Residents’ Section • Pat tern of the Month Shahrzad et al. Anterior Mediastinal Masses
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Residents’ Section Pattern of the Month
Anterior Mediastinal Masses Maryam Shahrzad1 Thi Som Mai Le Mario Silva Alexander A. Bankier Ronald L. Eisenberg Shahrzad M, Mai Le TS, Silva M, Bankier AA, Eisenberg RL
Keywords: anterior mediastinum, mediastinal masses DOI:10.2214/AJR.13.11998 Received September 17, 2013; accepted after revision March 9, 2014. A. A. Bankier is a consultant for Spiration (Olympus Medical Systems) and has received authorship honoraria from Elsevier. 1
All authors: Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 330 Brookline Ave, Boston, MA 02215. Address correspondence to R. L. Eisenberg ([email protected]).
WEB This is a web exclusive article. AJR 2014; 203:W128–W138 0361–803X/14/2032–W128 © American Roentgen Ray Society
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he anterior mediastinum is demarcated by the sternum anteriorly and by the brachiocephalic vessels, pericardium, and ascending aorta posteriorly. Its superior and inferior borders are the thoracic inlet and the diaphragm, respectively. The normal contents of the anterior mediastinum include the thymus, lymph nodes, adipose tissue, nerves, vessels, and sometimes downward extension of the thyroid from the neck. Anterior mediastinal masses generally arise from these structures. Clinically, an anterior mediastinal mass may be silent and incidentally discovered on imaging. There may be compression or local invasion of nerves (causing pain, paralysis of the diaphragm or vocal cords, or arrhythmias), vascular structures (superior vena cava syndrome), airways (dyspnea, cough), esophagus (dysphagia), and bone. In addition, some tumors can produce systemic symptoms related to excess hormone release, cytokines, or antibodies. Chest radiography can show an anterior mediastinal mass directly or indicate the presence of the mass indirectly by its compression or displacement of adjacent structures. However, radiography is of limited value for characterizing the lesion. CT is the most important imaging modality for showing the precise location, morphology, and pattern of contrast enhancement of an anterior mediastinal mass as well as its relationship to other mediastinal components or borders. A practical way of distinguishing among mediastinal masses is categorizing them according to their predominant CT attenuation values, which are primarily composed of fat, water, soft tissue, calcium, or vascular structures (Table 1). The imaging findings combined with clinical examination and laboratory studies can enable the radiologist to provide a reasonably narrow differential diagnosis and help guide diagnostic interventions.
Fat Attenuation Lipoma A lipoma is a mesenchymal tumor that originates from adipose tissue and resembles normal fat. Typically encapsulated, this benign tumor is composed of mature adipocytes, and malignant degeneration is extremely rare. Lipomas are mostly found in subcutaneous tissues and represent only about 2% of primary mediastinal tumors. They occur more often in the anterior mediastinum adjacent to the diaphragm, although they occasionally are found in other mediastinal compartments. As slow-growing tumors, lipomas typically remain asymptomatic until they cause sufficient compression of adjacent structures. Symptomatic tumors can be removed. On radiography, a lipoma in the anterior mediastinum is generally indistinguishable from other masses in this location. On CT, however, a lipoma has characteristic homogeneous fat attenuation (usually −50 to −100 HU), no contrast enhancement, and well-defined margins. If the diagnosis is equivocal, MRI may confirm the fatty nature of the tumor, which produces high T1 and low T2 signal intensity on fat-suppressed images. An extremely rare disorder that can mimic a lipoma is mesenteric fat necrosis. This self-limited condition is characterized by the triad of acute pleuritic chest pain, CT showing a fatty lesion in the mediastinum with intrinsic and surrounding soft-tissue stranding, and thickening of the adjacent pericardium. Liposarcoma Liposarcoma is a mesenchymal malignant tumor with fatty differentiation. Commonly encountered in the extremities and retroperitoneum, mediastinal involvement is exceedingly rare. Three histologic subtypes have been described: differentiated myxoid or round cell, dedifferentiated, and pleomorphic. The subtype and grade of the lesion are important
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TABLE 1: Categorization of Anterior Mediastinal Masses According to Predominant CT Attenuation Values Predominant Density Fat
Abnormality Lipoma Liposarcoma Mediastinal lipomatosis Hernia Thymolipoma Teratoma
Water
Thymic cyst Lymphangioma Abscess Others: pleuropericardial cyst, foregut duplication cyst, cystic teratoma, cystic degeneration of malignancy, pancreatic pseudocyst
Soft tissue
Thymic hyperplasia or neoplasm Thyroid goiter Ectopic parathyroid adenoma Germ cell tumor Mediastinitis Acute Fibrosing Lymphadenopathy Lymphoma Metastasis Others: hernia, Castleman disease, sarcoma
Calcification
Malignancy Teratoma Metastasis from serous or mucinous tumors Metastasis from bone tumor Treated lymphoma Goiter Granulomatous infection Sarcoidosis Silicosis
Vascular
Aortic aneurysm
prognostic features. Typically found in adults 50–60 years old, a large liposarcoma can produce both localized symptoms related to compression of adjacent structures and such systemic symptoms as fatigue and weight loss. Treatment options include surgery, chemotherapy, and radiotherapy, but the tumor has a high rate of recurrence. On CT, the appearance and aggressiveness of the tumor vary depending on its degree of differentiation, ranging from a fatty lesion that is difficult to distinguish from a benign lipoma to a pure soft-tissue tumor. In general, a mediastinal liposarcoma appears
as an inhomogeneously enhancing mass with variable amounts of fat and soft-tissue density. As with the imaging of benign lipomas, MRI is helpful to detect the fatty component and contrast enhancement of a liposarcoma. Mediastinal Lipomatosis Lipomatosis refers to excessive deposition of unencapsulated fat in the mediastinum, most often associated with exogenous steroid intake and obesity. Fatty deposition can also occur in patients with Cushing syndrome, adrenocorticotropic hormone–producing tumors, and in rare genetic or familial disorders.
At histology, there is an excessive accumulation of normal fat. Mediastinal lipomatosis usually is asymptomatic or has an indolent course but infrequently may produce symptoms by compressing adjacent structures. Therapy is generally focused on treating the underlying cause, although surgical decompression has been reported in severe cases. Radiography shows lipomatosis as smooth widening of the mediastinum, which may be accompanied by prominent epicardial fat pads and symmetric thickening of the extrapleural fat along the lateral chest wall. Although not a neoplastic process, mediastinal fat deposition localized to the superior portion of the anterior compartment may simulate a mass. CT shows abundant diffuse homogeneous fat-attenuation material that insinuates around anatomic structures without invasion or compression (Fig. 1). The presence of a soft-tissue component, septa, capsule, and contrast enhancement should suggest an underlying tumor. Omental Hernia Herniation of omental fat into the anterior chest can be due to a congenital Morgagni hernia or acquired postoperative or posttraumatic diaphragmatic defects. Usually an incidental finding, symptoms may develop due to incarceration or infarction of the herniated fat. Although infrequent, compressive respiratory or gastrointestinal symptoms have been described. Because of the risk of intestinal strangulation, surgical repair is the treatment of choice if the hernia contains bowel. Radiography shows a Morgagni hernia as a well-defined area of increased density along the heart border, usually at the right cardiophrenic angle. CT and MRI show fat attenuation or signal intensity within the lesion. Accompanying linear opacities correspond to omental vessels (Fig. 2). Coronal and sagittal reconstructions are useful to show the underlying diaphragmatic defect, which is a key finding in differentiating herniated omental fat from a lipoma or liposarcoma. Thymolipoma Representing up to 10% of thymic neoplasms, thymolipoma is a rare benign encapsulated tumor that is usually found in young adults and has no sex predilection. Histologically, it is composed of mature adipose and thymic tissue in variable proportion. Often discovered incidentally, a thymolipoma may displace adjacent structures and produce chest pain, arrhythmia, cough, or dyspnea. Thymolipomas have been associated with
