Fundamentals of Diagnostic Radiology
3rd Edition

Chapter 13
Mediastinum and Hila
Jeffrey S. Klein
This chapter will review the radiologic approach to mediastinal masses, diffuse mediastinal disease, and hilar abnormalities.
MEDIASTINAL MASSES
Localized mediastinal abnormalities are common diagnostic challenges for the radiologist. Patients with mediastinal masses tend to present in one of two fashions: with symptoms related to local mass effect or invasion of adjacent mediastinal structures (stridor in a patient with thyroid goiter), or incidentally with an abnormality on a routine chest radiograph. Occasionally, a mediastinal mass is discovered in the course of an evaluation for known malignancy (e.g., a patient with non-Hodgkin lymphoma) or for a condition such as myasthenia gravis, in which there is an association with thymoma. Multidetector-row CT (MDCT) and MR are the primary cross-sectional modalities used to evaluate mediastinal masses, with PET useful to assess response of mediastinal tumors to therapy, particularly lymphoma, and to distinguish residual or recurrent tumor from fibrosis (Table 13.1).
For the purposes of the following discussion, the mediastinum is divided into superior (thoracic inlet) and inferior components, with the inferior mediastinum subdivided into anterior, middle, and posterior compartments, as described in Chapter 12.
Thoracic Inlet Masses
The thoracic inlet is the region of the upper thorax marginated by the first rib and represents the junction between the neck and thorax. Masses in this region commonly present as neck masses or with symptoms of upper airway obstruction resulting from tracheal compression. Thyroid masses, lymphomatous nodes, and lymphangiomas are the most common thoracic inlet masses (Table 13.2).
Thyroid Masses
In a small percentage of patients with a cervical thyroid goiter, a thyroid carcinoma, or an enlarged gland from thyroiditis, extension of the thyroid through the thoracic inlet into the superior mediastinum may occur. These lesions are usually discovered as incidental findings on chest radiographs; a minority of patients will present with complaints of dyspnea or dysphagia as a result of tracheal or esophageal compression by the mass. Thyroid goiters arising from the lower pole of the thyroid or the thyroid isthmus can enter the superior mediastinum anterior to the trachea (80% of cases) or to the right and posterolateral to the trachea (20% of cases).
On chest radiographs, an anterosuperior mediastinal mass typically deviates the trachea laterally and either posteriorly (anterior masses) or anteriorly (posterior masses). Coarse, clumped calcifications are common in thyroid goiters. Radioiodine studies should be performed as the initial imaging procedure, although false-negative results do occur. CT usually shows characteristic findings: (1) well-defined margins, (2) continuity of the mass with the cervical thyroid, (3) coarse calcifications, (4) cystic or necrotic areas, (5) baseline high CT attenuation (because of intrinsic iodine content), and (6) intense enhancement (>25 H) as a result of the hypervascularity of most thyroid masses and prolonged enhancement (resulting from active uptake of iodine from contrast media) following intravenous
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contrast administration (Fig. 13.1) (1). MR is useful in depicting the longitudinal extension of thyroid goiters without the use of intravenous contrast.
TABLE 13.1 Utility of MDCT, MR, and PET in the Evaluation of Mediastinal Masses
Indication for Study Modalities
Confirming the presence of a mass versus tortuous vascular structures MDCT = MR
Localization of mass to anterior, middle, or posterior compartment MDCT = MR
Suspected aneurysm or vascular anomaly MDCT = MR
Tissue characterization of mass  
   Detection of fluid MDCT = MR = US (for anterior masses or periesophageal masses)
   Detection of calcium CT
   Distinction of tumor from fibrosis PET>MR>CT
Relationship to adjacent structures  
   Vascular invasion MDCT = MR
   Tracheal involvement MDCT > MR
   Involvement of spinal canal MR > MDCT
Thoracic inlet lesions MR = MDCT
Contraindication to iodinated contrast MR > MDCT
Percutaneous biopsy of mediastinal mass CT
US for anterior mediastinal masses
MDCT, multidetector-row CT.
Parathyroid Masses
In approximately 2% of patients, the parathyroid glands fail to separate from the thymus in the neck and descend with the gland into the anterosuperior mediastinum. These glands can be found near the thoracic inlet in or about the thymus. This becomes important in the small percentage of patients with persistent clinical and biochemical evidence of hyperparathyroidism following routine neck exploration and parathyroidectomy. Most of these ectopic parathyroidlesions are small (<3 cm) adenomas; rarely, they represent hyperplastic glands or parathyroid carcinoma. When US and nuclear medicine studies have failed to localize a lesion in the neck, CT, MR, or technetium99 sestamibi scanning may be useful in detecting mediastinal lesions (Fig. 13.2).
TABLE 13.2 Thoracic Inlet Masses
Thyroid mass Goiter
Malignancy
Thyromegaly resulting from thyroiditis
Parathyroid mass Hyperplasia
Adenoma
Carcinoma
Lymph node mass Lymphoma
   Hodgkin
   Non-Hodgkin
Metastases
Inflammatory
   Tuberculosis
Lymphangioma  
Lymphangiomas
These uncommon masses are tumors comprised of dilated lymphatic channels. The cystic or cavernous form (cystic hygroma) is most commonly discovered in infancy and is often associated with chromosomal abnormalities, including Turner syndrome and trisomies 13, 18, and 21. In infants, these lesions tend to extend from the neck into the anterior mediastinum; less commonly they may arise primarily within the anterior mediastinum in older patients. Histologically, these tumors are composed of cystic spaces lined by epithelium and contain clear, straw-colored fluid. Although these lesions are benign histologically, they tend to insinuate themselves between vascular structures and the trachea. This makes complete surgical resection of lymphangiomas difficult, and they frequently recur. CT demonstrates a well-defined cystic mass within the thoracic inlet or superior mediastinum. MR typically shows a mass of high signal intensity on T2WIs because of the fluid content.
Anterior Mediastinal Masses
A number of neoplasms and nonneoplastic conditions arise in the anterior mediastinum and produce anterior mediastinal masses. These include thymic neoplasms, lymphoma, germ cell neoplasms, and primary mesenchymal tumors (Table 13.3).
Thymomas or thymic epithelial neoplasms are the second most common primary mediastinal neoplasms in adults after lymphoma. These lesions are neoplasms that arise from thymic epithelium and contain varying numbers of intermixed lymphocytes. The traditional classification of these tumors is into thymomas, which are histologically benign but may be either encapsulated (noninvasive) or invasive, and thymic carcinomas, in which the epithelial component shows signs of frank malignancy. The World Health Organization has recently reclassified these neoplasms based upon the morphology of the epithelial component and the ratio of epithelial cells to lymphocytes. The classification system divides these neoplasms into types A, AB, B1, B2, B3, and C, with a spectrum of histologic changes ranging from the classic encapsulated thymoma (A), which has a favorable prognosis, to thymic carcinoma (C), which generally carries a poor prognosis (2).
The average age at diagnosis of thymoma is 45 to 50; these lesions are rare in patients under the age of 20. While most often associated with myasthenia gravis, thymoma has been associated with other autoimmune diseases, such
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as pure red cell aplasia, Graves disease, Sjögren syndrome, and hypogammaglobulinemia. Of patients with myasthenia gravis, 10% to 28% have a thymoma, while a larger percentage of patients with thymoma (30% to 54%) have or will develop myasthenia.
FIGURE 13.1. Thyroid Goiter. Posteroanterior (A) and lateral (B) radiographs show a right superior mediastinal mass (arrows) compressing the trachea from the posterior. C. Contrast-enhanced CT at the level of the sternoclavicular joints shows inhomogeneous increased attenuation and enlargement of the thyroid gland that extends retrotracheally. D. More superiorly, the mass is contiguous with the right lobe of an enlarged gland.
On chest radiographs, thymomas are seen as round or oval, smooth or lobulated soft tissue masses arising near the origin of the great vessels at the base of the heart (Fig. 13.3) (2). CT is best for characterizing thymomas and detecting local invasion preoperatively. As a result of their firm consistency, thymomas characteristically maintain their shape where they contact the sternum anteriorly and heart and great vessels posteriorly. Compared to type A tumors, higher-grade thymomas, particularly types B3 and C, tend to show larger size, more irregular margins, heterogeneous enhancement, regions of necrosis,
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mediastinal nodal metastases, and calcification. Invasion of tumor through the thymic capsule is present in 33% to 50% of patients. In the majority of these patients, this determination cannot be made by CT or MR and may even be difficult to determine on examination of the resected specimen. Local invasion of pleura, lung, pericardium, chest wall, diaphragm, and great vessels occurs in decreasing order of frequency in 10% to 15% of patients. Contiguity of a thymoma with the adjacent chest wall or mediastinal structures cannot be used as reliable evidence of invasion of these structures. Drop metastases to dependent portions of the pleural space are a recognized route of spread of thymoma that has invaded the pleura. Extrathoracic metastases are rare, although transdiaphragmatic spread of a pleural tumor into the retroperitoneum has been described. For these reasons, it is important to image the entire thorax and upper abdomen in any patient with suspected invasive disease.
FIGURE 13.2. Ectopic Parathyroid Adenoma. A. In a patient with recurrent hyperparathyroidism after parathyroidectomy, an enhanced CT shows a prevascular mediastinal nodule (arrow). B. Technetium99 sestamibi scan shows a focal area of increased activity in the superior mediastinum (arrow) corresponding to the nodule on CT.
In patients with myasthenia gravis who are being evaluated for thymoma, CT can demonstrate tumors that are invisible on conventional radiographs. However, very small thymic tumors may not be distinguishable from a normal or hyperplastic gland with CT, particularly in younger patients with a large amount of residual thymic tissue.
Thymic cysts may be congenital or acquired. Congenital unilocular thymic cysts are rare lesions that represent remnants of the thymopharyngeal duct and contain thin or gelatinous fluid. They are characterized histologically by an epithelial lining, with thymic tissue in the cyst wall, which distinguishes thymic cysts histologically from other congenital cystic lesions within the anterior mediastinum. Acquired multilocular thymic cysts are postinflammatory in nature and have been associated with AIDS, prior radiation or surgery, and autoimmune conditions such as Sjögren syndrome, myasthenia gravis, and aplastic anemia; in these latter conditions, clinical and radiologic distinction of multilocular thymic cyst from thymoma may be difficult; in fact, the two conditions can coexist. Large cysts will be evident as soft tissue masses on conventional radiographs, and CT or MR will demonstrate the cystic nature of the lesion. If the distinction between a true thymic cyst, cystic degeneration of a thymoma or lymphoma, a germ
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cell neoplasm, or lymphangioma is impossible on clinical and radiologic grounds, the lesion should be biopsied or resected.
