–9, 2009, and Detterbeck FC, Boffa DJ, Tanoue LT: The new lung cancer staging system. Chest 136:260–271, 2009.
The next section discusses the impact of mediastinal lymph node metastases and mediastinal invasion on lung cancer staging. Chapters 5 and 7 discuss other aspects of lung cancer staging.
Mediastinal Node Metastases in Lung Cancer
In the TNM lung cancer staging system, ipsilateral mediastinal node metastases and subcarinal node metastases are termed N2 and are considered potentially resectable (although this is not always the case); contralateral nodes are termed N3 and are considered unresectable (Fig. 4-9).
In patients with lung cancer, the likelihood that a mediastinal node is involved by tumor is directly proportional to its size. However, although large nodes are most likely to be involved by tumor (Fig. 4-9), they can be benign; similarly, although small nodes are usually normal, they can harbor metastases. Although a short-axis measurement of greater than 1 cm is used in clinical practice to identify abnormally enlarged nodes, it is important to realize that no node diameter clearly separates benign nodes from those involved by tumor.
Using a short-axis node diameter of 1 cm as the upper limit of node size, CT will detect mediastinal lymph node enlargement in about 60% of patients with node metastases (CT sensitivity), whereas about 70% of patients with normal nodes will be classified normal on CT (CT specificity). Although CT is not highly accurate in diagnosing node metastases, it is commonly used to guide subsequent procedures or treatment.
In patients without node enlargement on CT, thoracotomy may be performed without prior mediastinoscopy, although this varies with the surgeon. Although some such patients will be found to have microscopic or small intranodal metastases, their presence does not necessarily indicate that surgery was inappropriate. Some patients with ipsilateral intranodal mediastinal metastases (N2) can have successful treatment results after surgical excision of nodes and radiation or chemotherapy.
In contrast, if mediastinal lymph node enlargement is seen on CT, about 70% of patients will have node metastases; benign hyperplasia of mediastinal lymph nodes accounts for the other 30%. Patients with large mediastinal nodes usually have node sampling at mediastinoscopy or by CT-guided needle biopsy before surgery. If node metastases are found at mediastinoscopy, surgery is not generally performed, even though the nodes would be classified as N2; it has been shown that patients with node metastases diagnosed at mediastinoscopy have a poor prognosis after surgery. It should be kept in mind that in patients with enlarged nodes on CT and mediastinal lymph node metastases, the metastases are not always in the nodes that appear large; that is, in some cases, CT can be right for the wrong reason.
Positron emission tomography (PET) scanning is more accurate than CT in the assessment of mediastinal lymph nodes in lung cancer and has assumed a significant role in preoperative staging. PET has a sensitivity of about 80% for diagnosis of mediastinal node metastases (vs. 60% for CT) and a specificity of about 90% (vs. 70% for CT). PET is usually combined with CT (PET-CT) because of the poor anatomic detail provided by PET alone.
Mediastinoscopy is slightly more sensitive than CT, has much higher specificity (100%; that is, no normal nodes are called abnormal based on mediastinoscopy), and has higher accuracy for diagnosing mediastinal node metastases. However, the mediastinoscopist cannot evaluate all mediastinal compartments or lymph node groups, and a significant percentage (up to 25%) of patients with bronchogenic carcinoma who have a negative mediastinoscopy result are found to have mediastinal nodal metastases at surgery. Using a standard transcervical approach, the mediastinoscopist can only evaluate pretracheal lymph nodes, nodes in the anterior subcarinal space, and lymph nodes extending anterior to the right main bronchus. Lymph nodes in the anterior mediastinum (prevascular space), aortopulmonary window, and posterior portions of the mediastinum (e.g., posterior subcarinal space and azygoesophageal recess) are inaccessible using this technique, although some can be evaluated using a left parasternal mediastinoscopy (Chamberlain procedure). CT and PET-CT, in contrast, allow evaluation of all these areas, show where any enlarged nodes are, and can serve to guide needle aspiration biopsy or parasternal mediastinotomy if enlarged lymph nodes are visible in areas that cannot be evaluated using a standard approach.
Mediastinal Invasion by Lung Cancer
In addition to node metastasis, bronchogenic carcinoma can involve the mediastinum by direct extension, so-called mediastinal invasion. In current surgical practice, invasion of the mediastinal pleura, parietal pericardium, diaphragm, or chest wall, termed T3 in the lung cancer staging system, does not prevent surgery. However, significant invasion of mediastinal fat or other mediastinal structures, such as the heart, great vessels, trachea, esophagus, or vertebral body, may prevent resection; such lesions are classified as T4 (Fig. 4-10). Some T4 tumors with no node metastases or hilar node metastases are considered resectable following chemotherapy or radiation.
How accurate is CT in predicting mediastinal invasion? An obvious finding is that a lung mass not contacting the mediastinum is not invasive, and this is an important use of CT.