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Shahrzad et al. myasthenia gravis, aplastic anemia, RBC aplasia, hypogammaglobulinemia, Hodgkin lymphoma, and thyrotoxicosis or Graves disease. Surgical removal of the lesion has been reported to alleviate symptoms in some cases of myasthenia gravis. Radiography reveals a usually large, soft, and pliable anterior mediastinal mass that often conforms to the borders of the heart and mimics cardiomegaly or diaphragmatic elevation. On CT, fat constitutes 50–85% of this well-defined, encapsulated mass. Nonenhancing solid linear densities, representing thymic tissue, are interwoven within the fat. At times, a thymolipoma may not clearly lie within the thymus. In such cases, tracing a connection between the mass and the thymus in coronal or sagittal images permits the diagnosis of its thymic origin. On MRI, the fat component of a thymolipoma has high signal intensity on both T1- and T2weighted sequences and has low signal intensity on fat-suppressed imaging. This appearance differs from thymic tissue, which has intermediate signal intensity similar to skeletal muscle on all sequences. Teratoma Germ cell tumors are a heterogeneous group of lesions containing tissues originating from primitive germ cell layers: ectoderm, mesoderm, and endoderm. In adults, these tumors occur most commonly in the gonads, with only 3% arising in a mediastinal location. Teratoma is the most common germ cell tumor (70%), contains tissues from all three layers, and occurs in young adults. It most often contains fat, which derives from the mesoderm. Teratomas are classified into three subtypes: mature, immature, and those with additional malignant components. Mature teratomas contain well-differentiated tissue and represent the vast majority of cases. They are almost always benign but still have a low malignant potential. Immature teratomas contain various amounts of undifferentiated neuroectodermal or mesenchymal tissues. Often benign, the malignant potential is proportional to the degree of immature components. Teratomas with additional malignant components are rare lesions in which the teratoma is accompanied by foci of carcinoma, sarcoma, or component of other germ cell tumors. Patients with teratoma may be asymptomatic or present with chest pain, dyspnea, fever, or cough. After complete surgical excision, mature and immature teratomas have an excellent prognosis. Aggressive teratomas with malignant components, however, do not respond well to chemotherapy. In general,
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a substantial number of teratomas in women are benign, whereas a high proportion of these tumors in men are malignant. On radiography, a teratoma appears as a soft-tissue mediastinal mass that may contain calcification (20%), radiolucency suggestive of fat (5%), and a fat-fluid level (2%). On CT, mature and immature teratomas most often appear as well-defined lobulated cystic masses (90%). Fatty foci, mainly sebaceous fat, are encountered in more than 75% of the cases, with characteristic fat-fluid levels. Up to 50% of teratomas contain rimlike or toothlike calcification (Fig. 3). Differentiating between mature and immature teratomas on imaging studies is difficult. Small solid portions of the lesion often represent the immature element. Visualization of a large predominantly solid necrotic, hemorrhagic, and enhancing lesion with elevated α-fetoprotein should raise concern for malignant teratoma. Teratomas can rupture or fistulize to adjacent structures. A ruptured teratoma may present with more inhomogeneity of its internal elements as well as adjacent atelectasis, consolidation, and pleural or pericardial effusion. The macroscopic fatty component of a teratoma can be seen on MRI as high signal intensity on T1- and T2-weighted sequences and low signal intensity on fat-suppressed imaging. Furthermore, chemical-shift artifact can delineate lipid-rich cellular content on in-phase and opposed-phase sequences. Water Attenuation Thymic Cyst Congenital or acquired thymic cysts represent only 1–3% of all mediastinal masses. Derived from a patent thymopharyngeal duct, congenital thymic cysts are usually simple, unilocular, and thin-walled lesions containing clear fluid and without pathologic evidence of inflammation. Acquired thymic cysts have been associated with myasthenia gravis, systemic lupus erythematosus, Sjögren syndrome, aplastic anemia, radiation therapy, Hodgkin disease, thoracotomy, AIDS in children, and other thymic tumors. In contrast to congenital lesions, acquired thymic cysts are more often multilocular with variable wall thickness and typically have pathologic evidence of inflammation. It is important to differentiate between congenital and acquired thymic cysts because acquired cysts tend to recur after resection and may be associated with thymic malignancy. Both types may be asymptomatic and discovered incidentally or present with symptoms of compression of adjacent structures.
On radiography, a thymic cyst is indistinguishable from other mediastinal lesions. Therefore, CT or MRI is necessary to make the diagnosis. CT shows a congenital thymic cyst as a well-defined water-attenuation lesion without a perceptible wall (Fig. 4A). In contrast, an acquired unilocular or multilocular cyst is a heterogeneous lesion with a clearly evident border of variable thickness. If complicated by hemorrhage or infection, an acquired thymic cyst may resemble a solid mass and show higher CT attenuation and even calcification within its wall. In this situation, the finding of high signal intensity on both T1- and T2-weighted MR images can differentiate a complicated cyst from a typical cyst containing fluid, which has low T1 signal intensity (Fig. 4B) and uniform high T2 signal intensity (Fig. 4C). Lymphangioma Lymphangioma is a rare congenital malformation that contains a variable amount of lymphatic and vascular proliferation and is caused by embryologic failure of the lymphatic system to adequately drain into the venous system. It is classified according to the size of the lymphatic channels as simple-capillary (small lymphatic vessels), cavernous (dilated lymphatic channels), or cystic (cystic spaces with lymphatic fluid, also known as cystic hygroma). Lymphangiomas are mainly encountered in children under 2 years old. They often involve the cervical and axillary region, sometimes with intrathoracic extension. Fewer than 1% of cases are isolated to the thorax, with the anterior mediastinum the most frequently involved compartment. Large mediastinal lymphangiomas are symptomatic because of compression of adjacent structures. Infection, hemorrhage, rupture with chylothorax, and chylopericardium are potential complications. Total excision is the treatment of choice, but this may be impossible with large and infiltrative lesions. Other reported treatments include sclerotherapy, marsupialization, and radiotherapy. On radiography, lymphangioma appears as a well-defined mass that occasionally is associated with chylous pleural effusion. CT shows a smooth homogeneous lobulated lesion of water attenuation containing cysts of variable size. Calcification and contrast enhancement are uncommon. The soft plastic mass of a lymphangioma may envelop or mold to adjacent structures. If CT is equivocal, MRI is useful to confirm the high T2 signal intensity characteristic of this cystic mass and show internal septations. The signal intensity on T1-weighted images is variable de-
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pending on the amount of proteinaceous and hemorrhagic contents of the lesion. Abscess Although not commonly encountered in the anterior mediastinum, abscesses may result from an infected surgical bed, perforated esophagus, or contiguous spread from another infection site (neck, abdomen, adjacent bone, or joint). An abscess should be suspected when the collection contains air and has a thick rimenhancing wall. This will be discussed in more detail in the Mediastinitis section. Others Other entities that may appear as waterdensity lesions include cystic degeneration of tumor and lymph nodes, pancreatic pseudocyst, and cystic teratoma. Other fluid-containing lesions, such as pleuropericardial cyst (Fig. 5), bronchogenic cyst, and duplication cyst, are more frequently encountered in other mediastinal compartments and thus are not discussed in this article. Soft-Tissue Attenuation Thymic Disorders The thymus is an anterior mediastinal organ that extends over the heart and great vessels. Weighing 10–15 g at birth and enlarging up to 40 g at puberty, the thymus involutes later in life. It plays an essential role in the cellular immunity and serves as a maturation site for lymphocytes, especially T-lymphocytes. Differentiating between the normal and pathologic thymus is essential to avoid unnecessary biopsy or surgery, but doing so may be difficult with imaging. On radiography, the normal thymus may be quite large during childhood but should not be visible later in life. On CT, the normal thymus may have various configurations and shapes: bilobed, inverted V or triangular, and quadrilateral. The borders of the gland are usually straight or concave, and the maximum thickness perpendicular to the long axes should not be greater than 13 mm in adults. As the organ involutes over time, it is often completely replaced by fat. On MRI, the normal thymus in young patients has a signal intensity similar to muscle on both T1- and T2-weighted images. As the gland atrophies with age, the signal intensity becomes higher on both sequences due to fatty infiltration. Chemical shift artifact with in- and out-of-phase sequences is also useful for showing the fat content of the gland. Thymic disorders, ranging from benign to malignant, can present as anterior mediastinal