TABLE 13.3 Anterior Mediastinal Masses
Thymic masses Thymoma
Thymic cyst
Thymolipoma
Thymic hyperplasia
Thymic neuroendocrine tumors
Thymic carcinoma
Thymic lymphoma
Lymphoma Hodgkin
Non-Hodgkin
Germ cell neoplasms Teratoma (benign or malignant)
Seminoma
Embryonal cell carcinoma
Endodermal sinus tumor
Choriocarcinoma
Thyroid mass Goiter
Tumor
   Adenoma
   Carcinoma
Thyroiditis
Ectopic parathyroid mass Hyperplasia
Adenoma
Carcinoma
Mesenchymal tumor Lipoma
Hemangioma
Leiomyoma
Liposarcoma
Angiosarcoma
FIGURE 13.3. Thymoma. A. Posteroanterior chest radiograph reveals a left mediastinal mass (arrow). B. CT confirms a solid anterior mediastinal mass (arrow). Biopsy revealed a thymoma.
Thymolipoma is a rare, benign thymic neoplasm that consists primarily of fat with intermixed rests of normal thymic tissue. These masses are asymptomatic and therefore are typically large when first detected. Chest radiographs show a large anterior mediastinal mass that, because of its pliable nature, tends to envelope the heart and diaphragm. CT demonstrates a fatty mass with interspersed soft tissue densities. Resection is curative.
Thymic Carcinoid
Neuroendocrine tumors of the thymus are rare malignant neoplasms believed to arise from thymic cells of neural crest origin (amine precursor uptake and decarboxylation [APUD] or Kulchitsky cells). The most common histologic type is carcinoid tumor, which, as with similar lesions arising within the bronchi, ranges in differentiation and behavior from typical carcinoid to atypical carcinoid to small cell carcinoma. Approximately 40% of patients have Cushing syndrome as a result of adrenocorticotropic hormone secretion by the tumor; these patients tend to have smaller lesions at time of diagnosis since they present early with signs of corticosteroid excess. The carcinoid syndrome is uncommon. This lesion is indistinguishable from thymoma on plain radiographs and CT scans.
Thymic hyperplasia is defined as enlargement of a thymus that is normal on gross and histologic examination. This rare entity occurs primarily in children as a rebound effect in response to an antecedent stress, discontinuation
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of chemotherapy, or treatment of hypercortisolism. An association with Graves disease has also been noted. The term thymic hyperplasia has been used incorrectly to describe the histologic findings of lymphoid follicular hyperplasia of the thymus, found in 60% of patients with myasthenia gravis. In contrast to most cases of true thymic hyperplasia, lymphoid hyperplasia does not produce thymic enlargement. Most patients with thymic hyperplasia have normal or diffusely enlarged glands on CT (Fig. 13.4); occasionally thymic hyperplasia will present as a mass that is radiographically indistinguishable from thymoma. Most cases can be resolved by noting a decrease in size on follow-up studies, thereby obviating the need for biopsy.
FIGURE 13.4. Thymic Hyperplasia. A. Enhanced CT in a 12-year-old undergoing chemotherapy for rhabdomyosarcoma shows virtual absence of thymic tissue. B. Scan 3 months following completion of chemotherapy shows uniform enlargement of thymus (arrow), reflecting rebound hyperplasia.
Thymic Lymphoma
The thymus is involved in 40% to 50% of patients with the nodular sclerosing subtype of Hodgkin disease. Its radiographic appearance is indistinguishable from that of other solid neoplasms arising within the thymus. The presence of lymph node enlargement in other portions of the mediastinum or anterior chest wall involvement should suggest the diagnosis.
Lymphoma—either Hodgkin disease or non-Hodgkin lymphoma (NHL)—is the most common primary mediastinal neoplasm in adults. Hodgkin disease involves the thorax in 85% of patients at the time of presentation. The majority (90%) of patients with intrathoracic involvement have mediastinal lymph node enlargement; this most commonly involves the anterior mediastinal and hilar nodal groups. The anterior mediastinum is the most frequent site of a localized nodal mass in patients with Hodgkin disease, particularly those with the nodular sclerosing type (Fig. 13.5). Isolated enlargement of mediastinal or hilar nodes outside the anterior mediastinum should suggest an alternative diagnosis. Only 25% of patients with Hodgkin lymphoma have disease limited to the mediastinum at the time of diagnosis. NHL involves the thorax in approximately 40% of patients at presentation. In contrast to Hodgkin disease, only 50% of patients with NHL and intrathoracic disease have mediastinal nodal involvement, and only 10% of NHL patients have disease that is limited to the mediastinum. Of the various subtypes of NHL that present with mediastinal masses, lymphoblastic lymphoma and diffuse large B-cell lymphoma are the most common (Fig. 13.6). Lymphoma involving a single mediastinal or hilar nodal group is much more common in NHL than in Hodgkin disease. NHL most commonly involves middle mediastinal and hilar lymph nodes; juxtaphrenic and posterior mediastinal nodal involvement is uncommon but is seen almost exclusively in NHL. Patterns of pulmonary parenchymal involvement in lymphoma are discussed in Chapter 15.
While Hodgkin disease spreads in a fairly predictable pattern from one nodal group to an adjacent group, NHL is felt to be a multifocal disorder in which patterns of involvement are unpredictable. Localized intrathoracic Hodgkin disease is usually treated with radiation therapy, with 90% response rates. More widespread Hodgkin disease and
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NHL are treated with chemotherapy, with better response rates for Hodgkin disease than for NHL.
FIGURE 13.5. Hodgkin Lymphoma. A. Posteroanterior chest radiograph in a 35-year-old man shows a large, lobulated mediastinal mass. B. Contrast-enhanced CT at the level of the aortic arch shows bulky anterior and middle mediastinal lymphadenopathy.
On conventional radiographs, lymphoma involving the anterior mediastinum is indistinguishable from thymoma or germ cell neoplasm and presents as a lobulated mass projecting to one or both sides (Fig. 13.5). Calcification in untreated lymphoma is extremely uncommon, and its presence within an anterior mediastinal mass should suggest another diagnosis. Involvement of other lymph nodes in the mediastinum or hila makes lymphoma more likely. An enlarged spleen displacing the gastric air bubble medially, seen in the upper abdominal portion of the frontal chest film, provides an additional clue to the diagnosis.
FIGURE 13.6. Non-Hodgkin Lymphoma, Diffuse Large B-cell Type. Enhanced CT scan in a 34-year-old woman shows a large anterior mediastinal mass with mixed attenuation invading right upper lobe and anterior chest wall (arrow) with associated right pleural effusion. Core needle biopsy showed diffuse large B-cell lymphoma.
CT is performed in virtually all patients with lymphoma. The advantages of chest CT include the ability to better characterize and localize masses seen on chest radiographs; detection of subradiographic sites of involvement that can alter disease staging, prognosis, and therapy; guidance for transthoracic or open biopsy; the ability to monitor response to therapy; and detection of relapse. The appearance of nodal involvement in lymphoma varies; most commonly, discrete enlarged solid lymph nodes or conglomerate masses of nodes are seen (Fig. 13.5B). Central necrosis, seen in 20% of patients, has no prognostic significance. Nodal calcification is rare in the absence of previous mediastinal radiation or systemic chemotherapy. Parenchymal involvement is usually the result of direct extranodal extension of a tumor from hilar nodes along the bronchovascular lymphatics; this is better appreciated on axial CT images than on chest radiographs (3). Likewise, a tumor extending from the mediastinum to the pericardium, subpleural space, and chest wall is best appreciated on CT or MR. On MR, untreated lymphoma appears as a mass of uniform low signal intensity on T1WIs and uniform high signal intensity or intermixed areas of low and high signal intensity on T2WIs. The areas of low signal intensity on T2WIs of untreated patients may be a result of foci of fibrotic tissue in nodular sclerosing Hodgkin disease.
CT, MR, gallium scintigraphy, and fluorodeoxyglucose (FDG) PET have been used to monitor the response of lymphoma to therapy. While CT can accurately assess tumor regression and detect relapse within nodal groups outside the treated region, the ability to distinguish residual tumor from sterilized fibrotic masses is limited. Residual soft tissue masses have been reported in up to
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50% of patients, most commonly with nodular sclerosing Hodgkin disease, and are more common when the pretreatment mass is large. Some patients with residual masses on CT or MR will have tumor recurrence within 6 to 12 months after the completion of therapy. In general, the appearance of high-signal-intensity regions on T2WIs more than 6 months after treatment should suggest recurrence. Radionuclide scintigraphy with gallium-67, particularly SPECT, has been largely replaced by FDG-PET in the initial diagnosis and staging of thoracic lymphoma. PET is clearly superior to CT or MR in distinguishing recurrent tumor from fibrosis in both Hodgkin disease and NHL (4).
Germ cell neoplasms, which include teratoma, seminoma, choriocarcinoma, endodermal sinus tumor, and embryonal cell carcinoma, arise from collections of primitive germ cells that arrest in the anterior mediastinum on their journey to the gonads during embryologic development. Since they are histologically indistinguishable from germ cell tumors arising in the testes and ovaries, the diagnosis of a primary malignant mediastinal germ cell neoplasm requires exclusion of a primary gonadal tumor as a source of mediastinal metastases. A key in distinguishing primary from metastatic mediastinal germ cell neoplasm is the presence of retroperitoneal lymph node involvement in metastatic gonadal tumors.
The most common benign mediastinal germ cell neoplasm is teratoma, comprising 60% to 70% of mediastinal germ cell neoplasms (5). Teratomas may be cystic or solid. Cystic or mature teratoma is the most common type of teratoma seen in the mediastinum. In contrast to a dermoid cyst, which is an ovarian neoplasm containing only elements derived from the ectodermal germinal layer, a cystic teratoma of the mediastinum commonly contains tissues of ectodermal, mesodermal, and endodermal origins. For this reason, it is inaccurate to use the term “dermoid cyst” to describe cystic mediastinal germ cell neoplasms. Solid teratomas are usually malignant. Most germ cell neoplasms are detected in patients in the third or fourth decade of life. While benign tumors have a slight female preponderance (female/male, 60%/40%), malignant tumors are seen almost exclusively in men.
FIGURE 13.7. Malignant Germ Cell Tumor. A. Posteroanterior chest radiograph in a 38-year-old man reveals a right mediastinal mass with discrete right lung nodules (arrows). B. Contrast-enhanced CT demonstrates a large anterior mediastinal mass invading the superior vena cava (arrow) with right lung nodules and a small pleural effusion. CT-guided biopsy showed choriocarcinoma.