CT findings of mediastinal invasion (Fig. 4-10) include the following:
1. Replacement of mediastinal fat by a tumor of soft-tissue attenuation
2. Compression or displacement of mediastinal vessels by tumor
3. A tumor contacting more than 90° of the circumference of a structure, such as the aorta or pulmonary artery (the greater the extent of circumferential contact, i.e., 180°, the greater is the likelihood of invasion)
4. Obliteration of the mediastinal fat plane normally seen adjacent to most mediastinal structures
5. A tumor contacting more than 3 cm of the mediastinum
6. Obtuse angles where a tumor contacts the mediastinum
7. Mediastinal pleural or pericardial thickening
Lymphoma and Leukemia
Mediastinal lymph nodes are commonly involved in patients with lymphoma. A small percentage are first recognized because of mediastinal masses noted on chest radiographs, but these patients will often have systemic signs and symptoms, including fever, night sweats, weight loss, weakness, and fatigue.
Hodgkin’s Disease
Hodgkin’s disease (HD) has a predilection for thoracic involvement, both at the time of diagnosis and if the disease recurs. HD occurs in patients of all ages but peaks in incidence in the third and fifth decades of life.
More than 85% of patients with HD eventually develop intrathoracic disease, typically involving the superior mediastinal (prevascular, pretracheal, and aortopulmonary) lymph nodes (Figs. 4-3 and 4-11). An important rule is that intrathoracic lymphadenopathy not associated with superior mediastinal node enlargement is unlikely to be Hodgkin’s lymphoma.
In one study, it was uncommon for CT to show evidence of mediastinal adenopathy if the chest radiograph was normal, but if the chest radiograph was abnormal, CT detected additional sites of adenopathy in many cases (Table 4-5). CT was most helpful in diagnosing subcarinal, internal mammary, and aortopulmonary window node enlargement. Cardiophrenic angle (paracardiac) lymph nodes are present in about 10% of patients and are seen well on CT (Fig. 4-2). Adenopathy in this location is less common in other diseases. In a significant percentage of patients, the additional node involvement shown by CT changes therapy.
Enlargement of a single node group can be seen with HD, most commonly in the prevascular (anterior) mediastinum. This often indicates the presence of nodular sclerosing histology, which accounts for 50% to 80% of adult HD. In patients with lymphoma, mediastinal lymph nodes may become matted, being visible as a single large mass (Fig. 4-12) rather than individual discrete nodes. Mediastinal nodes or masses in patients with Hodgkin’s lymphoma can appear cystic or fluid-filled on CT (Fig. 4-12). Calcification is unusual and of limited extent, except after treatment.
Non-Hodgkin’s Lymphoma
Non-Hodgkin’s lymphoma (NHL) is a diverse group of diseases that vary in radiologic manifestation, clinical presentation, course, and prognosis. In comparison with HD, these tumors are less common and generally occur in an older group of patients (40 to 70 years of age). At the time of presentation, the disease is often generalized (85% are stage III or IV), and chemotherapy is most appropriate. For this reason, precise anatomic staging is less crucial than with HD.
In one series, 43% of patients with NHL had intrathoracic disease and 40% had involvement of only one node group, which is much more common than in patients with HD (Fig. 4-13). In addition, posterior mediastinal nodes were more frequently involved. Lung involvement was present in only 4%; in some patients, lung infiltration may be rapid.
NHL may occur as a primary mediastinal mass. The most common cell types presenting in this fashion are lymphoblastic lymphoma (60% of cases; Fig. 4-13) and large B-cell lymphoma. These resemble the appearance of mediastinal HD, with a large anterior mediastinal mass predominating, and occur in a similar age group, being most common in young patients.
CT, as in patients with HD, can show evidence of intrathoracic disease when it is unrecognizable on plain radiographs and can affect management in patients with localized (stage I or II) disease.
Leukemia
Leukemia, particularly the lymphocytic varieties, can cause hilar or mediastinal lymph node enlargement, pleural effusion, and occasionally infiltrative lung disease. Lymphadenopathy is generally confined to the middle mediastinum, and the larger masses seen with some lymphomas generally do not occur.
Metastases
Extrathoracic primary tumors can result in mediastinal node enlargement, either with or without hilar or lung metastases (Figs. 4-5 and 4-14). Node metastases can be present because of inferior extension from neck masses (thyroid carcinoma and head and neck tumors), extension along lymphatic channels from below the diaphragm (testicular carcinoma, renal cell carcinoma, and gastrointestinal malignancies), or dissemination via other routes (breast carcinoma and melanoma). Middle mediastinal (paratracheal) or paravertebral mediastinal nodes are most commonly involved when the tumor is subdiaphragmatic. With breast carcinoma, internal mammary node metastases occur.
Sarcoidosis
Mediastinal lymph node enlargement is common in patients with sarcoidosis, occurring in 60% to 90% of cases. Typically, node enlargement is extensive, involving the hila as well as the mediastinum, and masses appear bilateral and symmetrical in most patients (Figs. 4-1, 4-4, and 4-8); this sometimes allows differentiation of sarcoidosis from lymphoma, which more typically produces asymmetrical enlargement. In addition, lymph nodes can be quite large in patients with sarcoidosis, but large isolated masses, as seen in some patients with lymphoma, are uncommon. Paratracheal lymph nodes are typically involved. Even though it is commonly stated that sarcoidosis does not involve anterior mediastinal lymph nodes, this involvement is often visible on CT; paravertebral node enlargement is seen occasionally (Table 4-6).