masses. Although chest radiography usually shows the lesion, CT is necessary to characterize it. Thymic Hyperplasia Thymic hyperplasia can be divided into two histologic subtypes: true hyperplasia and lymphofollicular hyperplasia. True thymic hyperplasia is an increase in size and weight of the gland with normal histology. This condition is commonly due to a rebound from recent stress, such as pneumonia, corticosteroid therapy, radiation therapy, chemotherapy, surgery, or burns (Fig. 6). Lymphofollicular hyperplasia refers to the presence of a hyperplastic lymphoid germinal center in the medulla of the gland (Fig. 7). Occurring in more than 50% of patients with myasthenia gravis, lymphofollicular hyperplasia is also associated with other autoimmune disorders, such as thyrotoxicosis, systemic lupus erythematosus, polyarteritis nodosa, and Addison disease. On CT, thymic hyperplasia usually appears as symmetric diffuse enlargement of the gland. Asymmetric enlargement raises the possibility of a thymoma. Thymic Neoplasm Tumors involving the thymus are classified as epithelial (thymoma, thymus carcinoma), lymphoma, thymolipoma, carcinoid, germ cell tumors, sarcomas, and metastasis. Thymomas represent 20% of anterior mediastinal neoplasms in adults. It can be challenging to distinguish among thymic hyperplasia, normal thymus, and malignancy, especially in patients who have undergone chemotherapy and have possible tumor recurrence. Moreover, there are variations of thymic appearance with age and sex. A hyperplastic thymus can retain its normal shape but usually loses its bilobed appearance. There is diffuse homogeneous enlargement of the gland, which contains areas of fat and lymphoid attenuation with normal vessels and smooth contour. On CT, the presence of a convex round thymus with an irregular margin and heterogeneous content associated with necrosis, cystic changes, hemorrhage, or calcification should raise the possibility of malignancy (Fig. 8A). Local invasion, distant metastasis, and pleural or pericardial dissemination (drop metastasis) are also useful signs (Fig. 8B). MRI chemical-shift imaging detects fat within the thymus, which occurs in thymic hyperplasia but not in thymic malignancy. As with other lesions of the anterior mediastinum, especially if there is evidence of invasion, further man-
agement should be undertaken after discussion with medical and surgical colleagues. Thyroid Goiter Intrathoracic, substernal goiter accounts for 3–6% of mediastinal masses, with a majority located in the anterior compartment. The most common type is secondary intrathoracic goiter with direct downward extension from the neck. Fewer than 1% of cases are primary, in which the ectopic goiter has a blood supply that arises from mediastinal vessels with no connection to the neck. Most cases of thyroid goiter are asymptomatic, although they can produce life-threatening symptoms because of airway or neurovascular obstruction. Carcinoma has been reported in 5–17% of cases of substernal goiter, and surgery is the treatment of choice. On radiography, an intrathoracic goiter presents as a homogeneous smooth sharply marginated mass in a retrosternal, paratracheal, or retrotracheal location. The trachea and esophagus may be displaced. On unenhanced CT, the goiter has hyperattenuation because of the high iodine content of the mass, and focal calcifications often occur. After contrast injection, there is early and prolonged enhancement. Visualization of a connection between the mass and the cervical thyroid gland is the key feature for differentiating a substernal goiter from other mediastinal masses (Fig. 9). Ectopic Parathyroid Adenoma The four parathyroid glands are normally found at the level of the thyroid gland, but one may occur in an ectopic position as the result of aberrant migration during early stages of development. Fewer than 2% of ectopic glands are located in the anterior mediastinum. The ectopic gland may cause symptoms of hyperparathyroidism or be the site of parathyroid hyperplasia; a solitary adenoma; or, infrequently, a malignant neoplasm. Thoracoscopic or open resection is indicated for a symptomatic ectopic parathyroid gland in the anterior mediastinum. In a patient with primary hyperparathyroidism, ultrasound or 99mTc sestamibi (Fig. 10) examination of the neck is generally the first imaging modality. If this proves inconclusive, CT or MRI can be performed to detect an ectopic parathyroid tumor in the mediastinum. On CT, an ectopic parathyroid adenoma appears as a 1- to 3-cm oval or round well-defined mass of soft-tissue attenuation that occasionally can contain cystic de-
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Shahrzad et al. generation or hemorrhage. Although sometimes mimicking an enlarged lymph node, an ectopic parathyroid tumor typically shows more contrast enhancement, especially during the arterial phase. On MRI, ectopic parathyroid adenomas are iso- to hypointense on T1weighted imaging and iso- to hyperintense on T2-weighted imaging compared with the thyroid gland and enhance avidly. Germ Cell Tumors Germ cell tumors are a heterogeneous group of lesions originating from primitive germ cell layers (ectoderm, mesoderm, and endoderm) and normally affect the gonads. However, aberrant migration of primordial germ cells may permit these tumors to develop in an extragonadal location, most commonly the anterior mediastinum (15% of mediastinal masses in adults, 24% in children). Germ cell tumors are divided into two main categories: seminomatous and nonseminomatous (teratoma, yolk sac tumor, choriocarcinoma, and embryonal carcinoma). Symptoms reflect mass effect on adjacent organs; secretions of β-HCG and α-fetoprotein are used as tumor markers. The multiple histologic subtypes of germ cell tumors produce various appearances on CT. Teratoma, the most frequent type, has been discussed earlier. Seminomas generally present as large, lobulated, mildly enhancing, homogeneous, noncalcified soft-tissue masses. Conversely, nonseminomatous germ cell tumors tend to be more heterogeneous, with ill-defined margins, and may contain areas of necrosis and hemorrhage (Fig. 11). Mediastinitis Acute—Acute mediastinitis is most commonly an emergency condition caused by esophageal rupture or a complication of surgery, notably after sternotomy. At times, it may be caused by direct extension of infection from adjacent osteomyelitis or from the neck. Acute mediastinitis manifests with infectious symptoms, such as fever, chills, tachycardia, and sepsis. There may also be retrosternal pain with upward radiation to the neck or subcutaneous emphysema. Acute mediastinitis requires immediate medical or surgical intervention, with the precise treatment depending on the severity and clinical symptoms. On radiography, acute mediastinitis can produce mediastinal widening, diffuse or focal gas bubbles, pleural effusion, mediastinal air-fluid levels, or even a soft-tissue mass. CT
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findings suggesting mediastinitis include fat stranding, lymphadenopathy, abscess, and empyema. In addition to defining the extension of the process, CT is of value for determining the underlying cause. A diagnosis of esophageal perforation should be suggested if there is esophageal wall thickening and the presence of a fluid collection, extraluminal air, extravasation of contrast material into the pleural space or mediastinum, pneumomediastinum, or pneumothorax. Mediastinitis developing acutely after sternotomy can cause sternal dehiscence and misaligned sternum wires. CT also is the modality of choice for showing direct extension of infection from osteomyelitis and infections of the head and neck. Chronic—Chronic mediastinitis is a rare benign disease characterized by fibrous proliferation. Also known as fibrosing mediastinitis, it most often affects the middle mediastinum. Among the many reported causes are infection (histoplasmosis, other fungal disease, and tuberculosis), systemic autoimmune disorders, lymphoma, radiation therapy, trauma, and drugs (methylsergide). A familial form is associated with retroperitoneal fibrosis, sclerosing cholangitis, Reidel thyroiditis, and orbital pseudotumor. Fibrosing mediastinitis produces symptoms because of obstruction of vital structures (airways, esophagus, and vessels). On radiography, fibrosing mediastinitis shows such nonspecific findings as mediastinal widening, hilar and mediastinal lymphadenopathy, and calcification. On CT, the disease produces a soft-tissue mass that infiltrates the mediastinum and obliterates adjacent structures. In the more common focal pattern, which is thought to be linked to histoplasmosis, there are calcified fibrous masses located in the hila, paratracheal, and subcarinal areas. Diffuse disease, which is not associated with histoplasmosis, produces multiple noncalcified infiltrative masses that involve multiple mediastinal compartments. Lymphadenopathy Lymphoma—Primary lymphoma, which accounts for approximately 20% of mediastinal neoplasms in adults and 50% in children, usually occurs in the anterior mediastinum. About 50–70% of mediastinal lymphomas are due to Hodgkin disease, which has no sex predilection and presents with a bimodal age distribution in young adults and in patients older than 50 years, most of whom have constitutional symptoms, including fever, night