Radiographically, these tumors have a distribution similar to that of thymomas. While the majority are located in the anterior mediastinum, up to 10% are found in the posterior mediastinum. Benign lesions are often round or oval and smooth in contour; an irregular, lobulated, or ill-defined margin suggests malignancy. Calcification is present in 33% to 50% of tumors but is nonspecific unless in the form of a tooth. On CT, benign teratomas are usually cystic and may contain soft tissue, bone, teeth, fat, or, rarely, fat–fluid levels. Seminoma, choriocarcinoma, and endodermal sinus (yolk sac) tumors are malignant lesions seen primarily in young men. Seminoma is the most common malignant germ cell neoplasm, accounting for 30% of these tumors. The radiographic findings are nonspecific. CT typically shows a large lobulated soft tissue mass that may contain areas of hemorrhage, calcification, or necrosis (Fig. 13.7). Elevated serum levels of α-fetoprotein or
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human chorionic gonadotropin are helpful in the diagnosis of suspected malignant mediastinal germ cell neoplasm, while clinical and CT evidence of gynecomastia is an additional clue.
Thyroid Masses
While masses arising from the thyroid can present as anterior and superior mediastinal masses, these lesions are best considered as thoracic inlet masses, as discussed earlier.
Mesenchymal Tumors
Benign and malignant tumors arising from the fibrous, fatty, muscular, or vascular tissues of the mediastinum may present as mediastinal masses, most commonly in the anterior mediastinum. Lipomas can occur in any location in the mediastinum but are most often anterior. The diagnosis is made by recognition of a well-defined mass of uniform fatty attenuation (under -50 H). The presence of soft tissue elements should raise the possibility of a thymolipoma or liposarcoma; the latter may show evidence of invasion of adjacent structures at the time of diagnosis. Fat within a mature teratoma or transdiaphragmatic herniation of omental fat is usually easily distinguished from a lipoma.
Hemangiomas are benign tumors composed of vascular channels and may be associated with the syndrome of hereditary hemorrhagic telangiectasia. A pathognomonic sign on chest radiographs is the recognition of phleboliths within a smooth or lobulated soft tissue mass. Angiosarcomas are rare malignant vascular neoplasms that are indistinguishable from other invasive neoplasms arising within the anterior mediastinum.
Leiomyomas are rare benign neoplasms that arise from smooth muscle within the mediastinum. Similarly, fibromas and mesenchymomas (tumors that contain more than one mesenchymal element) can appear as anterior mediastinal masses.
Middle Mediastinal Masses
Lymph Node Enlargement and Masses (Table 13.4). Most middle mediastinal lymph node masses are malignant, representing metastases from bronchogenic carcinoma, extrathoracic malignancy, or lymphoma (6). Benign causes of middle mediastinal lymph node enlargement include sarcoidosis, mycobacterial and fungal infection, angiofollicular lymph node hyperplasia (Castleman disease), and angioimmunoblastic lymphadenopathy.
On plain radiographs, several findings suggest that a middle mediastinal mass represents lymph node enlargement. The presence of multiple bilateral mediastinal masses that distort the lung/mediastinal interface is relatively specific for lymph node enlargement. Solitary masses resulting from lymph node enlargement tend to be elongated and lobulated rather than spherical, since usually more than a single node in a vertical chain of nodes is involved. Occasionally, calcification can be detected within enlarged lymph nodes on plain radiographs; CT is more sensitive in detecting nodal calcification and its distribution within lymph nodes.
TABLE 13.4 Middle Mediastinal Masses
Lymph node masses Malignancy
   Bronchogenic carcinoma
   Lymphoma
   Leukemia
   Kaposi sarcoma
   Extrathoracic malignancy
      Head and neck tumors (squamous cell carcinoma of skin, larynx; thyroid carcinoma)
      Genitourinary tumors (renal cell carcinoma, seminoma)
      Breast carcinoma
      Melanoma
Infection
   Bacteria
      Anaerobic lung abscess
      Anthrax
      Plague
      Tularemia
      Tuberculosis
   Fungi
      Histoplasmosis
      Coccidioidomycosis
      Cryptococcosis
   Viral infection
      Measles
      Mononucleosis
Idiopathic
   Sarcoidosis
   Castleman disease
   Angioimmunoblastic lymphadenopathy
Foregut and mesothelial cysts Bronchogenic cyst
Pericardial cyst
Tracheal and central bronchial neoplasms Malignant
   Carcinoid tumor (bronchi)
   Adenoid cystic carcinoma (trachea)
   Squamous cell carcinoma
Diaphragmatic hernias Foramen of Morgagni hernia
Traumatic hernia
Vascular lesions Arterial
   Double arch/right arch
   Tortuous innominate/subclavian artery
   Aneurysm of the aortic arch
Venous
   Dilated azygos vein
   Dilated hemiazygos vein
   Dilated SVC
   Left-sided SVC
   Dilated left superior intercostal vein
   Dilatation of the main pulmonary artery
SVC, superior vena cava.
One of the prime indications for performing thoracic CT is to detect the presence of enlarged mediastinal lymph nodes. CT is most often obtained to confirm an abnormal
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chest radiographic finding or to evaluate a patient with suspected mediastinal disease despite normal radiographs (a patient with a suspicious solitary pulmonary nodule or with cervical Hodgkin disease). The ability of CT to image in the axial plane and its inherent high contrast resolution allow for the recognition of abnormally enlarged lymph nodes that would not be evident on chest radiographs. In general, abnormal lymph nodes are seen as round or oval soft tissue masses that measure >1.0 cm in their short axis diameter. Although CT is unable to distinguish between benign inflammatory nodes and those involved by malignancy based upon size criteria alone, CT can provide useful information about the internal density of the nodes (Table 13.5).
TABLE 13.5 Density of Mediastinal/Hilar Nodes on CT
Calcification
      Central Mycobacteria
Fungus
      Peripheral (eggshell) Silicosis
Sarcoidosis
Hypervascular Carcinoid tumor/small cell carcinoma
Kaposi sarcoma
Metastases
   Renal cell carcinoma
   Thyroid carcinoma
Castleman disease
Necrosis Mycobacteria
Fungus
Metastases
   Squamous cell carcinoma
   Seminoma
   Lymphoma
A standardized classification system for hilar and mediastinal lymph nodes has recently been advanced by the American Thoracic Society (Fig. 13.8) (7). This scheme correlates with easily identifiable CT and anatomic landmarks and is most important when reporting lymph node enlargement in patients with bronchogenic carcinoma.
MR is as sensitive as CT in detecting enlarged mediastinal lymph nodes. Advantages of MR include the absence of iodinated contrast, easy distinction between vascular and soft tissue structures, exquisite contrast resolution between mediastinal nodes and fat on T1W sequences, and the ability to image in the direct coronal or sagittal plane. The latter feature is an advantage in those mediastinal regions that parallel the axial plane (subcarinal space, aortopulmonary window) and therefore tend to suffer from partial volume averaging effects on CT. The major disadvantages of MR at present are the inability to detect nodal calcification and limited spatial resolution; the latter can result in an inability to distinguish between a group of normal size nodes and a single enlarged node, thereby leading to false-positive results.
In addition to the detection and characterization of enlarged mediastinal nodes, CT can help guide diagnostic nodal tissue sampling. This is usually most helpful in the setting of suspected bronchogenic carcinoma, where accurate staging of mediastinal nodal disease is important for prognostic purposes and treatment planning. The recognition of enlarged subcarinal or pretracheal nodes on CT may suggest biopsy via transcarinal Wang needle or mediastinoscopy, respectively.
As mentioned above, mediastinal lymph node enlargement is common in Hodgkin disease and NHL. Lymphoma accounts for 20% of all mediastinal neoplasms in adults, and most patients with intrathoracic lymphoma have concomitant extrathoracic disease. In most patients, the nodal enlargement is bilateral but asymmetric. Nodular sclerosing Hodgkin disease commonly results in lymph node enlargement, predominantly within the anterior mediastinum and thymus. Isolated posterior nodal enlargement is usually seen only in patients with NHL.
Leukemia, particularly the T-lymphocytic varieties, can cause intrathoracic lymph node enlargement. The lymph node enlargement is usually confined to the middle mediastinal and hilar nodes.
The most common source of metastases to middle mediastinal nodes is bronchogenic carcinoma. In the majority of patients, symptoms or plain radiographic findings suggest the presence of a primary tumor in the lung. In a small percentage of patients, particularly those with small cell carcinoma, the primary carcinoma may be inconspicuous or invisible on plain radiographs, with nodal metastases being the only visible abnormality. Lymph node enlargement is often unilateral on the side of the visible pulmonary or hilar abnormality. Paratracheal and aorticopulmonary nodes are most commonly involved. Since the accuracy of CT in predicting the presence or absence of mediastinal lymph node metastases is approximately 60% to 70%, PET—and in particular integrated CT/PET—should be performed in most patients with bronchogenic carcinoma. A more thorough discussion of mediastinal nodal involvement in bronchogenic carcinoma may be found in Chapter 15.
Lymph node metastases from extrathoracic malignancies can result in mediastinal node enlargement, either with or without concomitant pulmonary metastases. These mediastinal nodal metastases may result from inferior extension of neck masses (thyroid carcinoma, head and neck tumors); extension along lymphatic channels from below the diaphragm (testicular or renal cell carcinoma, GI malignancies); or hematogenous extension (breast carcinoma, melanoma, Kaposi sarcoma) (8).
Mediastinal lymph node enlargement is very common in patients with sarcoidosis, occurring in 60% to 90% of patients at some stage of their disease. Nodal enlargement is typically bilateral and symmetric and involves the hila as well as the mediastinum (Fig. 13.9); this usually
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allows for differentiation of sarcoidosis from lymphoma and metastatic disease. In sarcoidosis, the enlarged nodes produce a lobulated appearance on chest radiographs and CT, because the enlarged nodes do not coalesce. This is in contrast to lymphoma and nodal metastases, in which the intranodal tumor extends through the nodal capsule to form conglomerate enlarged nodal masses. Right and left paratracheal lymph nodes are typically involved; anterior or posterior mediastinal nodal enlargement has been described with greater frequency recently, probably as a
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result of the improved sensitivity of CT for detecting nodal involvement in these regions.
FIGURE 13.8. American Thoracic Society Nodal Stations. Ao, aorta; PA, pulmonary artery. From Mountain CF, Dresler CM. Regional lymph node classification for lung cancer staging. Chest 1997;111:1718–1723; reprinted with permission.
FIGURE 13.9. Lymphadenopathy in Sarcoidosis. Posteroanterior radiograph in a 56-year-old woman with sarcoidosis reveals discrete hilar, paratracheal, and aortopulmonary window lymphadenopathy.
