Compartment
Boundaries
Major contents
Prevascular
Superior: Thoracic inlet
Inferior: Diaphragm
Anterior: Sternum
Lateral: Parietal (mediastinal) pleural reflections; lateral margin of the bilateral internal thoracic arteries and veins, and superior and inferior pulmonary veins
Posterior: Anterior to the pericardium, along the anterior margin of the superior vena cava, ascending aorta, and the lateral rim of the aortic arch, superior and inferior pulmonary veins
Thymus
Fat
Lymph nodes
Left brachiocephalic vein
Visceral
Superior: Thoracic inlet
Inferior: Diaphragm
Anterior: Posterior boundaries of the prevascular compartment
Posterior: A vertical line connecting a point on each thoracic vertebral body at 1 cm posterior to its anterior margin
Nonvascular: Trachea, main bronchi, esophagus, lymph nodes
Vascular: Heart, ascending thoracic aorta, aortic arch, and descending thoracic aorta, superior vena cava, intrapericardial pulmonary arteries, thoracic duct
Paravertebral
Superior: Thoracic inlet
Inferior: Diaphragm
Anterior: Posterior boundaries of the visceral compartment
Posterolateral: Vertical line against the posterior margin of the
chest wall at the lateral margin of the transverse process of the thoracic spine
Paravertebral soft tissues
The boundaries of the prevascular compartment include: (1) superiorly—the thoracic inlet, (2) inferiorly—the diaphragm, (3) anteriorly—the sternum, (4) laterally—the parietal mediastinal pleura, and (5) posteriorly—the anterior aspect of the pericardium as it wraps around in a curvilinear fashion (Fig. 3.1). Based on these anatomic landmarks, the major contents of the prevascular mediastinum include the thymus, fat, lymph nodes, and the left brachiocephalic vein. The most common lesions originating in the prevascular compartment include thymic masses (cysts, hyperplasia, and thymic epithelial neoplasms), germ cell neoplasms, lymphoma, metastatic lymphadenopathy, and intrathoracic goiter.
Fig. 3.1
Contrast-enhanced axial CT images slightly below the aortic arch (a), at the level of the left pulmonary artery (b), and at the level of the left atrium (c) demonstrate the ITMIG classification system of mediastinal compartments. Green, prevascular compartment; blue, visceral compartment; red, paravertebral compartment; yellow line, visceral–paravertebral compartment boundary line
The boundaries of the visceral compartment include: (1) superiorly—the thoracic inlet, (2) inferiorly—the diaphragm, (3) anteriorly—the anterior aspect of the pericardium (which envelops the distal aspect of the superior vena cava, the proximal aspect of the ascending aorta and lateral rim of the aortic arch, and the intrapericardial pulmonary arteries), and (4) posteriorly—a vertical line connecting a point on the thoracic vertebral bodies 1 cm posterior to the anterior margin of the spine (referred to as the visceral–paravertebral compartment boundary line) (Fig. 3.1). The major contents of the visceral compartment fall into two main categories: (1) vascular (including the heart, superior vena cava, ascending thoracic aorta, aortic arch, and descending thoracic aorta, intrapericardial pulmonary arteries, and the thoracic duct) and (2) nonvascular (including the trachea, carina, esophagus, and lymph nodes). In the ITMIG description, all of the structures contained within the pericardium are included in the visceral compartment. The most common lesions originating in the visceral compartment include lymphadenopathy (related to lymphoma or metastatic disease), foregut duplication cysts, tracheal lesions, and esophageal neoplasms. Abnormalities of the heart, pericardium, and great vessels may also be encountered in this compartment.
The boundaries of the paravertebral compartment include: (1) superiorly—the thoracic inlet, (2) inferiorly—the diaphragm, (3) anteriorly—the visceral compartment, and (4) posterolaterally—a vertical line along the posterior margin of the chest wall at the lateral aspect of the transverse processes (Fig. 3.1). The major contents of the paravertebral compartment include the thoracic spine and paravertebral soft tissues; therefore, most abnormalities in this region are neurogenic neoplasms arising from the dorsal root ganglia/neurons adjacent to the intervertebral foramina, as well as lesions related to infection and trauma.