sweats, and weight loss. Mediastinal involvement of lymphoma may also present with cough, dyspnea, chest pain, pleural effusion, and superior vena cava syndrome. Non-Hodgkin disease is classified into large B cell lymphoma (typically in young and middle-aged adults) and T cell lymphoblastic lymphoma (generally affecting children and adolescents). On radiography, lymphoma usually produces bulky enlargement of mediastinal and hilar nodes (Fig. 12), although it can present as a solitary mediastinal mass. On CT, Hodgkin lymphoma appears as a homogeneous soft-tissue mass in the anterior mediastinum with surface lobulation and mild-to-moderate contrast enhancement. Occasionally, there may be cystic changes and necrosis (Fig. 13). Both types of non-Hodgkin lymphoma typically appear similar to Hodgkin lymphoma, although they tend to have more vascular involvement and associated mediastinal lymphadenopathy. Reactive lymphadenopathy—Enlargement of lymph nodes reflecting a reaction to infection (including tuberculosis) or autoimmune or inflammatory disease can develop in the anterior mediastinum. Unlike in lymphoma, reactive lymph nodes do not tend to coalesce into a large mass. Others In addition to fat, hernias can have softtissue density when they contain abdominal organs. Other rare disorders presenting this appearance include sarcomas and Castleman disease. Calcific Attenuation Malignancy Treated malignancy, especially lymphoma (20%), can show calcification (Fig. 14). Calcified untreated malignancy includes teratomas (50%), metastasis from serous or mucinous tumors (ovaries, gastrointestinal tract), and metastasis from bone tumor. Goiter Calcification develops in up to 75% of thyroid goiters. Lymphadenopathy in Granulomatous Infection or Sarcoidosis-Silicosis Some infectious or inflammatory processes can cause calcification in mediastinal lymph nodes. This condition most commonly reflects granulomatous infection (tuberculosis, histo-
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Anterior Mediastinal Masses plasmosis). Although typically found elsewhere, characteristic egg-shell calcifications in sarcoidosis and silicosis can sometimes occur in the anterior mediastinum. Vascular Structures An ascending aortic aneurysm (> 5 cm) may mimic an anterior mediastinal mass (Fig. 15). Predisposing factors include atherosclerosis, trauma, vasculitis, collagen-related disorders, and infection. Often an incidental finding in a patient without symptoms, an ascending aortic aneurysm can cause symptoms due to compression or rupture. Radiographic signs suggesting an aortic aneurysm include obliteration and enlarge-
ment of normal aortic contours (Figs. 15A and 15B). Curvilinear calcification of a lesion should raise the possibility of a vascular origin. Rupture of an aneurysm may lead to fluid in the pleural space, enlargement of the cardiac contour due to hemopericardium, and enlargement of the mediastinum due to hematoma. CT can confirm the diagnosis of aortic aneurysm (Figs. 15C and 15D), define its extent, and show signs of rupture or dissection. Suggested Reading 1. Brown LR, Aughenbaugh GL. Masses of the anterior mediastinum: CT and MR imaging. AJR 1991; 157:1171–1180
2. Dalton ML, Connally SR. Median sternotomy. Surg Gynecol Obstet 1993; 176:615–624 3. D’Cruz IA, Feghali N, Gross CM. Echocardiographic manifestations of mediastinal masses compressing or encroaching on the heart. Echocardiography 1994; 11:523–533 4. Devaraj A, Griffin N, Nicholson AG, Padley SP. Computed tomography findings in fibrosing mediastinitis. Clin Radiol 2007; 62:781–786 5. Kim JH, Goo JM, Lee HJ, et al. Cystic tumors in the anterior mediastinum: radiologic-pathological correlation. J Comput Assist Tomogr 2003; 27:714–723 6. Marom EM. Advances in thymoma imaging. J Thorac Imaging 2013; 28:69–80; quiz 81–83 7. Newman E, Shaha AR. Substernal goiter. J Surg Oncol 1995; 60:207–212
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Fig. 1—Mediastinal lipomatosis. A and B, Transverse (A) and coronal (B) reformatted CT images of chest show abundant diffuse homogeneous material bulging from normal mediastinal borders (arrows). Density appears as low as that in subcutaneous fat (asterisk). Fat-attenuation material surrounds anatomic structures without signs of invasion. Beam-hardening and reconstruction artifacts are seen.
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Fig. 2—Morgagni hernia in 68-year-old man with dyspnea. A, Posteroanterior chest radiograph shows large area of increased density in right inferior hemithorax that obscures right cardiophrenic angle (dotted line). B, Transverse CT image of chest shows voluminous hernia containing omentum and small bowel. C, Sagittal reformatted CT image of chest shows voluminous diaphragmatic hernia containing omentum, mesenteric vessels (arrowheads), and small (solid arrow) and large (open arrow) bowel. No diaphragmatic profile can be seen in bottom portion of image.
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Fig. 3—Teratoma. Transverse CT image of chest shows anterior mediastinal mass with heterogeneous density that indicates presence of encapsulated fatty component (solid arrow) associated with focal toothlike calcification (open arrow).
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Fig. 4—Thymic cyst. A, Transverse CT image of chest shows anterior mediastinal mass (arrow). Location and shape are typical of thymic structure, whereas near-water density and smooth margin reflect cystic nature. B and C, Transverse T1-weighted (B) and T2-weighted (C) fat-suppressed MR images of chest show anterior mediastinal mass (arrow). Hypointense signal intensity in T1weighted and hyperintense signal intensity in T2-weighted images confirm water content.
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Fig. 5—Pericardial cyst. A, Posteroanterior chest radiograph shows rounded opacity (arrow) obliterating right cardiophrenic angle. B and C, Transverse (B) and coronal reformatted (C) CT images of chest show mass (arrow) in right cardiophrenic angle. Water density and smooth margin reflect cystic nature, whereas contiguity to surface of pericardium suggests relation between this structure and mass.
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Anterior Mediastinal Masses
Fig. 6—Thymic hyperplasia in 31-year-old woman who presented after six cycles of chemotherapy for lymphoma. Transverse CT image of chest acquired in fully suspended inspiration shows soft-tissue density (arrow) within mediastinal fat. This finding has regular margin and is adjacent to mediastinal vessels without signs of invasion.
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Fig. 7—Lymphofollicular hyperplasia. Transverse CT image of chest at baseline shows persistent area of soft-tissue density in anterior mediastinum (arrow). Finding was stable over time.
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Fig. 8—Thymic malignancy. Different stages of thymoma. A, Transverse CT image of chest shows stage I thymoma. Soft-tissue mass in anterior mediastinum is partly calcified (solid arrow), encapsulated by fat, with regular margins (open arrows). B, Transverse CT image of chest shows advanced-stage thymoma. Heterogeneous density of mediastinal mass reflects presence of coarse calcifications and necrosis (solid arrow). Mediastinal deviation with unilateral pleural thickening (open arrows) and atelectasis (arrowhead) suggest left pleural and parenchymal involvement. Centrally necrotic subcarinal lymphadenopathy (asterisk) is consistent with mediastinal lymph node involvement. Image also shows scarring in left upper lobe, adjacent to left hilum.
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Fig. 9—Thyroid goiter in 57-year-old woman who presented with dysphagia. A, Axial CT image shows enlargement of left and right thyroid lobes (thick arrow) with small calcification on left (thin arrow). B, Axial CT image at lower level shows goiter in left paratracheal location (thick arrow) displacing supraaortic branches to left (thin arrows). C, Coronal CT reconstruction shows complete apical-basilar extension of goiter (thick arrow) and areas of calcification (thin arrow).
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Fig. 10—Parathyroid adenoma. A, Transverse enhanced CT image of chest shows solid lesion (arrows) with homogeneous soft-tissue density similar to lymph node. B, Transverse reconstructed image of 99mTc sestamibi scan shows high-signal-intensity spot in anterior mediastinum (arrow).
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Anterior Mediastinal Masses
Fig. 12—Lymphadenopathy from Hodgkin lymphoma. Transverse CT image of chest shows enlarged paraaortic lymph nodes in anterior mediastinum (lymph node region 6 (solid arrow) and region 4R (open arrow).
Fig. 11—Germ cell tumor in 37-year-old man who presented with chest pain and right upper extremity swelling. Transverse CT image of chest shows anterior mediastinal mass with mainly soft-tissue density and areas of reduced density. Compression and displacement of mediastinal veins are caused by mass (arrow).
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Fig. 13—Lymphoma. A, Transverse enhanced CT image of chest shows bulky mass of heterogeneous density (arrow) in anterior mediastinum. Encasement and compression of mediastinal veins are caused by growing mass. B, Transverse fused FDG PET/CT image of chest shows FDG-avid structure (solid arrow) corresponding to lymphoma. Signal heterogeneity reflecting necrosis of lesion is clearly detectable (open arrow).
Fig. 14—Lymphoma after chemotherapy. Transverse CT image of chest shows lymph nodes with coarse calcifications (arrow).
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Shahrzad et al. Fig. 15—Aortic aneurysm. A and B, Posteroanterior (A) and lateral (B) radiographs of chest show right mediastinal enlargement (arrow, A) and obliteration of retrosternal space. C and D, Transverse (C) and sagittal (D) reformatted images of enhanced CT of chest show severe dilatation of ascending aorta (asterisk). Compression and displacement of superior vena cava (arrow, C) are seen.