A variety of infections, most commonly histoplasmosis, coccidioidomycosis, cryptococcosis, and tuberculosis, can cause mediastinal nodal enlargement (Fig. 13.10). Typically these patients have parenchymal opacities on chest radiographs, but isolated lymph node enlargement may be seen, particularly in children and young adults. Bacterial infections such as anthrax, bubonic plague, and tularemia are uncommon causes of lymph node enlargement. Typically, there will be symptoms and signs of acute infection, and chest radiographs will show evidence of pneumonia. Bacterial lung abscesses also may be associated with reactive lymph node enlargement. Hilar and mediastinal lymph nodes may be enlarged in patients with measles pneumonia and infectious mononucleosis.
FIGURE 13.10. Tuberculous Lymphadenopathy. Contrast-enhanced CT at the level of the tracheal carina demonstrates enlarged precarinal and left peribronchial lymph nodes with central necrosis and peripheral enhancement. Material obtained by mediastinoscopy revealed Mycobacterium tuberculosis.
Angiofollicular lymph node hyperplasia (Castleman disease) is characterized by enlargement of hilar and mediastinal lymph nodes, predominantly in the middle and posterior mediastinal compartments. In the more common hyaline vascular type, the disease is localized to one lymph node region and presents as an asymptomatic mediastinal soft tissue mass. Histologically, there is replacement of normal nodal architecture with multiple germinal centers and multiple small vessels with hyalinized walls that course perpendicularly toward the germinal centers to give a characteristic “lollipop” appearance on light microscopy. The vascular nature of these masses accounts for the intense enhancement seen on contrast-enhanced CT or angiography. Calcification within these masses has been described. These lesions are cured by resection.
Angioimmunoblastic lymphadenopathy is a rare disorder seen in older adults; it is characterized by constitutional symptoms, lymphadenopathy, hepatosplenomegaly, and skin rash. Hemolytic anemia and hypergammaglobulinemia may be seen. Histologically, the enlarged nodes contain a chronic inflammatory infiltrate and are hypervascular. Chest radiographs and CT show hilar and mediastinal lymph node enlargement that are indistinguishable from other etiologies. As with Castleman disease, the vascular nature of the involved lymph nodes accounts for the contrast enhancement seen on CT. These patients manifest signs of immunodeficiency similar to those associated with AIDS, with one third developing high-grade lymphoma and many succumbing to opportunistic infections such as Pneumocystis carinii pneumonia and cytomegalovirus inclusion disease.
Foregut and mesothelial cysts are common mediastinal lesions that typically present as asymptomatic masses on routine chest radiographs in young adults. CT and MR show findings characteristic of the cystic nature of these lesions.
Congenital bronchogenic cysts result from anomalous budding of the tracheobronchial tree during development. To be characterized as bronchogenic in origin, the wall of the cyst must be lined by a respiratory epithelium with pseudostratified columnar cells and contain seromucous glands; some may contain cartilage and smooth muscle within their walls. It is often difficult to distinguish between bronchogenic and enteric cysts based on their location and pathologic appearance; the term foregut cyst has been used to describe those lesions that cannot be specifically characterized. The majority of bronchogenic cysts
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(80% to 90%) arise within the mediastinum in the vicinity of the tracheal carina. Most mediastinal lesions are asymptomatic; occasionally, compression of the tracheobronchial tree or esophagus may produce dyspnea, wheezing, or dysphagia. Rarely, mediastinal cysts become secondarily infected after communication with the airway or esophagus, or they cause symptomatic compression after rapid enlargement following hemorrhage. Bronchogenic cysts are seen as soft tissue masses in the subcarinal or right paratracheal space on frontal chest radiographs; less common sites of involvement include the hilum, posterior mediastinum, and periesophageal region. They appear as a single smooth, round, or elliptic mass; a minority are lobulated in contour. CT is the method of choice for the diagnosis of a mediastinal cyst. If a well-defined, thin-walled mass of fluid density (0 to 10 H) is seen that fails to enhance following intravenous contrast administration, it can be assumed to represent a benign cyst (Fig. 13.11) (9). High CT numbers (>40 H) suggesting a solid mass can be seen when the cyst is filled with mucoid material, milk of calcium, or blood. Calcification of the cyst wall has been described but is uncommon. MR shows characteristic low signal intensity on T1WIs and high signal intensity on T2WIs. The presence of proteinaceous material within the cyst will shorten T1 relaxation times, yielding high signal intensity on T1WIs. In many patients, resection is required for definitive diagnosis. Both transbronchoscopic and percutaneous needle aspiration and drainage have been used successfully for the diagnosis and treatment of these lesions.
FIGURE 13.11. Bronchogenic Cyst. Unenhanced (A) and enhanced (B) CT scans in a 38-year-old man demonstrate a smooth, low-attenuation paratracheal mass (arrows) that fails to enhance, consistent with a bronchogenic cyst.
Pericardial cysts arise from the parietal pericardium and contain clear serous fluid surrounded by a layer of mesothelial cells. Most often, they arise in the anterior cardiophrenic angles, with right-sided lesions being twice as common as left-sided lesions; approximately 20% arise more superiorly within the mediastinum. These lesions usually present as incidental asymptomatic round or oval masses in the cardiophrenic angle (Fig. 13.12). Their pliable nature can be demonstrated with a change in patient position. CT typically shows a unilocular cystic mass
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adjacent to the heart; MR or US via a subxiphoid approach shows findings characteristic of a simple cyst. As with bronchogenic cysts, there have been reports of cysts with high attenuation on CT that on resection are found to be filled with proteinaceous or mucoid material.
FIGURE 13.12. Pericardial Cyst. Enhanced CT scan through heart shows a smooth, sharply marginated, low-attenuation mass (arrow) in the right cardiophrenic angle, consistent with a pericardial cyst.
Tracheal and central bronchial masses commonly produce upper airway symptoms with obstructive pneumonitis and atelectasis and rarely present as asymptomatic mediastinal masses. Occasionally, central airway masses present as radiographic abnormalities when they distort the tracheal air column or mediastinal contour. These masses are discussed in Chapter 18.
Diaphragmatic hernias, which may present as pericardiac masses, are discussed in Chapter 19.
Vascular Lesions
Congenital or acquired anomalies of the heart and great vessels are common middle mediastinal masses and are discussed in Chapter 14.
Neurogenic Lesions
Rarely, a neurofibroma arising from the phrenic nerve may present as a middle mediastinal juxtacardiac mass.
Posterior Mediastinal Masses
Neurogenic Tumors (Table 13.6). Posterior mediastinal masses arising from neural elements are classified by their tissue of origin. Three groups have been recognized: (1) tumors arising from intercostal nerves (neurofibroma, schwannoma); (2) sympathetic ganglia (ganglioneuroma, ganglioneuroblastoma, and neuroblastoma); and (3) paraganglionic cells (chemodectoma, pheochromocytoma). Tumors in each of these three groups may be benign or malignant neoplasms (5). 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 schwannoma affect adults more frequently.
Histologically, both neurofibroma and schwannoma are comprised of spindle cells that arise from the Schwann cell. While neurofibroma is an encapsulated tumor that contains interspersed neurons, schwannoma is not encapsulated and contains no neuronal elements. Both tumors are more common in patients with neurofibromatosis. Multiple lesions in the mediastinum, particularly bilateral apicoposterior masses, are virtually diagnostic of neurofibromatosis. A small percentage of schwannomas (10%) are locally invasive (malignant schwannoma).
Radiographically, intercostal nerve tumors appear as round or oval paravertebral soft tissue masses. CT shows a smooth or lobulated paraspinal soft tissue mass, which may erode the adjacent vertebral body or rib. CT demonstration of tumor extension from the paravertebral space into the spinal canal via an enlarged intervertebral foramen is characteristic of a “dumbbell” neurofibroma. MR is the modality of choice for imaging a suspected neurofibroma. In addition to the occasional demonstration of both intra–and extra–spinal canal components, MR of neurofibromas shows typical high signal intensity on T2WIs.
TABLE 13.6 Posterior Mediastinal Masses
Neurogenic tumors Peripheral (intercostal) nerves
   Neurofibroma
   Schwannoma
Sympathetic ganglia
   Ganglioneuroma
   Ganglioneuroblastoma
   Neuroblastoma
Paraganglion cells
   Chemodectoma
   Pheochromocytoma
Esophageal lesions Duplication (enteric) cyst
Diverticulum
Neoplasm
   Leiomyoma
   Squamous cell carcinoma
Esophageal dilatation
   Achalasia
   Scleroderma
   Peptic stricture
   Carcinoma
Paraesophageal varices
Hiatal hernia
   Sliding
   Paraesophageal
Foregut cysts Enteric
Neurenteric
Vertebral lesion Trauma
   Paraspinal hematoma
Infection
   Paraspinal abscess
      Tuberculosis
      Staphylococcus
Tumor
   Metastases (bronchogenic, breast, renal cell carcinoma)
   Multiple myeloma
   Lymphoma
Degenerative disease (osteophytosis)
Extramedullary hematopoiesis
Lateral thoracic meningocele  
Pancreatic pseudocyst  
Tumors that arise from the sympathetic ganglia represent a continuum from the histologically benign ganglioneuroma found in adolescents and young adults to the highly malignant neuroblastoma seen almost exclusively in children under the age of 5. These tumors generally present as elongated, vertically oriented paravertebral soft tissue masses with a broad area of contact with the posterior mediastinum (Figs. 13.13, 13.14). These findings may help distinguish these lesions from neurofibromas, which
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usually maintain an acute angle with the vertebral column and posterior mediastinum and therefore tend to show sharp superior and inferior margins on lateral chest radiographs. Large masses may erode vertebral bodies or ribs. Calcification, seen in up to 25% of cases, is a helpful diagnostic feature of these tumors but does not help distinguish benign from malignant neoplasms. Because these tumors often produce catecholamines, urinary levels of vanillylmandelic acid or metanephrines, which are byproducts of catecholamine metabolism, may be elevated. Prognosis depends upon the histologic features of the tumor and the patient’s age and extent of disease at the time of diagnosis.
FIGURE 13.13. Neurofibroma. A. Frontal chest radiograph shows a left upper mediastinal mass (arrow). B. Contrast-enhanced CT confirms the presence of a left paravertebral soft tissue mass (arrow). Surgical resection confirmed a neurofibroma.
FIGURE 13.14. Ganglioneuroma. A. Posteroanterior radiograph in a 15-year-old woman reveals an oval, vertically oriented, right-sided mediastinal mass (arrows). B. Contrast-enhanced CT shows a low-attenuation posterior mediastinal mass (arrow) with calcification. This was surgically proven to be ganglioneuroma.