Infections
A variety of infectious agents can cause mediastinal lymph node enlargement during the acute stage of the infection. These include a number of fungal infections (commonly histoplasmosis and coccidioidomycosis), tuberculosis, bacterial infections, and viral infections. Typically, there will be symptoms and signs of acute infection, and chest radiographs will show evidence of pneumonia.
The lymph node enlargement will often be asymmetrical, involving hilar and middle mediastinal nodes. In patients with tuberculosis, enlarged nodes typically show rim enhancement and central necrosis after contrast medium injection; this appearance is nearly diagnostic in patients with an appropriate history. Lymph node calcification occurs in patients with chronic fungal or tuberculous infection.
Castleman’s Disease (Angiofollicular Lymph Node Hyperplasia)
An unusual disease of unknown cause, Castleman’s disease occurs in two forms. The more common localized form is characterized by enlargement of hilar or mediastinal lymph nodes, usually middle or posterior mediastinal nodes. A single smooth or lobulated mass, which can be large, is typically visible on CT, and dense opacification after contrast medium infusion is commonly visible. Localized Castleman’s disease is usually asymptomatic and has a benign course.
The rare diffuse form of Castleman’s disease results in generalized lymph node enlargement involving mediastinal and hilar nodes, and often axillary, abdominal, and inguinal node groups (Fig. 4-15). It is often associated with systemic symptoms and has a progressive course despite treatment. As with the localized form, marked node enhancement can be seen.
Diagnosis of Mediastinal Masses
Differential diagnosis of a mediastinal mass on CT is usually based on several factors. The location of the mass is fundamental in differential diagnosis. Although most mediastinal masses can occur in different parts of the mediastinum, most have characteristic locations (Table 4-7). Location is used in this chapter to classify masses. Lymph node masses, such as lymphoma, already discussed in this chapter, and abnormalities discussed in other chapters (e.g., aortic aneurysm) are not covered again but are included in Table 4-7.
Other considerations in differential diagnosis include whether a single mass or multifocal abnormalities are present (i.e., involving several areas of the mediastinum), the shape of the mass (round or lobulated), and findings such as pleural effusion. Attenuation of a mass (fat, fluid, soft-tissue, or a combination of these, and the presence, character, and amount of calcification) is also very important in differential diagnosis (Table 4-8).
TABLE 4-7
Differential Diagnosis of Mediastinal Masses Based on Common Sites of Origin
Prevascular space (anterior mediastinum)
Thymic masses
Thymoma
Thymic carcinoma
Thymic neuroendocrine tumor
Thymolipoma
Thymic cyst
Thymic hyperplasia
Thymic lymphoma
Germ cell tumors
Teratoma and dermoid cyst
Seminoma
Nonseminomatous germ cell tumors
Thyroid abnormalities (goiter and neoplasm)
Parathyroid tumor or hyperplasia
Lymph node masses (particularly Hodgkin’s lymphoma)
Vascular abnormalities (aorta and great vessels)
Mesenchymal abnormalities (e.g., lipomatosis, lipoma)
Foregut cyst
Lymphangioma
Hemangioma
Retrosternal space (anterior mediastinum)
Lymph node masses
Pretracheal space (middle mediastinum)
Lymph node masses
Lung carcinoma
Sarcoidosis
Lymphoma (particularly Hodgkin’s disease)
Metastases
Infections (e.g., tuberculosis)
Foregut cyst
Tracheal tumor
Mesenchymal masses (e.g., lipomatosis, lipoma)
Thyroid abnormalities
Vascular abnormalities (aorta and great vessels)
Lymphangioma and hemangioma
Aortopulmonary window (middle mediastinum)
Lymph node masses
Lung carcinoma
Sarcoidosis
Lymphoma
Metastases
Infections (e.g., tuberculosis)
Mesenchymal masses (e.g., lipomatosis, lipoma)
Vascular abnormalities (aorta or pulmonary artery)
Chemodectoma
Foregut cyst
Subcarinal space and azygoesophageal recess (middle mediastinum)
Lymph node masses
Lung carcinoma
Sarcoidosis
Lymphoma
Metastases
Infections (e.g., tuberculosis)
Foregut cyst
Dilated azygos vein
Esophageal masses
Varices
Hernia
Paravertebral masses (posterior mediastinum)
Neurogenic tumor
Nerve sheath tumors
Sympathetic ganglia tumors
Paraganglioma
Meningocele
Foregut cyst
Neurenteric cyst
Thoracic spine abnormalities
Extramedullary hematopoiesis
Fluid collections and pseudocyst
Vascular abnormalities
Hernias
Esophageal masses
Varices
Mesenchymal masses (e.g., lipomatosis, lipoma)
Lymph node masses
Lymphoma (particularly non-Hodgkin’s)
Metastases
Dilated azygos or hemiazygos vein
Hernia
Lymphangioma and hemangioma
Thymic mass or germ cell tumor
Anterior cardiophrenic angle masses
Lymph node masses (particularly lymphoma and metastases)