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Residents’ Section Pattern of the Month
Anterior Mediastinal Masses Maryam Shahrzad1 Thi Som Mai Le Mario Silva Alexander A. Bankier Ronald L. Eisenberg Shahrzad M, Mai Le TS, Silva M, Bankier AA, Eisenberg RL
Keywords: anterior mediastinum, mediastinal masses DOI:10.2214/AJR.13.11998 Received September 17, 2013; accepted after revision March 9, 2014. A. A. Bankier is a consultant for Spiration (Olympus Medical Systems) and has received authorship honoraria from Elsevier. 1
All authors: Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 330 Brookline Ave, Boston, MA 02215. Address correspondence to R. L. Eisenberg ([email protected]).
WEB This is a web exclusive article. AJR 2014; 203:W128–W138 0361–803X/14/2032–W128 © American Roentgen Ray Society
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he anterior mediastinum is demarcated by the sternum anteriorly and by the brachiocephalic vessels, pericardium, and ascending aorta posteriorly. Its superior and inferior borders are the thoracic inlet and the diaphragm, respectively. The normal contents of the anterior mediastinum include the thymus, lymph nodes, adipose tissue, nerves, vessels, and sometimes downward extension of the thyroid from the neck. Anterior mediastinal masses generally arise from these structures. Clinically, an anterior mediastinal mass may be silent and incidentally discovered on imaging. There may be compression or local invasion of nerves (causing pain, paralysis of the diaphragm or vocal cords, or arrhythmias), vascular structures (superior vena cava syndrome), airways (dyspnea, cough), esophagus (dysphagia), and bone. In addition, some tumors can produce systemic symptoms related to excess hormone release, cytokines, or antibodies. Chest radiography can show an anterior mediastinal mass directly or indicate the presence of the mass indirectly by its compression or displacement of adjacent structures. However, radiography is of limited value for characterizing the lesion. CT is the most important imaging modality for showing the precise location, morphology, and pattern of contrast enhancement of an anterior mediastinal mass as well as its relationship to other mediastinal components or borders. A practical way of distinguishing among mediastinal masses is categorizing them according to their predominant CT attenuation values, which are primarily composed of fat, water, soft tissue, calcium, or vascular structures (Table 1). The imaging findings combined with clinical examination and laboratory studies can enable the radiologist to provide a reasonably narrow differential diagnosis and help guide diagnostic interventions.
Fat Attenuation Lipoma A lipoma is a mesenchymal tumor that originates from adipose tissue and resembles normal fat. Typically encapsulated, this benign tumor is composed of mature adipocytes, and malignant degeneration is extremely rare. Lipomas are mostly found in subcutaneous tissues and represent only about 2% of primary mediastinal tumors. They occur more often in the anterior mediastinum adjacent to the diaphragm, although they occasionally are found in other mediastinal compartments. As slow-growing tumors, lipomas typically remain asymptomatic until they cause sufficient compression of adjacent structures. Symptomatic tumors can be removed. On radiography, a lipoma in the anterior mediastinum is generally indistinguishable from other masses in this location. On CT, however, a lipoma has characteristic homogeneous fat attenuation (usually −50 to −100 HU), no contrast enhancement, and well-defined margins. If the diagnosis is equivocal, MRI may confirm the fatty nature of the tumor, which produces high T1 and low T2 signal intensity on fat-suppressed images. An extremely rare disorder that can mimic a lipoma is mesenteric fat necrosis. This self-limited condition is characterized by the triad of acute pleuritic chest pain, CT showing a fatty lesion in the mediastinum with intrinsic and surrounding soft-tissue stranding, and thickening of the adjacent pericardium. Liposarcoma Liposarcoma is a mesenchymal malignant tumor with fatty differentiation. Commonly encountered in the extremities and retroperitoneum, mediastinal involvement is exceedingly rare. Three histologic subtypes have been described: differentiated myxoid or round cell, dedifferentiated, and pleomorphic. The subtype and grade of the lesion are important
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TABLE 1: Categorization of Anterior Mediastinal Masses According to Predominant CT Attenuation Values Predominant Density Fat
Abnormality Lipoma Liposarcoma Mediastinal lipomatosis Hernia Thymolipoma Teratoma
Water
Thymic cyst Lymphangioma Abscess Others: pleuropericardial cyst, foregut duplication cyst, cystic teratoma, cystic degeneration of malignancy, pancreatic pseudocyst
Soft tissue
Thymic hyperplasia or neoplasm Thyroid goiter Ectopic parathyroid adenoma Germ cell tumor Mediastinitis Acute Fibrosing Lymphadenopathy Lymphoma Metastasis Others: hernia, Castleman disease, sarcoma
Calcification
Malignancy Teratoma Metastasis from serous or mucinous tumors Metastasis from bone tumor Treated lymphoma Goiter Granulomatous infection Sarcoidosis Silicosis
Vascular
Aortic aneurysm
prognostic features. Typically found in adults 50–60 years old, a large liposarcoma can produce both localized symptoms related to compression of adjacent structures and such systemic symptoms as fatigue and weight loss. Treatment options include surgery, chemotherapy, and radiotherapy, but the tumor has a high rate of recurrence. On CT, the appearance and aggressiveness of the tumor vary depending on its degree of differentiation, ranging from a fatty lesion that is difficult to distinguish from a benign lipoma to a pure soft-tissue tumor. In general, a mediastinal liposarcoma appears
as an inhomogeneously enhancing mass with variable amounts of fat and soft-tissue density. As with the imaging of benign lipomas, MRI is helpful to detect the fatty component and contrast enhancement of a liposarcoma. Mediastinal Lipomatosis Lipomatosis refers to excessive deposition of unencapsulated fat in the mediastinum, most often associated with exogenous steroid intake and obesity. Fatty deposition can also occur in patients with Cushing syndrome, adrenocorticotropic hormone–producing tumors, and in rare genetic or familial disorders.
At histology, there is an excessive accumulation of normal fat. Mediastinal lipomatosis usually is asymptomatic or has an indolent course but infrequently may produce symptoms by compressing adjacent structures. Therapy is generally focused on treating the underlying cause, although surgical decompression has been reported in severe cases. Radiography shows lipomatosis as smooth widening of the mediastinum, which may be accompanied by prominent epicardial fat pads and symmetric thickening of the extrapleural fat along the lateral chest wall. Although not a neoplastic process, mediastinal fat deposition localized to the superior portion of the anterior compartment may simulate a mass. CT shows abundant diffuse homogeneous fat-attenuation material that insinuates around anatomic structures without invasion or compression (Fig. 1). The presence of a soft-tissue component, septa, capsule, and contrast enhancement should suggest an underlying tumor. Omental Hernia Herniation of omental fat into the anterior chest can be due to a congenital Morgagni hernia or acquired postoperative or posttraumatic diaphragmatic defects. Usually an incidental finding, symptoms may develop due to incarceration or infarction of the herniated fat. Although infrequent, compressive respiratory or gastrointestinal symptoms have been described. Because of the risk of intestinal strangulation, surgical repair is the treatment of choice if the hernia contains bowel. Radiography shows a Morgagni hernia as a well-defined area of increased density along the heart border, usually at the right cardiophrenic angle. CT and MRI show fat attenuation or signal intensity within the lesion. Accompanying linear opacities correspond to omental vessels (Fig. 2). Coronal and sagittal reconstructions are useful to show the underlying diaphragmatic defect, which is a key finding in differentiating herniated omental fat from a lipoma or liposarcoma. Thymolipoma Representing up to 10% of thymic neoplasms, thymolipoma is a rare benign encapsulated tumor that is usually found in young adults and has no sex predilection. Histologically, it is composed of mature adipose and thymic tissue in variable proportion. Often discovered incidentally, a thymolipoma may displace adjacent structures and produce chest pain, arrhythmia, cough, or dyspnea. Thymolipomas have been associated with
