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
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
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
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,
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
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.
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
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
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 neuroendocrine tumors
|Germ cell neoplasms
||Teratoma (benign or malignant)
Embryonal cell carcinoma
Endodermal sinus tumor
|Ectopic parathyroid mass
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.
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.
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
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
); 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.
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.
—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
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.
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
. 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.
, 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
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
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
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
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.
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.
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
). 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
Head and neck tumors (squamous cell carcinoma of skin, larynx; thyroid carcinoma)
Genitourinary tumors (renal cell carcinoma, seminoma)
Anaerobic lung abscess
|Foregut and mesothelial cysts
|Tracheal and central bronchial neoplasms
Carcinoid tumor (bronchi)
Adenoid cystic carcinoma (trachea)
Squamous cell carcinoma
||Foramen of Morgagni hernia
Double arch/right arch
Tortuous innominate/subclavian artery
Aneurysm of the aortic arch
Dilated azygos vein
Dilated hemiazygos vein
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
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
| Peripheral (eggshell)
||Carcinoid tumor/small cell carcinoma
Renal cell carcinoma
Squamous cell carcinoma
A standardized classification system for hilar and mediastinal lymph nodes has recently been advanced by the American Thoracic Society (Fig. 13.8
). 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
—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
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
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
(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
). 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.
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
adjacent to the heart; MR
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
, which may present as pericardiac masses, are discussed in Chapter 19
Congenital or acquired anomalies of the heart and great vessels are common middle mediastinal masses and are discussed in Chapter 14
Rarely, a neurofibroma arising from the phrenic nerve may present as a middle mediastinal juxtacardiac mass.
Posterior Mediastinal Masses
). 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
||Peripheral (intercostal) nerves
||Duplication (enteric) cyst
Squamous cell carcinoma
Metastases (bronchogenic, breast, renal cell carcinoma)
Degenerative disease (osteophytosis)
|Lateral thoracic meningocele
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
). These findings may help distinguish these lesions from neurofibromas, which
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.
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
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 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
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
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
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.
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
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.