Pericardial cyst
Fat pad
Morgagni hernia
Thymic masses
Germ cell tumors
TABLE 4-8
Attenuation Characteristics of Mediastinal Masses∗
| Mass | Air | Fat | Water | Tissue | >Tissue | Calcium |
| Thymoma | N | N | O | A | N | O |
| Thymolipoma | N | A | N | C | N | N |
| Lymphoma (thymic) | N | N | O | A | N | R |
| Dermoid cyst/teratoma | N | O | O | A | N | O |
| Germ-cell tumor | N | N | R | A | N | R |
| Thyroid tumor | N | N | O | A | C | C |
| Lipoma | N | A | N | N | N | N |
| Hygroma | N | C | C | C | N | N |
| Cysts (congenital) | R | N | C | O | N | R |
| Hernia | O | O | N | O | N | N |
| Lung cancer (nodes) | N | N | O | A | N | N |
| Tuberculosis (nodes) | N | N | C | A | N | C |
| Sarcoidosis (nodes) | N | N | R | A | N | O |
| Castleman’s (nodes) | N | N | N | A | N | O |
| Neurogenic tumor | N | O | C | C | N | O |
| Neurenteric cyst | R | N | A | N | N | N |
| Meningocele | N | N | A | N | N | N |
| Hematopoiesis | N | O | N | A | N | N |
∗ ACORN: A, always; C, common; O, occasionally; R, rare; N, never (“never” does not mean it never happens, but rather that it is so unlikely that practically the radiologist should “never” consider the diagnosis, and if the differential diagnosis turns out to be wrong, will “never” be blamed).
Prevascular Space Masses
Masses in the prevascular space, when large, tend to displace the aorta and great arterial branches posteriorly (Fig. 4-13), but distinct compression or narrowing of these relatively thick-walled structures is unusual. Within the supra-aortic mediastinum, displacement, compression, or obstruction of the brachiocephalic veins is not uncommon. In the subaortic mediastinum, posterior displacement or compression of the superior vena cava is typical only with right-sided masses. On the left, compression of the main pulmonary artery can be seen.
Although we are taught that the differential diagnosis of anterior mediastinal masses includes the “4 Ts” (thymoma, teratoma, thyroid tumor, and terrible lymphoma), the differential diagnosis should be extended to include (1) thymoma and other thymic tumors; (2) teratoma and other germ-cell tumors; (3) thyroid masses; (4) lymphoma and other lymph node masses; and (5) parathyroid masses, cysts, fatty masses, and lymphangioma (hygroma).
Thymic Tumors
Tumors of various histology arise from cells of thymic origin, including thymoma, thymic carcinoma, thymic carcinoid tumor, thymolipoma, thymic cyst, lymphoma, and leukemia. Thymic hyperplasia may mimic a mass.
Thymoma
Thymoma is a tumor of thymic epithelial origin and is a common cause of anterior mediastinal mass in adults. Occasionally, these lesions arise in the middle or posterior mediastinum. It is extremely difficult to determine if thymomas are benign or malignant by histologic criteria, but the World Health Organization (WHO) has developed an alphanumeric histologic classification system (i.e., A, AB, B1-3, C) that correlates with the presence of invasion, metastasis, and survival. Thymomas are generally classified as invasive or noninvasive based on their appearance at surgery. Approximately 30% of thymomas are pathologically and surgically invasive. Invasion of mediastinal structures or the pleural space is most typical. Distant metastases are not common with invasive thymoma.
From 10% to 30% of patients with myasthenia gravis will be found to have a thymoma, whereas a larger percentage of patients with thymoma (30% to 50%) have myasthenia. Other syndromes associated with thymoma include red blood cell hypoplasia and hypogammaglobulinemia.
On CT, thymomas are usually visible in the prevascular space (Fig. 4-16), but they can also be seen in a paracardiac location. They appear as a localized mass distorting or replacing the normally arrowhead-shaped thymus. Typically, they are unilateral. Calcification and cystic degeneration can be present. On CT, bilaterality, large size, lobulated contours, poor definition of the tumor margin, and associated pleural effusion or nodules suggest the presence of an invasive thymoma, but a definite diagnosis is difficult to make on CT.
In patients suspected of having thymoma because of myasthenia gravis, CT can demonstrate tumors that are invisible on plain radiographs. However, small thymic tumors may not be distinguishable from a normal or hyperplastic gland with CT.
Thymic Carcinoma
Similar to invasive thymoma, thymic carcinoma arises from thymic epithelial cells. However, unlike invasive thymoma, thymic carcinoma can be diagnosed as malignant on the basis of histology. It is classified as C in the WHO system previously described. This tumor is aggressive and is more likely to result in distant metastases than invasive thymoma. Thymic carcinoma cannot be distinguished accurately from thymoma on CT unless metastases are visible.