General Considerations
More than half of all mediastinal masses are located in the prevascular compartment. One-fourth of mediastinal lesions are present in the visceral mediastinum, and another one-fourth of masses occur in the paravertebral mediastinum [3–12]. The prevalence of specific mediastinal lesions is difficult to establish from the medical literature for multiple reasons. For instance, different radiologic and/or clinical classification systems may be used to define the mediastinal compartments. Additionally, the inclusion of non-neoplastic lesions such as thymic, pericardial, and foregut duplication cysts and various types of lymphoma is variable between different studies.
Despite the aforementioned limitations in determining the true prevalence of mediastinal abnormalities, several studies have shown that the most common prevascular mediastinal masses include the following entities: thymic malignancy (35%), lymphoma (25% overall; 13% Hodgkin lymphoma or HL and 12% non-Hodgkin lymphoma or NHL), thyroid and other endocrine neoplasms (15%), benign teratoma (10%), malignant germ cell tumors (10% overall; 4 seminoma and 7% non-seminomatous germ cell tumor or NSGCT), and benign thymic cysts (5%). [3, 6, 8].
Role of Imaging
Mediastinal lesions may be first detected on chest radiography given the widespread use and availability of the modality. Small lesions may not be readily identified, whereas large masses may manifest as an extra soft tissue mass or opacity. One particular tool referred to as the “silhouette sign,” which describes the loss of normal borders of intrathoracic structures on radiography, increases the sensitivity of detecting mediastinal abnormalities (Fig. 3.2). Primary mediastinal neoplasms are usually focal, unilateral lesions that abut the mediastinum and preclude radiographic visualization of a medial lesion border (Fig. 3.3). These neoplasms typically displace the normal mediastinal interfaces and manifest as mediastinal contour abnormalities. Once a lesion is identified on chest radiography, cross-sectional imaging techniques are used to characterize the abnormality, generate a differential diagnosis, assess for other abnormalities, and guide further management. CT with intravenous (IV) contrast has traditionally been the imaging modality of choice for the evaluation and characterization of mediastinal masses. In an analysis of 127 anterior mediastinal masses of various etiologies, investigators demonstrated that CT was equal or superior to magnetic resonance imaging (MRI) in diagnosing specific lesions except for thymic cysts [13]. MRI is the most useful imaging modality to evaluate a suspected cystic mediastinal lesion, because MRI is superior to CT in distinguishing cystic from solid masses (e.g., thymic cysts from thymic neoplasms) and discerning cystic and/or necrotic components within solid masses [14]. Specific chemical shift techniques can be used to differentiate thymic hyperplasia from thymic epithelial neoplasms and other malignancies [15, 16]. Unenhanced MRI may also be performed to characterize masses in patients who are unable to undergo contrast-enhanced CT due to renal failure or severe allergy to IV contrast. 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT is not routinely performed to evaluate or characterize mediastinal masses, but may be used for staging, assessing response, and evaluating for disease recurrence. However, it is important to note that imaging with FDG-PET/CT can be misleading, given that normal and hyperplastic thymus and infectious and inflammatory lesions in the mediastinum may be FDG-avid [12].