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Shahrzad et al. myasthenia gravis, aplastic anemia, RBC aplasia, hypogammaglobulinemia, Hodgkin lymphoma, and thyrotoxicosis or Graves disease. Surgical removal of the lesion has been reported to alleviate symptoms in some cases of myasthenia gravis. Radiography reveals a usually large, soft, and pliable anterior mediastinal mass that often conforms to the borders of the heart and mimics cardiomegaly or diaphragmatic elevation. On CT, fat constitutes 50–85% of this well-defined, encapsulated mass. Nonenhancing solid linear densities, representing thymic tissue, are interwoven within the fat. At times, a thymolipoma may not clearly lie within the thymus. In such cases, tracing a connection between the mass and the thymus in coronal or sagittal images permits the diagnosis of its thymic origin. On MRI, the fat component of a thymolipoma has high signal intensity on both T1- and T2weighted sequences and has low signal intensity on fat-suppressed imaging. This appearance differs from thymic tissue, which has intermediate signal intensity similar to skeletal muscle on all sequences. Teratoma Germ cell tumors are a heterogeneous group of lesions containing tissues originating from primitive germ cell layers: ectoderm, mesoderm, and endoderm. In adults, these tumors occur most commonly in the gonads, with only 3% arising in a mediastinal location. Teratoma is the most common germ cell tumor (70%), contains tissues from all three layers, and occurs in young adults. It most often contains fat, which derives from the mesoderm. Teratomas are classified into three subtypes: mature, immature, and those with additional malignant components. Mature teratomas contain well-differentiated tissue and represent the vast majority of cases. They are almost always benign but still have a low malignant potential. Immature teratomas contain various amounts of undifferentiated neuroectodermal or mesenchymal tissues. Often benign, the malignant potential is proportional to the degree of immature components. Teratomas with additional malignant components are rare lesions in which the teratoma is accompanied by foci of carcinoma, sarcoma, or component of other germ cell tumors. Patients with teratoma may be asymptomatic or present with chest pain, dyspnea, fever, or cough. After complete surgical excision, mature and immature teratomas have an excellent prognosis. Aggressive teratomas with malignant components, however, do not respond well to chemotherapy. In general,
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a substantial number of teratomas in women are benign, whereas a high proportion of these tumors in men are malignant. On radiography, a teratoma appears as a soft-tissue mediastinal mass that may contain calcification (20%), radiolucency suggestive of fat (5%), and a fat-fluid level (2%). On CT, mature and immature teratomas most often appear as well-defined lobulated cystic masses (90%). Fatty foci, mainly sebaceous fat, are encountered in more than 75% of the cases, with characteristic fat-fluid levels. Up to 50% of teratomas contain rimlike or toothlike calcification (Fig. 3). Differentiating between mature and immature teratomas on imaging studies is difficult. Small solid portions of the lesion often represent the immature element. Visualization of a large predominantly solid necrotic, hemorrhagic, and enhancing lesion with elevated α-fetoprotein should raise concern for malignant teratoma. Teratomas can rupture or fistulize to adjacent structures. A ruptured teratoma may present with more inhomogeneity of its internal elements as well as adjacent atelectasis, consolidation, and pleural or pericardial effusion. The macroscopic fatty component of a teratoma can be seen on MRI as high signal intensity on T1- and T2-weighted sequences and low signal intensity on fat-suppressed imaging. Furthermore, chemical-shift artifact can delineate lipid-rich cellular content on in-phase and opposed-phase sequences. Water Attenuation Thymic Cyst Congenital or acquired thymic cysts represent only 1–3% of all mediastinal masses. Derived from a patent thymopharyngeal duct, congenital thymic cysts are usually simple, unilocular, and thin-walled lesions containing clear fluid and without pathologic evidence of inflammation. Acquired thymic cysts have been associated with myasthenia gravis, systemic lupus erythematosus, Sjögren syndrome, aplastic anemia, radiation therapy, Hodgkin disease, thoracotomy, AIDS in children, and other thymic tumors. In contrast to congenital lesions, acquired thymic cysts are more often multilocular with variable wall thickness and typically have pathologic evidence of inflammation. It is important to differentiate between congenital and acquired thymic cysts because acquired cysts tend to recur after resection and may be associated with thymic malignancy. Both types may be asymptomatic and discovered incidentally or present with symptoms of compression of adjacent structures.
On radiography, a thymic cyst is indistinguishable from other mediastinal lesions. Therefore, CT or MRI is necessary to make the diagnosis. CT shows a congenital thymic cyst as a well-defined water-attenuation lesion without a perceptible wall (Fig. 4A). In contrast, an acquired unilocular or multilocular cyst is a heterogeneous lesion with a clearly evident border of variable thickness. If complicated by hemorrhage or infection, an acquired thymic cyst may resemble a solid mass and show higher CT attenuation and even calcification within its wall. In this situation, the finding of high signal intensity on both T1- and T2-weighted MR images can differentiate a complicated cyst from a typical cyst containing fluid, which has low T1 signal intensity (Fig. 4B) and uniform high T2 signal intensity (Fig. 4C). Lymphangioma Lymphangioma is a rare congenital malformation that contains a variable amount of lymphatic and vascular proliferation and is caused by embryologic failure of the lymphatic system to adequately drain into the venous system. It is classified according to the size of the lymphatic channels as simple-capillary (small lymphatic vessels), cavernous (dilated lymphatic channels), or cystic (cystic spaces with lymphatic fluid, also known as cystic hygroma). Lymphangiomas are mainly encountered in children under 2 years old. They often involve the cervical and axillary region, sometimes with intrathoracic extension. Fewer than 1% of cases are isolated to the thorax, with the anterior mediastinum the most frequently involved compartment. Large mediastinal lymphangiomas are symptomatic because of compression of adjacent structures. Infection, hemorrhage, rupture with chylothorax, and chylopericardium are potential complications. Total excision is the treatment of choice, but this may be impossible with large and infiltrative lesions. Other reported treatments include sclerotherapy, marsupialization, and radiotherapy. On radiography, lymphangioma appears as a well-defined mass that occasionally is associated with chylous pleural effusion. CT shows a smooth homogeneous lobulated lesion of water attenuation containing cysts of variable size. Calcification and contrast enhancement are uncommon. The soft plastic mass of a lymphangioma may envelop or mold to adjacent structures. If CT is equivocal, MRI is useful to confirm the high T2 signal intensity characteristic of this cystic mass and show internal septations. The signal intensity on T1-weighted images is variable de-
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pending on the amount of proteinaceous and hemorrhagic contents of the lesion. Abscess Although not commonly encountered in the anterior mediastinum, abscesses may result from an infected surgical bed, perforated esophagus, or contiguous spread from another infection site (neck, abdomen, adjacent bone, or joint). An abscess should be suspected when the collection contains air and has a thick rimenhancing wall. This will be discussed in more detail in the Mediastinitis section. Others Other entities that may appear as waterdensity lesions include cystic degeneration of tumor and lymph nodes, pancreatic pseudocyst, and cystic teratoma. Other fluid-containing lesions, such as pleuropericardial cyst (Fig. 5), bronchogenic cyst, and duplication cyst, are more frequently encountered in other mediastinal compartments and thus are not discussed in this article. Soft-Tissue Attenuation Thymic Disorders The thymus is an anterior mediastinal organ that extends over the heart and great vessels. Weighing 10–15 g at birth and enlarging up to 40 g at puberty, the thymus involutes later in life. It plays an essential role in the cellular immunity and serves as a maturation site for lymphocytes, especially T-lymphocytes. Differentiating between the normal and pathologic thymus is essential to avoid unnecessary biopsy or surgery, but doing so may be difficult with imaging. On radiography, the normal thymus may be quite large during childhood but should not be visible later in life. On CT, the normal thymus may have various configurations and shapes: bilobed, inverted V or triangular, and quadrilateral. The borders of the gland are usually straight or concave, and the maximum thickness perpendicular to the long axes should not be greater than 13 mm in adults. As the organ involutes over time, it is often completely replaced by fat. On MRI, the normal thymus in young patients has a signal intensity similar to muscle on both T1- and T2-weighted images. As the gland atrophies with age, the signal intensity becomes higher on both sequences due to fatty infiltration. Chemical shift artifact with in- and out-of-phase sequences is also useful for showing the fat content of the gland. Thymic disorders, ranging from benign to malignant, can present as anterior mediastinal