Thymic Neuroendocrine Tumor
Thymic neuroendocrine tumors, which may be further classified as carcinoid, atypical carcinoid, or small-cell neuroendocrine carcinoma, are usually malignant and aggressive. This type of lesion does not differ significantly from thymoma in its CT appearance, but it has a worse prognosis. Approximately 40% of patients have Cushing’s syndrome as a result of tumor secretion of adrenocorticotropic hormone, and nearly 20% have been associated with multiple endocrine neoplasia syndromes I and II.
Thymolipoma
Thymolipoma is a rare, benign thymic tumor consisting primarily of fat but also containing strands or islands of thymic tissue. The tumor is generally unaccompanied by symptoms and can be large when first detected, usually on chest radiographs. Because of its fatty content and pliability, it tends to drape around the heart and can simulate cardiac enlargement. On CT, its fatty composition, with wisps of soft tissue within it, can permit a preoperative diagnosis (Fig. 4-17).
Thymic Cyst
Thymic cysts are either congenital or acquired and can be diagnosed using CT if they are thin-walled and their contents have an attenuation close to that of water. In some cases, they will show soft-tissue attenuation. Calcification of the cyst margin can occur. Notably, thymoma can have cystic components but also demonstrates solid areas or a thick or irregular wall.
An important general rule in diagnosing mediastinal masses is that cysts can appear solid, and solid (malignant) masses can have cystic or necrotic components. A true cyst has a thin wall; a mass with cystic degeneration usually has a thick, irregular wall.
Thymic Hyperplasia and Thymic Rebound
Thymic hyperplasia may result in thymic enlargement or a focal thymic mass. It is associated with myasthenia gravis. Distinction from thymoma on CT may be impossible.
The thymus may appear enlarged and relatively dense (containing little fat) in patients with thymic hyperplasia. In young patients, the thymus may show significant rebound hyperplasia 3 months to a year after cessation of chemotherapy for malignancy. This can result in a distinctly enlarged thymus.
Thymic Lymphoma
Anterior mediastinal lymph node enlargement (Figs. 4-3 and 4-9) or thymic involvement (Fig. 4-18) is present in more than half of patients with HD. In patients with thymic involvement, lymphoma can present as a single spherical or lobulated mass or as thymic enlargement. In such cases, lymphoma can be indistinguishable from thymoma or other causes of prevascular mass. However, if the abnormality is multifocal (indicating its origin from nodes) or is associated with other sites of lymph node enlargement, the diagnosis is made more easily (Fig. 4-18). Cystic areas of necrosis may be visible at CT in patients with lymphoma (Fig. 4-12). Except in rare cases, calcification does not occur in the absence of radiation. HD limited to the prevascular mediastinum is typically of the nodular sclerosing cell type.
Germ-Cell Tumors
Several different tumors originating from rests of primitive germ cells can occur in the anterior mediastinum. These include teratoma, dermoid cysts, seminoma, choriocarcinoma, and endodermal sinus tumor. These tumors are less common than thymoma. Approximately 80% of germ-cell tumors are benign. Germ-cell tumors are usually considered in three categories: teratoma and dermoid cysts, seminoma, and nonseminomatous germ-cell tumors.
Teratoma and Dermoid Cyst
Teratomas can be cystic or solid and are most commonly benign. A teratoma contains tissues of ectodermal, mesodermal, and endodermal origins. A dermoid cyst is a specific type of teratoma derived primarily from epidermal tissues, although other tissues are usually present. Teratomas are classified histologically as mature or immature. Mature teratomas are benign; immature teratomas usually behave in a malignant fashion in adults but may be benign in children.
Teratomas occur in a distribution similar to that of thymomas; they rarely originate in the posterior mediastinum. Benign lesions are often round, oval, and smooth in contour; as with thymoma, an irregular, lobulated, or ill-defined margin suggests malignancy. On average, these tumors are larger than thymomas, but they can be any size. Calcification can be seen (Fig. 4-19) but is nonspecific except in the unusual instance when a bone or tooth is present within the mass. They may appear cystic or contain visible fat, a finding that is of great value in differential diagnosis (Fig. 4-19). A fat–fluid level can also be seen.
Seminoma
Seminoma occurs almost entirely in young men. It is the most common malignant mediastinal germ-cell tumor, accounting for 30% of such cases. On CT, seminoma presents as a large, smooth or lobulated homogeneous soft-tissue mass, although small areas of low attenuation can be seen. Obliteration of fat planes is common, and pleural or pericardial effusion may be present. Seminomas are radiosensitive, and the 5-year survival rate for affected patients is 50% to 75%.
Nonseminomatous Germ-Cell Tumors
Nonseminomatous germ-cell tumors, namely embryonal carcinoma, endodermal sinus (yolk sac) tumor, choriocarcinoma, and mixed types, are often grouped together because of their rarity, similar appearance, aggressive behavior, and poor prognosis. The tumors are usually unresectable at the time of diagnosis because of local invasion or distant metastasis. Unlike seminoma, radiotherapy is of limited value.