Paragangliomas are tumors that arise in the aorticopulmonary paraganglia of the middle mediastinum or the aorticosympathetic ganglia of the posterior mediastinum. They are divided into nonfunctioning neoplasms (chemodectomas), which occur almost exclusively in or about the aortopulmonary window, and functioning neoplasms (pheochromocytomas), which are found in the posterior sympathetic chain or in or about the heart or pericardium. Approximately 2% of all pheochromocytomas arise in the mediastinum. The posterior mediastinum is the site of fewer than 25% of mediastinal paragangliomas, with the majority arising in the anterior or middle mediastinum. Radiographically, these tumors are indistinguishable from other neurogenic tumors. However, most patients have hypertension and biochemical evidence of excess catecholamine production. CT and angiography demonstrate hypervascular masses; radionuclide iodine-131-meta-iodobenzylguanidine (MIBG) scanning is diagnostic in functioning tumors.
Esophageal Lesions
Because most of the intrathoracic esophagus is intimately associated with the thoracic spine and descending thoracic aorta, lesions in the middle or distal third of the esophagus may present as posterior mediastinal masses. Common presenting symptoms include dysphagia and aspiration pneumonia, although many patients are asymptomatic.
The majority of esophageal neoplasms, excluding lesions that arise at the esophagogastric junction, are squamous cell carcinomas. Unlike benign neoplasms of the posterior mediastinum, these lesions, when seen on chest radiographs, are rarely asymptomatic. Typically these patients have a history of dysphagia and significant weight loss. Difficulty in detecting asymptomatic lesions and the absence of a serosa account for the advanced stage of most esophageal carcinoma at presentation and a 5-year survival rate of less than 20%. Most patients with esophageal carcinoma have abnormal plain radiographic findings, including an abnormal azygoesophageal interface, widening of the mediastinum (resulting from the tumor itself or a dilated esophagus proximal to the obstructing lesion), abnormal thickening of the tracheoesophageal stripe, and tracheal deviation and compression. The diagnosis is usually made on barium esophagram and confirmed by endoscopic biopsy. CT scanning has proved accurate for staging esophageal carcinoma: findings include an intraluminal mass; thickening of the esophageal wall; loss of fat planes between the esophagus and adjacent mediastinal structures (usually the trachea, with upper esophageal lesions, and the descending aorta, with lower esophageal lesions); and evidence of nodal and distant metastases.
Several benign esophageal neoplasms, including leiomyoma, fibroma, and lipoma, can present as smooth, solitary mediastinal masses projecting laterally from the posterior mediastinum on frontal chest radiographs. They generally involve the lower third of the esophagus from the level of the subcarinal space to the esophageal hiatus. Initial evaluation is with barium studies, which show a smooth, broad-based mass forming obtuse margins with the esophageal wall. CT demonstrates a smooth, well-defined soft tissue mass adjacent to the esophagus without obstruction. The absence of esophageal dilatation above the mass helps distinguish benign tumors from carcinoma.
Pulsion diverticula arising at the cervicothoracic esophageal junction or distal esophagus are false diverticula representing mucosal outpouchings through defects in the muscular layer of the esophagus. A large proximal pulsion diverticulum (Zenker) may extend through the thoracic inlet and appear as a retroesophageal superior mediastinal mass containing an air–fluid level on upright chest radiographs. A distal pulsion diverticulum appears as a juxtadiaphragmatic mass with an air–fluid level projecting to the right of midline. Barium swallow is diagnostic.
A dilated esophagus resulting from functional (achalasia, scleroderma) or anatomic (stricture, carcinoma) obstruction may produce a mass that courses vertically over the length of the mediastinum, projecting toward the right side on frontal chest radiographs. An air–fluid level on upright films is usually present. A completely air-filled, dilated esophagus appears as a thin curvilinear line along the medial right thorax, because the right lateral wall of the esophagus is outlined by intraluminal air medially and the right lung laterally. Barium study or CT will confirm the diagnosis of a dilated esophagus; determination of the cause of obstruction often requires endoscopy or esophageal manometry.
Esophageal varices may produce a round or lobulated retrocardiac mass in patients with portal hypertension. The diagnosis is usually made by endoscopic recognition of submucosal varices involving the distal esophagus. The varices are readily recognized on contrast CT, MR, or portal venography.
A common cause of a mass in the posteroinferior mediastinum is a hiatal hernia. This results from a separation of the superior margins of the diaphragmatic crura
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and stretching of the phrenicoesophageal ligament. The stomach is by far the most common structure in the hernia sac; the gastric cardia (sliding hernia) or fundus (paraesophageal hernia) may be involved. Rarely, omental fat, ascitic fluid, or a pancreatic pseudocyst herniates through the esophageal hiatus into the mediastinum. The characteristic location at the esophageal hiatus and the presence of a rounded density containing an air or air–fluid level on upright films are diagnostic. Barium swallow or a CT scan will confirm the diagnosis (see Fig. 19.25).
Enteric/Neurenteric Cysts
Enteric cysts are fluid-filled masses lined by enteric epithelium. Esophageal cysts usually arise intramurally or immediately adjacent to the esophagus. When an enteric cyst has a persistent communication with the spinal canal (canal of Kovalevski) and is associated with congenital defects of the thoracic spine (anterior spina bifida, hemivertebrae, or butterfly vertebrae), it is termed a neurenteric cyst. CT or MR can confirm the cystic nature of these masses (Fig. 13.15). If the cyst communicates with the GI tract, it may contain air or an air–fluid level or opacify with contrast during an upper GI series.
Vertebral Abnormalities
A variety of conditions that affect the thoracic spine may manifest as posterior mediastinal masses. These lesions typically produce lateral deviation of the paraspinal reflection on frontal radiographs. Often, the bony origin of these lesions is not obvious on initial examination, making distinction from neurogenic tumors and other posterior mediastinal masses difficult.
FIGURE 13.15. Esophageal Duplication Cyst. Enhanced CT in an 18-year-old man with a posterior mediastinal mass on chest radiography (not shown) demonstrates a low-attenuation right paraesophageal mass (arrow), consistent with an esophageal duplication cyst.
Neoplastic, infectious, metabolic, traumatic, or degenerative processes of the thoracic spine may produce a paraspinal mass by one of four mechanisms: (1) expansion of vertebral body or posterior elements (multiple myeloma, aneurysmal bone cyst); (2) extraosseous extension of infection, tumor, or marrow elements (infectious spondylitis, metastatic carcinoma, extramedullary hematopoiesis, respectively); (3) pathologic fracture and paraspinal hematoma formation (any destructive neoplastic or inflammatory process, trauma); or (4) protrusion of degenerative osteophytes. Neoplastic processes are usually easily identified by expansion and destruction of vertebral bodies, with sparing of intervertebral disks. Bronchogenic, breast, or renal cell carcinoma are the most common primary sites of thoracic spinal metastases. Infectious spondylitis is distinguished from neoplastic processes by the presence of a paravertebral mass centered at the point of maximal bone destruction. In patients with a paravertebral abscess secondary to tuberculosis or bacterial infection, narrowing of the adjacent disk space and destruction of vertebral endplates are important clues to the diagnosis. Extramedullary hematopoiesis is seen almost exclusively in conditions associated with ineffective production or excessive destruction of erythrocytes, such as thalassemia major, congenital spherocytosis, and sickle cell anemia. It is recognized by noting expansion of the medullary space and cyst formation within long bones, ribs, and vertebral bodies, with associated lobulated paraspinal soft tissue masses. These masses, which represent hyperplastic bone marrow that has extruded from the vertebral bodies and posterior ribs, are typically seen in the lower thoracic and upper lumbar region. Traumatic injuries to the thoracic spine are usually obvious from the patient’s history and recognition of spine fracture on conventional and CT studies of the spine. Degenerative disk disease may produce a localized paraspinal mass on frontal radiographs. Well-penetrated films will show the characteristic inferolaterally projecting osteophytes at the level of the mass, which are most commonly right-sided because of the inhibitory effect of the pulsating descending aorta on left-sided osteophyte formation.
Lateral thoracic meningoceles represent an anomalous herniation of the spinal meninges through an intervertebral foramen, resulting in a paravertebral soft tissue mass. Most meningoceles are discovered in middle-aged patients as asymptomatic masses. They are slightly more common on the right, and are multiple in 10% of cases. There is a high association between lateral thoracic meningoceles and neurofibromatosis. A meningocele is the most common posterior mediastinal mass in
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patients with neurofibromatosis; conversely, approximately two thirds of patients with meningoceles have neurofibromatosis. Chest radiographs typically reveal a round, well-defined paraspinal mass that is indistinguishable from a neurofibroma. Additional clues to the diagnosis include rib erosion, enlargement of the adjacent neural foramen, vertebral anomalies, or kyphoscoliosis. When a lateral meningocele is associated with kyphoscoliosis, it is usually found at the apex of the scoliotic curve on the convex side. MR demonstration of a herniated subarachnoid space is the diagnostic technique of choice; conventional or CT myelography, which demonstrates filling of the meningocele with contrast, is reserved for equivocal cases.
FIGURE 13.16. Pancreatic Pseudocyst as Posterior Mediastinal Mass. A. Portable chest radiograph in a 62-year-old man with an episode of severe pancreatitis 7 months earlier shows a posteroinferior mediastinal mass (arrows). B. Unenhanced CT through the lower chest shows a thick-walled cystic posterior mediastinal mass. C. Scan through the upper abdomen shows communication of the abdominal and thoracic components of the pseudocyst (arrows) via the esophageal hiatus.
Miscellaneous Conditions
A pancreatic pseudocyst rarely produces a posterior mediastinal mass by extending cephalad from the retroperitoneum through the esophageal or aortic hiatus of the diaphragm. The diagnosis relies on CT demonstration of continuity of a predominantly cystic mass with its retroperitoneal portion (Fig. 13.16).
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The presence of a left pleural effusion is a further clue to the diagnosis. Hernias through the foramen of Bochdalek, which produce a posterior mediastinal mass, are discussed in Chapter 19.
Rarely, malignant lymph node enlargement may produce a recognizable paraspinal mass. This is most often seen in NHL and metastatic lung cancer; other mediastinal or extrathoracic sites of involvement are invariably present.
Despite the advances in detection and characterization of mediastinal masses with cross-sectional imaging, most patients will require tissue sampling for definitive diagnosis. However, the radiologist can use the information provided by CT or MR to help limit the differential diagnosis and thereby guide the appropriate evaluation and treatment. In a large percentage of cases, when tissue sampling is required, it can be accomplished by CT- or US-guided transthoracic biopsy.