masses. Although chest radiography usually shows the lesion, CT is necessary to characterize it. Thymic Hyperplasia Thymic hyperplasia can be divided into two histologic subtypes: true hyperplasia and lymphofollicular hyperplasia. True thymic hyperplasia is an increase in size and weight of the gland with normal histology. This condition is commonly due to a rebound from recent stress, such as pneumonia, corticosteroid therapy, radiation therapy, chemotherapy, surgery, or burns (Fig. 6). Lymphofollicular hyperplasia refers to the presence of a hyperplastic lymphoid germinal center in the medulla of the gland (Fig. 7). Occurring in more than 50% of patients with myasthenia gravis, lymphofollicular hyperplasia is also associated with other autoimmune disorders, such as thyrotoxicosis, systemic lupus erythematosus, polyarteritis nodosa, and Addison disease. On CT, thymic hyperplasia usually appears as symmetric diffuse enlargement of the gland. Asymmetric enlargement raises the possibility of a thymoma. Thymic Neoplasm Tumors involving the thymus are classified as epithelial (thymoma, thymus carcinoma), lymphoma, thymolipoma, carcinoid, germ cell tumors, sarcomas, and metastasis. Thymomas represent 20% of anterior mediastinal neoplasms in adults. It can be challenging to distinguish among thymic hyperplasia, normal thymus, and malignancy, especially in patients who have undergone chemotherapy and have possible tumor recurrence. Moreover, there are variations of thymic appearance with age and sex. A hyperplastic thymus can retain its normal shape but usually loses its bilobed appearance. There is diffuse homogeneous enlargement of the gland, which contains areas of fat and lymphoid attenuation with normal vessels and smooth contour. On CT, the presence of a convex round thymus with an irregular margin and heterogeneous content associated with necrosis, cystic changes, hemorrhage, or calcification should raise the possibility of malignancy (Fig. 8A). Local invasion, distant metastasis, and pleural or pericardial dissemination (drop metastasis) are also useful signs (Fig. 8B). MRI chemical-shift imaging detects fat within the thymus, which occurs in thymic hyperplasia but not in thymic malignancy. As with other lesions of the anterior mediastinum, especially if there is evidence of invasion, further man-
agement should be undertaken after discussion with medical and surgical colleagues. Thyroid Goiter Intrathoracic, substernal goiter accounts for 3–6% of mediastinal masses, with a majority located in the anterior compartment. The most common type is secondary intrathoracic goiter with direct downward extension from the neck. Fewer than 1% of cases are primary, in which the ectopic goiter has a blood supply that arises from mediastinal vessels with no connection to the neck. Most cases of thyroid goiter are asymptomatic, although they can produce life-threatening symptoms because of airway or neurovascular obstruction. Carcinoma has been reported in 5–17% of cases of substernal goiter, and surgery is the treatment of choice. On radiography, an intrathoracic goiter presents as a homogeneous smooth sharply marginated mass in a retrosternal, paratracheal, or retrotracheal location. The trachea and esophagus may be displaced. On unenhanced CT, the goiter has hyperattenuation because of the high iodine content of the mass, and focal calcifications often occur. After contrast injection, there is early and prolonged enhancement. Visualization of a connection between the mass and the cervical thyroid gland is the key feature for differentiating a substernal goiter from other mediastinal masses (Fig. 9). Ectopic Parathyroid Adenoma The four parathyroid glands are normally found at the level of the thyroid gland, but one may occur in an ectopic position as the result of aberrant migration during early stages of development. Fewer than 2% of ectopic glands are located in the anterior mediastinum. The ectopic gland may cause symptoms of hyperparathyroidism or be the site of parathyroid hyperplasia; a solitary adenoma; or, infrequently, a malignant neoplasm. Thoracoscopic or open resection is indicated for a symptomatic ectopic parathyroid gland in the anterior mediastinum. In a patient with primary hyperparathyroidism, ultrasound or 99mTc sestamibi (Fig. 10) examination of the neck is generally the first imaging modality. If this proves inconclusive, CT or MRI can be performed to detect an ectopic parathyroid tumor in the mediastinum. On CT, an ectopic parathyroid adenoma appears as a 1- to 3-cm oval or round well-defined mass of soft-tissue attenuation that occasionally can contain cystic de-
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Shahrzad et al. generation or hemorrhage. Although sometimes mimicking an enlarged lymph node, an ectopic parathyroid tumor typically shows more contrast enhancement, especially during the arterial phase. On MRI, ectopic parathyroid adenomas are iso- to hypointense on T1weighted imaging and iso- to hyperintense on T2-weighted imaging compared with the thyroid gland and enhance avidly. Germ Cell Tumors Germ cell tumors are a heterogeneous group of lesions originating from primitive germ cell layers (ectoderm, mesoderm, and endoderm) and normally affect the gonads. However, aberrant migration of primordial germ cells may permit these tumors to develop in an extragonadal location, most commonly the anterior mediastinum (15% of mediastinal masses in adults, 24% in children). Germ cell tumors are divided into two main categories: seminomatous and nonseminomatous (teratoma, yolk sac tumor, choriocarcinoma, and embryonal carcinoma). Symptoms reflect mass effect on adjacent organs; secretions of β-HCG and α-fetoprotein are used as tumor markers. The multiple histologic subtypes of germ cell tumors produce various appearances on CT. Teratoma, the most frequent type, has been discussed earlier. Seminomas generally present as large, lobulated, mildly enhancing, homogeneous, noncalcified soft-tissue masses. Conversely, nonseminomatous germ cell tumors tend to be more heterogeneous, with ill-defined margins, and may contain areas of necrosis and hemorrhage (Fig. 11). Mediastinitis Acute—Acute mediastinitis is most commonly an emergency condition caused by esophageal rupture or a complication of surgery, notably after sternotomy. At times, it may be caused by direct extension of infection from adjacent osteomyelitis or from the neck. Acute mediastinitis manifests with infectious symptoms, such as fever, chills, tachycardia, and sepsis. There may also be retrosternal pain with upward radiation to the neck or subcutaneous emphysema. Acute mediastinitis requires immediate medical or surgical intervention, with the precise treatment depending on the severity and clinical symptoms. On radiography, acute mediastinitis can produce mediastinal widening, diffuse or focal gas bubbles, pleural effusion, mediastinal air-fluid levels, or even a soft-tissue mass. CT
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findings suggesting mediastinitis include fat stranding, lymphadenopathy, abscess, and empyema. In addition to defining the extension of the process, CT is of value for determining the underlying cause. A diagnosis of esophageal perforation should be suggested if there is esophageal wall thickening and the presence of a fluid collection, extraluminal air, extravasation of contrast material into the pleural space or mediastinum, pneumomediastinum, or pneumothorax. Mediastinitis developing acutely after sternotomy can cause sternal dehiscence and misaligned sternum wires. CT also is the modality of choice for showing direct extension of infection from osteomyelitis and infections of the head and neck. Chronic—Chronic mediastinitis is a rare benign disease characterized by fibrous proliferation. Also known as fibrosing mediastinitis, it most often affects the middle mediastinum. Among the many reported causes are infection (histoplasmosis, other fungal disease, and tuberculosis), systemic autoimmune disorders, lymphoma, radiation therapy, trauma, and drugs (methylsergide). A familial form is associated with retroperitoneal fibrosis, sclerosing cholangitis, Reidel thyroiditis, and orbital pseudotumor. Fibrosing mediastinitis produces symptoms because of obstruction of vital structures (airways, esophagus, and vessels). On radiography, fibrosing mediastinitis shows such nonspecific findings as mediastinal widening, hilar and mediastinal lymphadenopathy, and calcification. On CT, the disease produces a soft-tissue mass that infiltrates the mediastinum and obliterates adjacent structures. In the more common focal pattern, which is thought to be linked to histoplasmosis, there are calcified fibrous masses located in the hila, paratracheal, and subcarinal areas. Diffuse disease, which is not associated with histoplasmosis, produces multiple noncalcified infiltrative masses that involve multiple mediastinal compartments. Lymphadenopathy Lymphoma—Primary lymphoma, which accounts for approximately 20% of mediastinal neoplasms in adults and 50% in children, usually occurs in the anterior mediastinum. About 50–70% of mediastinal lymphomas are due to Hodgkin disease, which has no sex predilection and presents with a bimodal age distribution in young adults and in patients older than 50 years, most of whom have constitutional symptoms, including fever, night