On CT, these tumors often show heterogeneous opacity, including ill-defined areas of low attenuation secondary to necrosis and hemorrhage or cystic areas. They often appear infiltrative, with obliteration of fat planes, and may be spiculated. Calcification may be seen.
Thyroid Masses
A small percentage of patients with a thyroid mass have some extension of the mass into the superior mediastinum, and, rarely, a completely intrathoracic mass can arise from ectopic mediastinal thyroid tissue. In most patients, such masses represent a goiter (Fig. 4-20), but other diseases (Graves’ disease and thyroiditis) and neoplasms can result in an intrathoracic mass. Masses are often asymmetrical.
Most patients with intrathoracic goiter are asymptomatic, but symptoms of tracheal or esophageal compression can be present. CT usually shows anatomic continuity of the visible mass with the cervical thyroid gland. The location of the mass on CT is somewhat variable, and it can be anterior or posterior to the trachea. Masses anterior to the trachea splay the brachiocephalic vessels, whereas masses that are primarily posterior and lateral to the trachea displace the brachiocephalic vessels anteriorly. A location anterior to the great vessels is somewhat unusual (Fig. 4-20). Calcifications and low-attenuation cystic areas are common in patients with goiter. In addition, because of their high iodine content, the CT attenuation of goiters, Graves’ disease, and thyroiditis can be greater than that of soft-tissue and thyroid tumors, but less dense than normal thyroid tissue.
As a rule, if a thyroid mass is suspected clinically, CT should be performed without contrast injection. This allows subsequent injection of radioactive iodine for diagnosis. Injection of iodinated radiographic contrast agents delays radionuclide imaging.
Mesenchymal Abnormalities
Lipomatosis and Lipoma
A diffuse accumulation of unencapsulated fat in the mediastinum, so-called mediastinal lipomatosis, can occur in patients with Cushing’s syndrome, after long-term corticosteroid therapy, or as a result of exogenous obesity. It produces no symptoms. CT shows a generalized increase in anterior mediastinal fat surrounding the great vessels, with some lateral bulging of the mediastinal pleural reflections. On CT, fat has a characteristic low attenuation, measuring from –50 to –100 Hounsfield units (HU).
As with other mesenchymal tumors, lipomas can occur in any part of the mediastinum but are most common anteriorly. Because of their pliability, they rarely cause symptoms. A lipoma, although of the same attenuation as lipomatosis, is localized. Most fatty masses are benign. Liposarcoma, teratoma, and thymolipoma, which are other masses that can contain fat, also contain soft-tissue elements and thus can be distinguished from lipoma or lipomatosis.
Lymphangioma (Hygroma)
Lymphangiomas are classified histologically as simple, cavernous, and cystic. Simple lymphangiomas are composed of small, thin-walled lymphatic channels with considerable connective tissue stroma. Cavernous lymphangiomas consist of dilated lymphatic channels, whereas cystic lymphangiomas (hygromas) contain single or multiple cystic masses filled with serous or milky fluid and having little, if any, communication with normal lymphatic channels. Most commonly, these lesions are detected in children and may extend into the neck. However, they can be seen in adults as well. On CT, the mass can appear as a single cyst, be multicystic, or envelop, rather than displace, mediastinal structures. Discrete cysts may not be visible; calcification does not occur. Abnormal vessels that are opacified by contrast medium may be present.
Retrosternal Space Masses
Enlargement of internal mammary nodes results in a convexity in the expected position of this node chain (Fig. 4-1). Other than lymph node enlargement, masses in this region are unusual. Metastases and lymphoma are most common. The anterior mediastinal masses previously listed can project into this space.
Pretracheal Space Masses
Masses that occupy the pretracheal compartment characteristically replace or displace normal pretracheal fat. Because the pretracheal space is limited by the relatively immobile aortic arch anteriorly and to the left, large masses extend preferentially to the right, displacing and compressing the superior vena cava anteriorly and laterally. In the presence of a pretracheal mass, the superior vena cava appears crescentic and convex laterally. Lateral displacement of the superior vena cava results in most of the mediastinal widening visible on plain films. Large masses also displace the trachea posteriorly, but tracheal cartilage usually prevents significant tracheal narrowing. Masses in this compartment are almost always of lymph node origin, but masses more typical in other parts of the mediastinum may involve the pretracheal space.
Aortopulmonary Window Masses
Masses in the aortopulmonary region typically replace mediastinal fat; when large, they displace the mediastinal pleural reflection laterally. Displacement or compression of the aorta, pulmonary artery, and trachea is sometimes seen.
Subcarinal Space and Azygoesophageal Recess
Large masses in the subcarinal space can (1) produce a convexity of the azygoesophageal recess, (2) splay the carina, (3) displace the carina anteriorly, (4) displace the esophagus to the left, and/or (5) displace the right pulmonary artery anteriorly and compress its lumen. The most common masses involving this compartment are lymph node masses, cysts, and esophageal lesions.