Fig. 3.2
Thymoma in a 43-year-old woman. a and b Frontal chest radiograph (a) demonstrates a mass-like opacity (M) in the right mediastinum silhouetting the right heart border suggesting an anterior location, which is confirmed on the lateral radiograph (b). Multiple pleural metastases (arrows) are present in the right hemithorax. c Contrast-enhanced axial CT of the same patient shows the right prevascular mass (M) consistent with biopsy-proven thymoma and right pleural metastases (arrows). The “silhouette sign” describes the loss of normal borders of intrathoracic structures on radiography and increases the sensitivity of detecting mediastinal abnormalities
Fig. 3.3
Chondrosarcoma in a 52-year-old woman. a and b Frontal chest radiograph (a) shows an ill-defined mediastinal mass (arrow). Since it does not silhouette the anterior aorta interface (arrowhead) or the right paratracheal line (curved arrow), it has to be located posteriorly in the paravertebral compartment, confirmed on the lateral radiograph (b). c Unenhanced axial CT of the same patient demonstrates a soft tissue mass (M) in the right paravertebral mediastinum invading the adjacent rib (arrow). CT-guided biopsy revealed primary chondrosarcoma
Imaging Approach to Mediastinal Lesions
In many cases, the identification of certain characteristics on CT may suggest a fairly specific diagnosis on imaging alone. Findings such as hyperdense and enhancing lesions that communicate with the thyroid gland, intralesional fat, cystic components, and regions of soft tissue attenuation may be used to narrow the differential diagnosis. Calcification, whether punctate, coarse, or curvilinear, cannot discriminate benign from malignant mediastinal lesions [12, 17]. In other cases, while the imaging findings may not enable a definitive diagnosis, a combination of clinical information/context and cross-sectional imaging features can help narrow the differential diagnosis and guide further management.
Fat-Containing Lesions
Teratoma
The presence of visible areas of intralesional fat on CT, measuring between −40 and −120 Hounsfield units, within a heterogeneous prevascular mediastinal mass is highly suggestive of a benign teratoma. These germ cell neoplasms typically demonstrate various amounts of fat, fluid, calcification (including bone and tooth-like elements), and soft tissue (Fig. 3.4) [17, 18]. Fat is identified in approximately 50% of cases [17]. Although a fat–fluid level is highly specific for teratoma, this finding is much less common [19]. Benign mediastinal teratomas are usually seen in young patients and account for approximately 25% of prevascular lesions in ages 10–19, 10–15% in ages 20–49, and less than 5% over age 50 in both men and women.
Fig. 3.4
Benign teratoma in a 34-year-old man. Contrast-enhanced axial CT demonstrates a left prevascular mediastinal mass composed of macroscopic fat (white arrow), calcification, (black arrow), and fluid. Surgical resection was performed due to severe chest pain and pathology revealed benign teratoma. When a complex mass containing various amounts of fat, fluid, calcification, and soft tissue is present in the prevascular mediastinum, the most likely diagnosis is a benign teratoma
Thymolipoma
When a large predominantly fat-containing mass is identified in the prevascular mediastinum or in one of the cardiophrenic angles, thymolipoma should be considered. Thymolipoma is a benign encapsulated lesion that usually contains 50–85% fat and a small amount of solid tissue and fibrous septa, and may be very large at presentation with an average size of 20 cm (Fig. 3.5). The presence of a direct connection with the thymus is confirmatory [18, 20, 21]. Although patients are usually asymptomatic at presentation, anecdotal associations between thymolipoma and myasthenia gravis, Grave’s disease, and hematological disorders have been reported [21]. Thymolipoma is a relatively uncommon lesion, representing <5% of prevascular mediastinal lesions, but the diagnosis can be suggested when the lesion consists almost entirely of fat [12].
Fig. 3.5
Thymolipoma in a 41-year-old man. Contrast-enhanced axial CT shows a predominantly fat-containing mass in the left prevascular mediastinum with only a small amount of internal soft tissue (arrow). Given the presence of internal soft tissue, mediastinal liposarcoma could not be excluded and surgical resection was performed. Pathology revealed thymolipoma, a benign neoplasm typically containing 50–85% fat
Lipoma and Liposarcoma
Additional but less common mediastinal neoplasms with intralesional fat include lipoma and liposarcoma. Lipomas account for 2% of all primary mediastinal tumors and manifest as encapsulated lesions predominantly composed of fat with a small amount of soft tissue and vessels in the prevascular mediastinum [22]. Liposarcomas represent more aggressive malignancies that can be distinguished from lipomas through features such as increased soft tissue components, local invasion, lymphadenopathy, and metastatic disease (Fig. 3.6) [22, 23].