DIFFUSE MEDIASTINAL DISEASE
The differential diagnosis of diffuse widening of the mediastinum is reviewed in Table 13.7.
TABLE 13.7 Diffuse Mediastinal Widening
Smooth Mediastinal lipomatosis
Malignant infiltration
   Lymphoma
   Small cell carcinoma
   Adenocarcinoma
Mediastinal hemorrhage
   Arterial bleeding
      Traumatic aortic arch/great vessel laceration
      Aneurysmal rupture
   Venous bleeding
   SVC/right atrial laceration
Mediastinitis
   Acute (suppurative)
   Chronic (sclerosing)
      Histoplasmosis
      Tuberculosis
      Idiopathic
Lobulated Lymph node enlargement (see Table 13.4)
Thymic mass (see Table 13.3)
Germ cell neoplasm (see Table 13.3)
Vascular lesions
   Tortuosity of great vessels
   SVC occlusion (dilated venous collaterals)
      Malignancy
      Sclerosing mediastinitis
      Catheter-induced thrombosis
Neurofibromatosis
SVC, superior vena cava.
Mediastinal infection is an uncommon condition that may be divided into acute and chronic forms based upon etiology, clinical features, and radiologic findings. The distinction between acute and chronic infection is important because there are considerable differences in treatment and prognosis.
Acute mediastinitis is caused by bacterial infection that most often develops following esophageal perforation or is a complication of cardiothoracic surgery. Esophageal perforation may complicate esophageal instrumentation (e.g., endoscopy, biopsy, dilatation, or stent placement); penetrating chest trauma; esophageal carcinoma; foreign body or corrosive ingestion; or vomiting. Spontaneous esophageal perforation following prolonged vomiting is termed Boerhaave syndrome. In this condition, a vertical tear occurs along the left posterolateral wall of the distal esophagus, just above the esophagogastric junction, leading to signs and symptoms of acute mediastinitis. Less commonly, acute mediastinitis may develop from intramediastinal extension of infection in the neck, retropharyngeal space, lungs, pleural space, pericardium, or spine.
The clinical presentation of acute mediastinitis is usually dramatic and is characterized by severe retrosternal chest pain, fever, chills, and dysphagia, often accompanied by evidence of septic shock. Physical examination may reveal findings associated with pneumomediastinum, with subcutaneous emphysema in the neck and an apical, systolic crunching sound on chest auscultation (Hamman sign).
The most common chest radiographic findings are widening of the superior mediastinum in 66% of patients and pleural effusion in 50% of patients. Specific findings such as mediastinal air or air–fluid levels are less common. When mediastinitis occurs in association with Boerhaave syndrome, pneumoperitoneum and left hydropneumothorax may be seen.
When esophageal perforation is suspected, an esophagram should be performed to detect leakage of contrast into the mediastinum and to localize the exact site of perforation. In a patient not at risk for aspiration, a water-soluble contrast agent is administered initially. Once gross contrast extravasation has been excluded, barium is then given for superior radiographic detail. The sensitivity of the esophagram for detecting contrast leakage is highest when the study is obtained within 24 hours of the perforation.
MDCT is the radiologic study of choice for the diagnosis of acute mediastinitis (10). CT findings include extra luminal gas, bulging of the mediastinal contours, and focal or diffuse soft tissue infiltration of mediastinal fat. Localized fluid collections suggest focal abscess formation. Associated findings include mediastinal venous thrombosis, pneumothorax, pleural effusion or empyema, subphrenic abscess, and vertebral osteomyelitis.
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While the clinical and radiographic diagnosis of mediastinitis is often straightforward, it may be difficult in postoperative patients who have undergone recent median sternotomy. In these patients, infiltration of mediastinal fat and focal air or fluid collections may be normal findings on postoperative CT scans performed days to weeks following the removal of intraoperatively placed mediastinal drains. In such patients, the progression of findings on follow-up CT scans will correctly identify the majority of those with postoperative mediastinal infection.
The prognosis for patients with acute mediastinitis varies with the underlying etiology and the extent of mediastinal involvement at the time of diagnosis. Esophageal perforation is associated with the poorest outcome, with a mortality approaching 50%. A delay in diagnosis and treatment of the mediastinal infection of greater than 24 hours is associated with a significant increase in overall morbidity and mortality.
In addition to its sensitivity in the diagnosis of mediastinitis, CT can be used to guide treatment and predict outcome. Those patients with evidence of extensive mediastinal infection, seen on CT as diffuse infiltration of the mediastinal fat without evidence of abscess formation, have a mortality approaching 50%. In contrast, patients with discrete mediastinal abscesses that are amenable to surgical or percutaneous drainage, or with small localized abscesses that are amenable to antibiotic therapy alone, have a more favorable prognosis. In addition, patients with mediastinal abscesses and contiguous empyema or subphrenic abscess may respond favorably to drainage of these extramediastinal collections.
Chronic Sclerosing (Fibrosing) Mediastinitis
The hallmarks of chronic sclerosing mediastinitis are chronic inflammatory changes and mediastinal fibrosis. The most common cause of this rare condition is granulomatous infection, usually secondary to Histoplasma capsulatum. Tuberculous infection, radiation therapy, and drugs (methysergide) are less common causes. Idiopathic mediastinal fibrosis, which is probably an autoimmune process, is related to fibrosis in other regions, including the retroperitoneum, intraorbital fat, and thyroid gland.
Several theories have been advanced to explain the pathogenesis of sclerosing mediastinitis owing to histoplasmosis. The most widely accepted theory suggests that affected patients develop an idiosyncratic hypersensitivity response to a fungal antigen that “leaks” from infected mediastinal lymph nodes.
Clinically, this condition occurs in adults and presents with a variety of symptoms, depending upon the extent of fibrosis and the mediastinal structures compromised by the fibrotic process. The superior vena cava (SVC) is the most commonly affected structure, with involvement in over 75% of symptomatic patients. The SVC syndrome manifests with headache, epistaxis, cyanosis, jugular venous distention, and edema of the face, neck, and upper extremities. The most serious and potentially fatal manifestation of sclerosing mediastinitis is obstruction of the central pulmonary veins, which produces pulmonary edema that may mimic severe mitral stenosis. Patients with involvement of the tracheobronchial tree may have cough, dyspnea, wheezing, hemoptysis, and obstructive pneumonitis. Dysphagia or hematemesis can be seen with esophageal involvement. Less commonly, pulmonary arterial hypertension and cor pulmonale can develop from narrowing of the pulmonary arteries.
The most common finding noted on chest radiographs is asymmetric lobulated widening of the upper mediastinum, most often on the right. When the process is secondary to granulomatous infection, enlarged calcified lymph nodes may be seen. Narrowing of the tracheobronchial tree may be evident. The sequelae of vascular involvement may be seen, including oligemia from pulmonary arterial compression or venous hypertension and pulmonary edema from involvement of the central pulmonary veins. Postobstructive atelectasis or consolidation may also be seen.
CT is the modality of choice for the diagnosis and assessment of chronic sclerosing mediastinitis. Enlarged lymph nodes with calcification are the most common finding (Fig. 13.17). The fibrotic infiltration of the mediastinal fat that is characteristic of this condition is seen as abnormal soft tissue density replacing the normal mediastinal fat with obliteration of the normal mediastinal interfaces. CT delineates the degree of involvement of the mediastinal vessels, trachea, and central bronchi. In patients with significant SVC involvement, collateral venous channels within the mediastinum and chest wall are well demonstrated.
MR is superior to CT for the assessment of vascular involvement. The ability to examine the mediastinal vessels in both the axial and coronal planes without the need for intravenous contrast helps detect vascular compromise. A significant disadvantage of MR is its inability to detect nodal calcification, a finding that is key to the diagnosis. For this reason, MR is most often utilized as an adjunct to CT when findings of vascular involvement are equivocal.
A definitive diagnosis of chronic sclerosing mediastinitis and the establishment of the underlying etiology are difficult. Skin tests for histoplasmosis and tuberculosis may add additional information but are usually not helpful. The precise diagnosis, and more important the distinction from infiltrating malignancy, usually requires biopsy.
Mediastinal Hemorrhage
Injury to mediastinal vessels resulting from blunt or penetrating thoracic trauma is the most common cause of mediastinal hemorrhage. Blunt chest trauma most often occurs in the setting of a motor
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vehicle accident, when rapid deceleration and thoracic cage compression produce shearing effects at the aortic isthmus. Iatrogenic trauma, usually from attempts at central line placement, can also cause mediastinal hemorrhage. Spontaneous hemorrhage may develop in patients with a coagulopathy, or with aortic rupture from aneurysm or dissection. Chronic hemodialysis, radiation vasculitis, and bleeding into a mediastinal mass are rare causes of mediastinal hemorrhage.
FIGURE 13.17. Sclerosing Mediastinitis From Histoplasmosis. A. Posteroanterior chest film in an asymptomatic 68-year-old man shows lobulated widening of the upper mediastinum. B. Contrast-enhanced CT reveals marked dilatation of the left superior intercostal vein (arrows), high-attenuation material in and around the superior vena cava, and numerous collaterals within the mediastinal fat. C. A noncontrast scan at approximately the same level reveals mediastinal calcification obliterating the superior vena cava. The patient was a former resident of Ohio where histoplasmosis is endemic.
In the nontraumatic setting, the symptoms and signs of mediastinal hemorrhage are often mild or absent. The patient may complain of retrosternal chest pain radiating toward the back. Rarely, SVC compression may result in the SVC syndrome. Extension of blood from the mediastinum superiorly into the retropharyngeal space may result in neck stiffness, odynophagia, or stridor.
The main radiographic finding in mediastinal hemorrhage of any cause is a focal or diffuse widening of the mediastinum that obscures the normal mediastinal contours (11). In mediastinal hemorrhage, the mediastinum develops a flat or slightly convex outward contour, unlike the round, lobulated, or irregular contour seen with enlarged lymph nodes or a localized mediastinal mass. Blood extending from the mediastinum into the pleural or extrapleural space produces a free-flowing effusion or a loculated extrapleural collection, respectively. Rarely, extension of blood into the lungs via the bronchovascular interstitium produces interstitial opacities that mimic pulmonary edema. Serial radiographs may show rapid changes in mediastinal or pleural fluid collections in patients with persistent hemorrhage. CT demonstrates abnormal soft tissue within the mediastinum that obliterates the normal interfaces between the mediastinal fat, the vessels, and the airways (Fig. 13.18). Freshly clotted blood is high in attenuation and is usually easily appreciated on helical CT. CT is also superior to plain radiography in
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demonstrating the extramediastinal extent of hemorrhage and is useful in demonstrating associated thoracic injuries in patients following blunt chest trauma.