Bronchogenic and Esophageal Duplication Cysts
Congenital bronchogenic cysts result from anomalous budding of the foregut during development. Most commonly, they are visible in the subcarinal space, but they can occur in any part of the mediastinum. They appear as single smooth masses that are round or elliptical (Fig. 4-21) and occasionally show calcification of their walls or contents. Air–fluid levels occurring because of communication with the trachea or bronchi are rare. When large, bronchogenic cysts can produce symptoms due to compression of mediastinal structures. A rapid increase in size can occur because of infection or hemorrhage.
Esophageal duplication cysts are indistinguishable from bronchogenic cysts, but they always contact the esophagus. They usually appear as well-defined solitary masses and occasionally contain an air–fluid level when they communicate with the esophagus.
CT can be of great value in diagnosing a mediastinal cyst. If a mass is thin-walled and is of fluid attenuation (approximately 0 HU), it can be assumed to represent a benign cyst. However, high CT numbers (20–40 HU) suggesting a solid mass can also be found in patients with foregut duplication cysts. These cysts contain a thick gelatinous material or blood. In such patients, surgery is usually required for diagnosis, but magnetic resonance imaging may sometimes help.
Esophageal Lesions
Esophageal lesions are discussed in Chapter 17.
Paravertebral Masses
Paravertebral masses may be seen to replace paravertebral fat. On the left, the normal concave mediastinal pleural reflection, posterior to the aorta, becomes convex in the presence of a significant mass. On the right, a paravertebral convexity is visible in a region where little tissue normally exists (Fig. 4-5).
Neurogenic Tumors
Neurogenic tumors are divided into three groups arising from (1) peripheral nerves or a nerve sheath (neurofibroma or neurilemmoma), (2) sympathetic ganglia (ganglioneuroma or neuroblastoma), and (3) paraganglionic cells (pheochromocytoma or chemodectoma). Tumors in each of these three groups may be benign or malignant. Although neurogenic tumors can occur at any age, they are most common in young patients. Neuroblastoma and ganglioneuroma are most common in children, whereas neurofibroma and neurilemmoma more frequently affect young adults.
Radiographically, neurogenic tumors appear as well-defined round or oval soft-tissue masses, typically in a paravertebral location (Fig. 4-22). Although the different tumors are by no means always distinguishable, a ganglioneuroma tends to be elongated, lying adjacent to the spine, whereas neurofibroma and neurilemmoma are smaller and more spherical in shape. Although neural tumors are frequently of soft-tissue attenuation, they can be low in attenuation because of the presence of lipid-rich Schwann cells, fat, or cystic regions. Although benign tumors tend to be sharply marginated and fairly homogeneous, and malignant tumors tend to be infiltrating and irregular, these findings are not sufficiently reliable for diagnosis. Calcification can occur, particularly in neuroblastoma; the presence of calcium does not help in distinguishing benign from malignant lesions.
A neurofibroma arising in a nerve root can be dumbbell-shaped, that is, partially inside and partially outside the spinal canal. In such cases, the intervertebral foramen may be enlarged. CT can be helpful in determining the extent of the mass and associated vertebral abnormalities, and can distinguish the mass from an aortic aneurysm or other vascular lesion if an intravenous contrast agent is given. CT after injection of myelographic contrast medium may be useful in demonstrating intraspinal extension.
Anterior or Lateral Thoracic Meningocele
A thoracic meningocele represents anomalous herniation of the spinal meninges through an intervertebral foramen or a defect in the vertebral body. It results in a soft-tissue mass visible on chest radiographs. In most patients, this abnormality is associated with neurofibromatosis; most cases are detected in adults. It is said that meningocele is the most common cause of a posterior mediastinal mass in patients with neurofibromatosis, but I doubt it.
Meningoceles are described as lateral or anterior, depending on their relationship to the spine. They are slightly more common on the right. Findings that suggest the diagnosis include rib or vertebral anomalies at the same level or an association with scoliosis. The mass is often visible at the apex of the scoliotic curve. CT after intraspinal contrast medium injection shows filling of the meningocele and is diagnostic (Fig. 4-23); magnetic resonance imaging may also be diagnostic.
Neurenteric Cyst
A neurenteric cyst, which is rare, is composed of both neural and gastrointestinal elements and is frequently attached to both the meninges and the gastrointestinal tract. It appears as a homogeneous posterior mediastinal mass and rarely contains air because of communication with the abdominal viscera. As with meningocele, it is frequently associated with a vertebral anomaly or scoliosis. As opposed to meningocele, it frequently causes pain and is generally diagnosed at a young age.
Diseases of the Thoracic Spine
Tumors (either benign or malignant), infectious spondylitis, or vertebral fracture with associated hemorrhage can produce a paravertebral mass. Frequently, the abnormality is bilateral and fusiform, allowing it to be distinguished from solitary masses such as a neurogenic tumor. Associated abnormalities of the vertebral bodies or disks assist in diagnosis and should be sought. Preservation of disks in association with vertebral body destruction suggests neoplasm or tuberculosis; disk destruction suggests infection other than tuberculosis.
