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MEDIASTINUM: L YMPH N ODE A BNORMALITIES AND M ASSES
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.
FIGURE 4-5 � Paravertebral lymph node enlargement in metastatic testicular carcinoma. Large lymph nodes on the right (large arrow ) can be considered paraesophageal or inferior pulmonary ligament nodes. They appear inhomogeneous and are necrotic. An enlarged left paravertebral lymph node (small arrows ) is also visible posterior to the aorta.
TABLE 4-1
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
International Association for the Study of Lung Cancer (IASLC) Lymph Node Zones compared to AJCC/UICC Node Stations
IASLC Nodal Groups Mediastinal zone Upper zone
Aortopulmonary zone Subcarinal zone Lower zone Hilar/interlobar zone Peripheral zone
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AJCC/UICC Description Right upper paratracheal Left upper paratracheal Prevascular Right lower paratracheal Left lower paratracheal Subaortic Paraaortic Subcarinal Paraesophageal Pulmonary ligament Hilar Interlobar Lobar Segmental Subsegmental
AJCC/UICC Station 2R 2L 3 4R 4L 5 6 7 8 9 10 11 12 13 14
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T HE T HORAX
Brachiocephalic (innominate) a.
3
2R
Ligamentum arteriosum
Ao 4R
Azygos v.
L. pulmonary a. Phrenic n.
4L 10R
6
PA Ao 5
7 11R
PA
11L 8
12,13,14R
10L
9
12,13,14L Inf. pulm. ligt.
A
B
FIGURE 4-6 � American Joint Committee on Cancer and Union Internationale Contre le Cancer lymph node stations. A and B , Ao, aorta; PA, pulmonary artery; a., artery; v., vein; Inf. pulm. ligt., inferior pulmonary ligament; n., nerve; L. pulmonary a., left pulmonary artery. (Reproduced from Mountain CF, Dresler CM: Regional lymph node classification for lung cancer staging. Chest 111:1718–1723, 1997, with permission.)
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.
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 sub jects. Pretracheal nodes are also commonly visible, but these nodes are typically smaller than normal subcarinal nodes. Nodes in the supraaortic mediastinum are usually smaller than lower pretracheal nodes, and left paratracheal nodes are usually smaller than right paratracheal nodes.
FIGURE 4-7 � Normal mediastinal nodes. Small lymph nodes are visible in the aortopulmonary window (small arrows ) with a short-axis diameter of less than 1 cm. A normal-sized pretracheal lymph node (large arrow ) has a fatty hilum and contains a large amount of fat. This is a benign appearance.
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TABLE 4-2
MEDIASTINUM: L YMPH N ODE A BNORMALITIES AND M ASSES
Upper Limits of Normal for the Short-Axis Node Diameter
Node Group
Short-Axis Node Diameter* (mm)
Supra-aortic paratracheal Subaortic paratracheal Aortopulmonary window Prevascular Subcarinal Paraesophageal
7 9 9 8 12 8
*Mean normal node diameter plus two standard deviations.
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
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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-shelllike 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, lowattenuation 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