FIGURE 13.18. Mediastinal Hematoma From Ruptured Thoracic Aortic Aneurysm. A. Portable chest radiograph in an 83-year-old woman with chest pain shows marked mediastinal widening. B. Contrast-enhanced CT demonstrates aneurysmal dilatation of the descending aorta, with active extravasation (arrow) into a large mediastinal hematoma. The patient was not a surgical candidate and expired shortly after the study.
Mediastinal lipomatosis is a benign, asymptomatic condition characterized by excessive deposition of fat in the mediastinum. Predisposing conditions include obesity, Cushing disease, and corticosteroid therapy. However, this entity is unassociated with identifiable conditions in approximately 50% of patients.
On conventional radiographs, the most common finding is smooth, symmetric widening of the superior mediastinum. If the amount of fat deposition is marked, the mediastinum may show lobulated margins. Unlike mediastinal tumor infiltration or hemorrhage, which usually cause tracheal deviation or narrowing, the trachea remains at midline in mediastinal lipomatosis. Fat may also accumulate in the paraspinal regions, chest wall, and cardiophrenic angles; the latter produces enlargement of the epipericardial fat pads that is a clue to the proper diagnosis.
CT provides a definitive diagnosis by demonstrating abundant, homogeneous, unencapsulated fat that bulges the mediastinal contours (Fig. 13.19). Displacement or compression of mediastinal structures, particularly the trachea, is notable by its absence. Heterogeneity within the fat suggests other primary or superimposed conditions, such as neoplastic infiltration, infection, hemorrhage, or fibrosis.
Multiple symmetric lipomatosis is a rare entity that resembles simple mediastinal lipomatosis radiographically. The distinction between these two conditions is made by the distribution of abnormal fat and mass effect on mediastinal structures. In multiple symmetric lipomatosis, the cardiophrenic angles, paraspinal areas, and the anterior mediastinum are spared; periscapular lipomas may also be seen. The trachea is often compressed or displaced by fat in patients with this condition, whereas this is not seen in simple lipomatosis.
Malignancy
Malignant involvement of the mediastinum is typically seen as discrete masses or lymph node enlargement. Rarely, diffuse soft tissue infiltration of the mediastinal fat may occur, either alone or in association with focal lesions. Plain radiographs are nonspecific, usually demonstrating mediastinal widening. CT shows soft tissue infiltration of the normal mediastinal fat and obliteration of the normal tissue planes. This pattern is most common with extracapsular spread of lymphoma or small cell carcinoma of the lung. The latter disease has a high propensity to invade mediastinal structures and therefore may present with symptoms of airway obstruction or SVC syndrome.
Pneumomediastinum is the presence of extraluminal gas within the mediastinum. Possible sources of such gas include the lungs, trachea, central bronchi, esophagus, and extension of gas from the neck or abdomen (Table 13.8) (Fig. 13.20) (12).
Air from the lungs is the most common source of pneumomediastinum. The mechanism of pneumomediastinum formation involves a sudden rise in intrathoracic and intra-alveolar pressure that leads to alveolar rupture. The
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extra-alveolar air first collects within the bronchovascular interstitium and then dissects centrally to the hilum and mediastinum (the Macklin effect). Less commonly, the air may dissect peripherally toward the subpleural interstitium and rupture through the visceral pleura to produce a pneumothorax.
FIGURE 13.19. Mediastinal Lipomatosis. A. Frontal chest radiograph shows a widened superior mediastinum, particularly on the right (arrow). B. Unenhanced CT at the level of the aortic arch shows abundant mediastinal fat, responsible for the mediastinal widening.
Pneumomediastinum most commonly complicates mechanical ventilation in patients with ARDS, because the combination of positive pressure ventilation and abnormally stiff lungs predisposes to alveolar rupture. Spontaneous pneumomediastinum can occur with deep inspiratory or Valsalva maneuvers during strenuous exercise, childbirth, weightlifting, and inhalation of drugs such as marijuana, nitrous oxide, and crack cocaine. Patients with asthma are prone to pneumomediastinum; this is related to the airways obstruction that characterizes this disease. Prolonged vomiting from any cause may lead to intrathoracic pressures that are sufficiently high to produce pneumomediastinum. In patients with diabetic ketoacidosis, the increased respiratory effort that accompanies attempts at correcting the underlying metabolic acidosis can lead to pneumomediastinum. Blunt chest trauma can result in pneumomediastinum as a result of an abrupt increase in intra-alveolar pressure and shearing forces affecting the alveolar walls.
TABLE 13.8 Pneumomediastinum
Intrathoracic source Alveoli
   Valsalva maneuver
   Positive pressure ventilation
Esophagus
   Boerhaave syndrome
   Endoscopic interventions (biopsy, dilatation, sclerotherapy)
   Carcinoma
Tracheobronchial tree
   Bronchial stump dehiscence
   Tracheobronchial laceration
Fistula formation
   Tracheal/esophageal malignancy
   Infection (tuberculosis, histoplasmosis)
Extrathoracic source Recent sternotomy/thoracotomy
Pneumoperitoneum/pneumoretroperitoneum
Subcutaneous emphysema in neck
Stab wound
Laryngeal fracture
Pneumomediastinum arising from the tracheobronchial tree or esophagus usually is a result of traumatic disruption of these structures. The marked shearing forces that develop with blunt trauma may lead to fracture of the trachea or mainstem bronchi. Penetrating trauma to the tracheobronchial tree is usually iatrogenic and may follow endotracheal intubation, bronchoscopy, or tracheostomy. Rarely, neoplasms or inflammatory lesions (e.g., tuberculosis) may erode through the tracheal wall and into the peritracheal fat. Esophageal rupture is most often spontaneous, usually in the setting of severe, prolonged vomiting (Boerhaave syndrome). In addition to pneumomediastinum, a left hydropneumothorax and
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pneumoperitoneum may be present in this condition. Spontaneous esophageal rupture may occur during childbirth, during a severe asthmatic episode, or with blunt chest trauma. Endoscopic procedures, stent placement, esophageal dilatation, corrosive ingestion, and carcinoma may lead to esophageal perforation. Mediastinal gas may be produced by bacterial organisms in acute mediastinitis.
FIGURE 13.20. Pneumomediastinum. Cone-down view of a patient with spontaneous pneumomediastinum shows vertically oriented lucencies (arrowheads) outlining the aorta, esophagus, and heart and extending into the thoracic inlet superiorly.
Air within the soft tissues of the neck from penetrating trauma or laryngeal fracture may lead to pneumomediastinum by extending inferiorly through the retropharyngeal and prevertebral spaces, or along the sheaths of the great vessels. Deep space infections in the neck can spread along the same fascial planes and lead to mediastinitis. The term Ludwig angina describes the substernal chest pain caused by the intramediastinal extension of such infections. Rarely, pneumomediastinum develops as air dissects superiorly from the retroperitoneum through the aortic hiatus or from the peritoneal cavity along the internal mammary vascular sheaths.
The symptoms associated with pneumomediastinum vary with the underlying etiology, extent of mediastinal air, and presence of mediastinitis. Mediastinal air without infection is generally asymptomatic and does not require treatment. In some patients with spontaneous pneumomediastinum, there may be substernal, pleuritic-type chest pain of sudden onset that can be related to a specific inciting incident, such as vomiting or the Valsalva maneuver. Dyspnea may be present. In adults, mediastinal air under pressure usually escapes into the neck, producing crepitus over the neck, supraclavicular regions, and chest wall. Rarely, mediastinal air under pressure may produce a tension pneumomediastinum in which the clinical findings are those of cardiac tamponade. Patients with mediastinitis and pneumomediastinum are usually seriously ill with chest pain, high fevers, dyspnea, and signs of sepsis. The radiographic findings of pneumomediastinum are reviewed in Chapter 12.
THE HILA
Hilar abnormalities are first appreciated on conventional posteroanterior and lateral chest radiographs. CT and MR are used to confirm and characterize hilar masses or to detect subradiographic involvement of the hila; the latter most often in patients with bronchogenic carcinoma.
Unilateral Hilar Enlargement
Malignancy (Table 13.9). A hilar mass usually represents bronchogenic carcinoma or confluent lymph node metastases (Fig. 13.21). Unilateral hilar enlargement may be the presenting radiographic feature of squamous cell carcinoma, where the hilar mass represents the central extension of an endobronchial tumor from its origin within a segmental bronchus. Concomitant hilar lymph node involvement may contribute to hilar enlargement in some of these patients. Approximately 20% of patients with squamous cell carcinoma have a hilar mass on chest radiograph. In contrast, patients with adenocarcinoma and large cell carcinoma more commonly present with a peripheral pulmonary nodule or mass. In many patients, the hilar mass may be obscured by adjacent lung collapse or obstructive pneumonitis.
Unilateral hilar enlargement resulting from metastatic lymph node involvement is most often seen in small cell carcinoma. The propensity of this tumor for early invasion of the bronchial submucosa and peribronchial lymphatics accounts for the high incidence of widespread hematogenous and hilar and mediastinal lymph node metastases
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at the time of diagnosis. Plain film evidence of enlarged hilar lymph nodes resulting from metastases from adenocarcinoma of lung or large cell carcinoma are seen in only 10% to 15% of patients. Contrast-enhanced CT or MR is more sensitive for detecting enlarged hilar nodes and should be performed in all patients to guide further staging procedures and for proper preoperative or treatment planning.
TABLE 13.9 Unilateral Hilar Enlargement
Lymph node enlargement  
Malignancy Bronchogenic carcinoma
Lymph node metastases
   Bronchogenic carcinoma
   Head and neck malignancy
      Squamous cell carcinoma of skin, larynx
      Thyroid carcinoma
   Breast carcinoma
   Melanoma
   Genitourinary malignancy
      Renal cell carcinoma
      Testicular neoplasm
   Lymphoma
   Infection Tuberculosis
Histoplasmosis
Coccidioidomycosis
Pneumonic plague
Tularemia
Anaerobic lung abscess
Measles
Mononucleosis
Pulmonary artery enlargement Valvular pulmonic stenosis
Pulmonary artery aneurysm
   Infection
      Tuberculosis (Rasmussen aneurysm)
   Left-to-right shunts
      Patent ductus arteriosis
      Atrial and ventricular septal defects
   Arteritis (see below)
Tetralogy of Fallot
Central pulmonary embolus
Chronic thromboembolic disease
Pulmonary arteritis
   Behc¸et disease
   Hughes-Stovin syndrome
   Takayasu arteritis
Cyst Bronchogenic cyst
Metastases to hilar and mediastinal lymph nodes from extrathoracic malignancies are uncommon, occurring in approximately 2% of patients. The malignancies that are most often associated with intrathoracic nodal metastases are genitourinary (renal and testicular); head and neck (skin, larynx, and thyroid); breast; and melanoma (7). In renal cell carcinoma and seminoma, lymphatic spread of tumor to retroperitoneal nodes and up the thoracic duct to the posterior mediastinum is the mode of spread to thoracic nodes. Although there is no direct communication between the thoracic duct and anterior mediastinal lymph nodes, reflux of tumor emboli through incompetent valves may allow tumor spread to hilar, paratracheal, and intraparenchymal lymphatics. Head and neck tumors reach the mediastinum via lymphatic spread from cervical lymph nodes. Intrathoracic nodal metastases from breast carcinoma are often seen late in the course of disease, often years after the initial diagnosis. Malignant melanoma is the extrathoracic neoplasm with the highest incidence of intrathoracic nodal metastases; patients with nodal disease will almost invariably have radiographic evidence of parenchymal metastases.