Extramedullary Hematopoiesis
Extramedullary hematopoiesis can result in paravertebral masses in patients with severe anemia (usually congenital hemolytic anemia or thalassemia). These masses are of unknown origin but perhaps arise from lymph nodes, veins, or an extension of rib marrow. Masses can be multiple and bilateral and are most commonly associated with the lower thoracic spine. They have no specific CT characteristics. With resolution they may appear as fat density.
Fluid Collections and Pseudocyst
Occasionally, posterior pleural fluid collections can simulate a paravertebral mediastinal mass. Mediastinal extension of a pancreatic pseudocyst through the aortic or esophageal hiatus can occur, but it is rare.
Vascular Abnormalities
Posteriorly located aortic aneurysms can occupy this part of the mediastinum. Azygos and hemiazygos vein dilatation also produces abnormalities in this region. Dilated azygos or hemiazygos veins, because they are visible on a number of contiguous slices, are easily distinguished from a focal mass.
Paracardiac Masses
Compression of the atria or right ventricle can be seen in the presence of a paracardiac mass, but left ventricular compression is uncommon because of the thickness of its wall and the relatively high pressure of its contents. A variety of masses can be seen in this location. Large anterior mediastinal masses often project posteriorly into a paracardiac location.
Anterior Cardiophrenic Angle Masses
Although a number of the mediastinal masses already described can occur at the level of the anterior cardiophrenic angle, differential diagnosis of lesions occurring in this location includes several specific entities. These include a pericardial cyst, a large epicardial fat pad, a Morgagni hernia, and enlargement of paracardiac lymph nodes (Fig. 4-2).
Pericardial Cyst
Most commonly, pericardial cysts touch the diaphragm, 60% in the anterior right cardiophrenic angle and 30% in the left cardiophrenic angle; 10% occur higher in the mediastinum. Most patients are asymptomatic. The cysts typically appear as smooth, round, homogeneous masses (Fig. 4-24). They range up to 15 cm in diameter. Although they are usually low in attenuation (i.e., near 0 HU), they may have similar attenuation to soft tissue.
Fat Pad
Deposition of fat in either cardiophrenic angle is not uncommon, particularly in obese patients, and can simulate a mass on plain radiographs. CT, of course, is diagnostic.
Morgagni Hernia
Hernias of abdominal contents through the anteromedial diaphragmatic foramen of Morgagni can result in a cardiophrenic angle mass; 90% of these occur on the right. The hernia usually contains omentum or liver; bowel is less common. When the hernia contains fat, CT can confirm its benign nature but does not allow its differentiation from a fat pad. When it contains liver, CT may allow diagnosis by showing hepatic vessels or bile ducts. If bowel is present in the hernia sac, gas is usually visible.
Diffuse Mediastinal Abnormalities
Mediastinitis
Mediastinal infections (mediastinitis) can be acute or chronic.
Acute Mediastinitis
Acute mediastinitis usually results from esophageal perforation or spread of infection from adjacent tissue spaces, including the pharynx, lungs, pleura, and lymph nodes. The primary symptoms are substernal chest pain and fever. CT shows mediastinal widening, replacement of normal fat by fluid attenuation, or localized fluid collections. Gas bubbles may be seen (Fig. 4-25).
Granulomatous Mediastinitis
In patients with histoplasmosis, tuberculosis, or sarcoidosis, chronic mediastinal lymph node enlargement and associated fibrosis can result in so-called granulomatous mediastinitis. In these patients, the large nodes and associated fibrous tissue form a mediastinal mass that can compress the superior vena cava, pulmonary arteries or veins, bronchi, and esophagus.
The node enlargement tends to be asymmetrical, except in patients with sarcoidosis. Fibrosis can replace normally visible mediastinal fat. Calcification of the nodes can be seen in some patients, indicating the benign nature of the disease process. Compression of the main bronchi (usually the left) or pulmonary arteries (usually the right) can sometimes be recognized.
Fibrosing Mediastinitis
In some patients, similar mediastinal fibrosis is not associated with obvious granulomatous disease. This occurrence is less common than granulomatous mediastinitis. In a few patients, this is associated with fibrosis elsewhere (retroperitoneal fibrosis). Symptoms and radiographic findings are similar to those for granulomatous mediastinitis, but calcification does not occur.
Mediastinal Hemorrhage
Mediastinal hemorrhage usually results from trauma such as venous or arterial laceration, from aortic rupture or dissection, or from anticoagulation (see Chapter 3). Superior mediastinal widening associated with blurring of normal mediastinal contours is usually present. Mediastinal fluid visible on CT is high in attenuation (> 50 HU). Blood can dissect extrapleurally over the lung apex, resulting in a so-called apical cap. In some patients, blood will also be present in the left pleural space. Contrast-enhanced CT may be of value in diagnosing associated aortic aneurysm, dissection, or rupture.
Suggested Readings are available on Expert Consult.