Fig. 3.6
Mediastinal lipoma and liposarcoma. a.Contrast-enhanced axial CT of a 73-year-old woman demonstrates a prevascular mediastinal mass (M) composed predominantly of fat with only minimal internal soft tissue, consistent with a lipoma. Note the narrowing of the superior vena cava (arrow), which remained patent. b Contrast-enhanced axial CT of a 57-year-old woman shows a prevascular mediastinal mass (M) that contains a large amount of fat but demonstrates more internal soft tissue components (arrows) than a thymolipoma or lipoma. CT-guided biopsy demonstrated primary mediastinal liposarcoma
Cystic Lesions
Thymic Cyst
When a well-circumscribed lesion that is homogeneous and round, oval, or saccular in shape is present in the prevascular compartment near the thymic bed, thymic cyst should be included in the differential diagnosis. Thymic cysts usually measure water or fluid attenuation on CT, between 0 and 20 Hounsfield units; however, they can demonstrate regions of high attenuation that can make differentiation between solid and cystic lesions/components difficult. In this scenario, MRI should be performed, as the modality is superior to CT in this regard. Cystic lesions with no soft tissue nodules or internal septations on MRI can reliably be diagnosed as unilocular thymic cysts (Fig. 3.7). Those that do demonstrate soft tissue components represent either multilocular thymic cysts or cystic thymoma, the latter of which should be strongly considered when symptoms related to myasthenia gravis or other paraneoplastic syndrome are present (Fig. 3.8) [12, 24].
Fig. 3.7
Unilocular thymic cyst in a 43-year-old man. a Contrast-enhanced axial CT shows a well-circumscribed, fluid/water attenuation mass (arrow) in the prevascular mediastinum. MRI was subsequently performed to evaluate for internal soft tissue components. b and c T2-weighted (b) and T1-weighted post-contrast (c) MRI of the same patient demonstrates a purely cystic lesion with no evidence of enhancing soft tissue components. Cystic lesions with no soft tissue nodules or internal septations on MRI can reliably be diagnosed as unilocular thymic cysts or cystic teratomas
Fig. 3.8
Multilocular thymic cyst in a 74-year-old woman. a Contrast-enhanced axial CT demonstrates a low-attenuation lesion (M) in the right prevascular mediastinum. MRI was subsequently performed to evaluate for internal soft tissue components. b T2-weighted MRI of the same patient shows a cystic lesion with multiple internal septations which, one of them with focal thickening (arrow). Based on the imaging appearance alone, the mass could represent a multilocular thymic cyst or a cystic thymoma; the former was demonstrated at the time of surgical resection
Teratoma
Although many benign mediastinal teratomas are composed of various amounts of fat, calcification, fluid, and soft tissues as previously stated, mature cystic teratomas, or dermoid cysts, manifest as predominantly cystic masses (Fig. 3.9). Although these lesions can arise from any mediastinal compartment, most originate in the prevascular mediastinum, with only 3–8% occurring in the paravertebral mediastinum [17].
Fig. 3.9
Cystic teratoma in a 30-year-old woman. Contrast-enhanced axial CT demonstrates a well-circumscribed, homogeneous, fluid/water attenuation lesion (M) in the left prevascular mediastinum. MRI (not shown) was subsequently performed to evaluate for internal soft tissue components and revealed a purely cystic lesion. Based on this imaging appearance alone, the differential diagnosis would include a unilocular thymic cyst or cystic teratoma; the latter was demonstrated at the time of surgical resection
Pericardial Cyst
Pericardial cysts are benign lesions composed of connective tissue and a single layer of mesothelial cells. Although these lesions are always connected to the pericardium, only a few cases show visible communication with the pericardial sac on imaging [25]. A homogeneous, low-density lesion measuring water or fluid attenuation with thin or imperceptible walls in one of the cardiophrenic angles can be confidently diagnosed as a pericardial cyst without the need for further evaluation (Fig. 3.10) [25, 26].