Although 75% of patients presenting with Hodgkin lymphoma have evidence of intrathoracic lymph node enlargement, isolated unilateral hilar lymph node enlargement is uncommon. The thoracic manifestations in NHL differ in primary pulmonary lymphoma, versus lymphoma that primarily involves extrathoracic sites with secondary pulmonary involvement. Thoracic involvement in primary pulmonary lymphoma is largely limited to parenchymal and pleural disease, whereas secondary pulmonary lymphoma generally manifests as intrathoracic lymph node enlargement, with 35% showing hilar or middle mediastinal lymph node enlargement and some presenting as an isolated finding.
Infection
Unilateral hilar or mediastinal lymph node enlargement is a characteristic feature in primary pulmonary tuberculosis in distinction to postprimary tuberculosis; an exception is the severely immunocompromised patient with AIDS. Isolated lymph node enlargement as a manifestation of primary tuberculosis is more common in children than in adults. There is almost always concomitant parenchymal disease in immunocompetent patients with lymph node enlargement. Fungal infections such as histoplasmosis and coccidioidomycosis may present with hilar lymph node enlargement, typically associated with patchy or lobar airspace consolidation in the ipsilateral lung. A variety of bacterial infections have been associated with unilateral hilar lymph node enlargement, including plague, tularemia, and anaerobic lung abscess. A characteristic finding in patients with pneumonic plague is the detection on unenhanced CT of increased attenuation within hilar and mediastinal nodes that drain regions of parenchymal involvement owing to intranodal hemorrhage. Tularemia (Francisella tularensis) causes parenchymal consolidation in association with hilar lymph node enlargement and pleural effusion.
The viral infections most commonly associated with hilar lymph node enlargement are infectious mononucleosis and measles pneumonia. The thorax is infrequently involved in mononucleosis, but hilar lymph node
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enlargement is the most common manifestation of intrathoracic disease. Lymph node enlargement may accompany the reticular interstitial opacities of typical measles pneumonia, or it may be associated with nodular, segmental, or lobar opacities and pleural effusion in atypical measles pneumonia.
FIGURE 13.21. Hilar Nodal Metastases From Melanoma. A. Posteroanterior radiograph in a patient with melanoma shows left hilar enlargement (arrow). B. Enhanced CT scan shows enlarged left hilar lymph nodes (arrows) from metastatic disease.
FIGURE 13.22. Unilateral Hilar Enlargement From Idiopathic Dilatation of the Pulmonary Artery. A. Scout view from chest CT shows abnormal convexity in the region of the main pulmonary artery (arrow). Note thoracic scoliosis. B. Enhanced CT scan shows dilated main pulmonary artery with normal right and left pulmonary arteries. Physical examination and echocardiogram showed no evidence of pulmonic valve disease.
Pulmonary Artery Enlargement
Although unilateral hilar enlargement is most often the result of a mass or enlarged lymph nodes, abnormal enlargement of the right or left pulmonary artery may cause hilar prominence (Fig. 13.22). Vascular disorders that produce unilateral pulmonary artery enlargement include poststenotic
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dilatation from valvular or postvalvular pulmonic stenosis, pulmonary artery aneurysms, and distension of the pulmonary artery by thrombus or tumor. Patients with congenital valvular pulmonic stenosis may develop poststenotic dilatation or aneurysms of the main and left pulmonary arteries from the jet effect of blood upon these vessels. Rarely, stenoses resulting from pulmonary artery vasculitis, congenital rubella, or Williams syndrome may lead to poststenotic dilatation of a pulmonary artery. Aneurysms of the central pulmonary arteries are usually associated with congenital heart disease, such as pulmonic stenosis and left-to-right shunts from ventricular septal defect and patent ductus arteriosis. Rare vasculitides such as Behçet disease and Hughes-Stovins syndrome may present with pulmonary artery aneurysms. A large pulmonary embolus lodging in the proximal portion of a pulmonary artery may cause proximal dilatation. Obviously, these patients are symptomatic and will show characteristic findings on perfusion lung scan, helical CT, and pulmonary arteriography.
Bronchogenic cyst is an uncommon cause of a hilar mass. CT and MR will show a round, smooth, thin-walled cyst, usually found in an asymptomatic young adult. Because the hilum is an unusual location for a bronchogenic cyst, and distinction from a necrotic tumor or lymph node mass cannot be made radiographically, these lesions should be biopsied or removed.
Bilateral Hilar Enlargement
Bilateral hilar enlargement is the result of enlargement of either the hilar lymph nodes or the central pulmonary arteries (Table 13.10).
Malignancy
The malignancies producing bilateral hilar lymph node enlargement are similar to those producing unilateral enlargement. In distinction to unilateral nodal enlargement, metastases are uncommon causes of bilateral hilar nodal enlargement. The most frequent solid tumors producing bilateral hilar disease are small cell carcinoma of the lung and malignant melanoma.
TABLE 13.10 Bilateral Hilar Enlargement
Lymph node enlargement Malignancy (see Table 13.2)
Infection (see Table 13.2)
Inflammatory disease
   Sarcoidosis
   Berylliosis
   Angioimmunoblastic lymphadenopathy
Inhalational disease
   Silicosis
Pulmonary artery enlargement Pulmonary arterial hypertension
Left-to-right intracardiac shunt
High output state
   Anemia
   Thyrotoxicosis
Cystic fibrosis
Bilateral hilar lymph node involvement by lymphoma is more common in Hodgkin disease than NHL. Hilar involvement is virtually never seen without concomitant anterior mediastinal nodal enlargement in Hodgkin disease, whereas NHL may produce isolated hilar disease.
The most common chest radiographic manifestation of leukemic involvement of the thorax is hilar and mediastinal lymph node enlargement; it is seen in up to 25% of patients. Lymph node enlargement is much more common in the lymphocytic than the myelogenous form, particularly in chronic lymphocytic leukemia.
Infection
Mediastinal and hilar lymph node enlargement from infection is most often seen in tuberculous and fungal infection with histoplasmosis and coccidioidomycosis. In these diseases, the lymph node enlargement may be unilateral or bilateral. With bilateral disease, the enlargement is asymmetric in distinction to sarcoidosis, which is typically symmetric. Bacterial infection from Bacillus anthracis (anthrax) and Yersinia pestis (plague) may produce bilateral hilar enlargement. In anthrax infection, the lymph node enlargement is often associated with patchy airspace opacities in the lower lobes. The bubonic form of plague may produce marked hilar and mediastinal adenopathy without pneumonia. Recurrent bacterial infection complicating cystic fibrosis is often associated with bilateral hilar lymph node enlargement, and distinction from pulmonary artery enlargement owing to pulmonary hypertension may be difficult.
Sarcoidosis is associated with bilateral hilar lymph node enlargement in 80% of patients. Most of these patients have concomitant paratracheal lymph node enlargement, and nearly half have concomitant radiographic parenchymal disease. The pattern of lymph node involvement in sarcoidosis has been termed the 1-2-3 sign, with 1 = right paratracheal, 2 = right hilar, and 3 = left hilar lymph node enlargement (Fig. 13.9; see Fig. 17.25). The enlarged nodes produce symmetric, lobulated hilar masses on plain film, since the enlarged nodes remain separate. In 20% of patients, the involved lymph nodes will calcify; usually the calcifications are punctate in appearance, but occasionally peripheral “eggshell” calcification is seen. In some patients, the involved nodes can be seen to enhance after contrast administration on CT. In the majority of patients, the enlarged nodes resolve within 2 years of discovery; in a small percentage, the nodes remain enlarged for many years.
Berylliosis and Silicosis
The hilar and mediastinal lymph node enlargement of chronic berylliosis is radiographically indistinguishable from that of sarcoidosis. Similarly, silicosis can produce hilar and mediastinal lymph node enlargement; eggshell calcification of hilar nodes is highly suggestive of this entity, although
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peripheral nodal calcification may also be seen with sarcoidosis, histoplasmosis, or amyloidosis.
TABLE 13.11 Small Hilum (Hila)
Unilateral Absence or hypoplasia of the pulmonary artery
Hypoplastic or hypogenetic lung
Swyer-James syndrome
Lobar atelectasis
Lobar resection
Compression/invasion of the pulmonary artery
   Cyst
   Neoplasm
   Fibrosing mediastinitis
Bilateral Emphysema
Obstruction to pulmonary flow
   Fibrosing mediastinitis
   Tetralogy of Fallot
   Valvular pulmonic stenosis
   Ebstein anomaly
Bilateral pulmonary artery enlargement is seen with increased flow or increased resistance in the pulmonary circulation. The conditions associated with bilateral pulmonary arterial enlargement are reviewed in Chapter 14.
Small Hila
Bilaterally small hila (Table 13.11) can be seen in some adults with severe pulmonary overinflation from emphysema or in those with diminished pulmonary blood flow due to congenital pulmonary outflow obstruction (tetralogy of Fallot, Ebstein anomaly).
The most common causes of a small hilum are atelectasis and resection of a portion of lung, which leave a small residual hilar artery supplying the remaining lobe or lobes. Hypoplasia of the pulmonary artery, often with associated abnormalities of the ipsilateral lung (hypogenetic lung syndrome, Swyer-James syndrome), is another cause of a small hilum. Less commonly, invasion of the proximal pulmonary artery by mediastinal tumor, or obstruction of the pulmonary artery on account of fibrosing mediastinitis, can produce a diminutive hilar shadow. In any patient in whom a small hilum is a new radiographic finding, a CT scan should be performed to assess the mediastinum for central obstructing lesions. The left hilum can appear small in patients in whom the hilar shadow is obscured by the upper left heart margin or by fat in the region of the aortopulmonic interface. In these cases, the lateral radiograph will usually show a left pulmonary artery of normal size.
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