sweats, and weight loss. Mediastinal involvement of lymphoma may also present with cough, dyspnea, chest pain, pleural effusion, and superior vena cava syndrome. Non-Hodgkin disease is classified into large B cell lymphoma (typically in young and middle-aged adults) and T cell lymphoblastic lymphoma (generally affecting children and adolescents). On radiography, lymphoma usually produces bulky enlargement of mediastinal and hilar nodes (Fig. 12), although it can present as a solitary mediastinal mass. On CT, Hodgkin lymphoma appears as a homogeneous soft-tissue mass in the anterior mediastinum with surface lobulation and mild-to-moderate contrast enhancement. Occasionally, there may be cystic changes and necrosis (Fig. 13). Both types of non-Hodgkin lymphoma typically appear similar to Hodgkin lymphoma, although they tend to have more vascular involvement and associated mediastinal lymphadenopathy. Reactive lymphadenopathy—Enlargement of lymph nodes reflecting a reaction to infection (including tuberculosis) or autoimmune or inflammatory disease can develop in the anterior mediastinum. Unlike in lymphoma, reactive lymph nodes do not tend to coalesce into a large mass. Others In addition to fat, hernias can have softtissue density when they contain abdominal organs. Other rare disorders presenting this appearance include sarcomas and Castleman disease. Calcific Attenuation Malignancy Treated malignancy, especially lymphoma (20%), can show calcification (Fig. 14). Calcified untreated malignancy includes teratomas (50%), metastasis from serous or mucinous tumors (ovaries, gastrointestinal tract), and metastasis from bone tumor. Goiter Calcification develops in up to 75% of thyroid goiters. Lymphadenopathy in Granulomatous Infection or Sarcoidosis-Silicosis Some infectious or inflammatory processes can cause calcification in mediastinal lymph nodes. This condition most commonly reflects granulomatous infection (tuberculosis, histo-
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Anterior Mediastinal Masses plasmosis). Although typically found elsewhere, characteristic egg-shell calcifications in sarcoidosis and silicosis can sometimes occur in the anterior mediastinum. Vascular Structures An ascending aortic aneurysm (> 5 cm) may mimic an anterior mediastinal mass (Fig. 15). Predisposing factors include atherosclerosis, trauma, vasculitis, collagen-related disorders, and infection. Often an incidental finding in a patient without symptoms, an ascending aortic aneurysm can cause symptoms due to compression or rupture. Radiographic signs suggesting an aortic aneurysm include obliteration and enlarge-
ment of normal aortic contours (Figs. 15A and 15B). Curvilinear calcification of a lesion should raise the possibility of a vascular origin. Rupture of an aneurysm may lead to fluid in the pleural space, enlargement of the cardiac contour due to hemopericardium, and enlargement of the mediastinum due to hematoma. CT can confirm the diagnosis of aortic aneurysm (Figs. 15C and 15D), define its extent, and show signs of rupture or dissection. Suggested Reading 1. Brown LR, Aughenbaugh GL. Masses of the anterior mediastinum: CT and MR imaging. AJR 1991; 157:1171–1180
2. Dalton ML, Connally SR. Median sternotomy. Surg Gynecol Obstet 1993; 176:615–624 3. D’Cruz IA, Feghali N, Gross CM. Echocardiographic manifestations of mediastinal masses compressing or encroaching on the heart. Echocardiography 1994; 11:523–533 4. Devaraj A, Griffin N, Nicholson AG, Padley SP. Computed tomography findings in fibrosing mediastinitis. Clin Radiol 2007; 62:781–786 5. Kim JH, Goo JM, Lee HJ, et al. Cystic tumors in the anterior mediastinum: radiologic-pathological correlation. J Comput Assist Tomogr 2003; 27:714–723 6. Marom EM. Advances in thymoma imaging. J Thorac Imaging 2013; 28:69–80; quiz 81–83 7. Newman E, Shaha AR. Substernal goiter. J Surg Oncol 1995; 60:207–212
A
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Fig. 1—Mediastinal lipomatosis. A and B, Transverse (A) and coronal (B) reformatted CT images of chest show abundant diffuse homogeneous material bulging from normal mediastinal borders (arrows). Density appears as low as that in subcutaneous fat (asterisk). Fat-attenuation material surrounds anatomic structures without signs of invasion. Beam-hardening and reconstruction artifacts are seen.
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C
Fig. 2—Morgagni hernia in 68-year-old man with dyspnea. A, Posteroanterior chest radiograph shows large area of increased density in right inferior hemithorax that obscures right cardiophrenic angle (dotted line). B, Transverse CT image of chest shows voluminous hernia containing omentum and small bowel. C, Sagittal reformatted CT image of chest shows voluminous diaphragmatic hernia containing omentum, mesenteric vessels (arrowheads), and small (solid arrow) and large (open arrow) bowel. No diaphragmatic profile can be seen in bottom portion of image.
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Fig. 3—Teratoma. Transverse CT image of chest shows anterior mediastinal mass with heterogeneous density that indicates presence of encapsulated fatty component (solid arrow) associated with focal toothlike calcification (open arrow).
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Fig. 4—Thymic cyst. A, Transverse CT image of chest shows anterior mediastinal mass (arrow). Location and shape are typical of thymic structure, whereas near-water density and smooth margin reflect cystic nature. B and C, Transverse T1-weighted (B) and T2-weighted (C) fat-suppressed MR images of chest show anterior mediastinal mass (arrow). Hypointense signal intensity in T1weighted and hyperintense signal intensity in T2-weighted images confirm water content.
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Fig. 5—Pericardial cyst. A, Posteroanterior chest radiograph shows rounded opacity (arrow) obliterating right cardiophrenic angle. B and C, Transverse (B) and coronal reformatted (C) CT images of chest show mass (arrow) in right cardiophrenic angle. Water density and smooth margin reflect cystic nature, whereas contiguity to surface of pericardium suggests relation between this structure and mass.
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Anterior Mediastinal Masses
Fig. 6—Thymic hyperplasia in 31-year-old woman who presented after six cycles of chemotherapy for lymphoma. Transverse CT image of chest acquired in fully suspended inspiration shows soft-tissue density (arrow) within mediastinal fat. This finding has regular margin and is adjacent to mediastinal vessels without signs of invasion.
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Fig. 7—Lymphofollicular hyperplasia. Transverse CT image of chest at baseline shows persistent area of soft-tissue density in anterior mediastinum (arrow). Finding was stable over time.
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Fig. 8—Thymic malignancy. Different stages of thymoma. A, Transverse CT image of chest shows stage I thymoma. Soft-tissue mass in anterior mediastinum is partly calcified (solid arrow), encapsulated by fat, with regular margins (open arrows). B, Transverse CT image of chest shows advanced-stage thymoma. Heterogeneous density of mediastinal mass reflects presence of coarse calcifications and necrosis (solid arrow). Mediastinal deviation with unilateral pleural thickening (open arrows) and atelectasis (arrowhead) suggest left pleural and parenchymal involvement. Centrally necrotic subcarinal lymphadenopathy (asterisk) is consistent with mediastinal lymph node involvement. Image also shows scarring in left upper lobe, adjacent to left hilum.
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Shahrzad et al.
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Fig. 9—Thyroid goiter in 57-year-old woman who presented with dysphagia. A, Axial CT image shows enlargement of left and right thyroid lobes (thick arrow) with small calcification on left (thin arrow). B, Axial CT image at lower level shows goiter in left paratracheal location (thick arrow) displacing supraaortic branches to left (thin arrows). C, Coronal CT reconstruction shows complete apical-basilar extension of goiter (thick arrow) and areas of calcification (thin arrow).
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Fig. 10—Parathyroid adenoma. A, Transverse enhanced CT image of chest shows solid lesion (arrows) with homogeneous soft-tissue density similar to lymph node. B, Transverse reconstructed image of 99mTc sestamibi scan shows high-signal-intensity spot in anterior mediastinum (arrow).
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Anterior Mediastinal Masses
Fig. 12—Lymphadenopathy from Hodgkin lymphoma. Transverse CT image of chest shows enlarged paraaortic lymph nodes in anterior mediastinum (lymph node region 6 (solid arrow) and region 4R (open arrow).
Fig. 11—Germ cell tumor in 37-year-old man who presented with chest pain and right upper extremity swelling. Transverse CT image of chest shows anterior mediastinal mass with mainly soft-tissue density and areas of reduced density. Compression and displacement of mediastinal veins are caused by mass (arrow).
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Fig. 13—Lymphoma. A, Transverse enhanced CT image of chest shows bulky mass of heterogeneous density (arrow) in anterior mediastinum. Encasement and compression of mediastinal veins are caused by growing mass. B, Transverse fused FDG PET/CT image of chest shows FDG-avid structure (solid arrow) corresponding to lymphoma. Signal heterogeneity reflecting necrosis of lesion is clearly detectable (open arrow).
Fig. 14—Lymphoma after chemotherapy. Transverse CT image of chest shows lymph nodes with coarse calcifications (arrow).
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Shahrzad et al. Fig. 15—Aortic aneurysm. A and B, Posteroanterior (A) and lateral (B) radiographs of chest show right mediastinal enlargement (arrow, A) and obliteration of retrosternal space. C and D, Transverse (C) and sagittal (D) reformatted images of enhanced CT of chest show severe dilatation of ascending aorta (asterisk). Compression and displacement of superior vena cava (arrow, C) are seen.
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