was a Radiology Resident at the University of Texas Health
Science Center Medical School at the time of this submission and
is currently a radiologist with Greater Houston Radiology
is a Professor, the Department Chairman ad interim of the
Department of Radiology, Division of Diagnostic Imaging, The
University of Texas M.D. Anderson Cancer Center.
is a Professor, the Associate Division Head of Translational and
Clinical Research, and the Section Chief of Thoracic Imaging,
Division of Diagnostic Imaging, The University of Texas M.D.
Anderson Cancer Center, Houston, TX.
The mediastinum is composed of various structures within the
central portion of the thorax that are bounded by the lungs, the
diaphragm, and the thoracic inlet. Because of this wide variety of
anatomic structures, mediastinal tumors constitute a heterogeneous
group of neoplasms, both primary and metastatic. Detection of
mediastinal abnormalities requires familiarity with the chest
radiograph, as most mediastinal tumors are discovered in
asymptomatic patients on routine chest radiography. Once a
mediastinal abnormality is detected by a chest radiograph, further
radiologic evaluation involves cross-sectional imaging with
computed tomography (CT) or magnetic resonance imaging (MRI) and
may employ other imaging techniques, such as positron emission
tomography (PET) or integrated PET/CT. This article will review
fundamental radiologic findings of mediastinal abnormalities on
chest radiographs and will address the salient features of
mediastinal tumors on other imaging modalities, including CT, MRI,
PET, and integrated PET/CT.
Division of the mediastinum
The diagnostic evaluation of mediastinal disease requires a
thorough understanding of mediastinal anatomy on chest radiography
and cross-sectional imaging. Localizing and understanding the
relationship of an abnormality to a mediastinal compartment or
normal structure can provide a valuable clue for the diagnosis of a
mediastinal mass. The chest radiograph is the most often performed
radiographic examination, constituting more than 40% of the total
volume of radiographs, with >50 million chest radiographs
performed per year in the United States.
Therefore, it is important for radiologists to understand the
appearance of normal structures and how they can be altered by
disease. In particular, the detection of mediastinal disease can be
difficult with conventional chest radiographs, as many masses are
of soft tissue density and are surrounded by structures composed of
To facilitate the detection and classification of mediastinal
lesions, most radiologists divide the mediastinum into 3
compartments, as proposed by Felson
(Figure 1). The anterior mediastinum is bounded anteriorly by the
sternum and posteriorly by a line drawn from the anterior aspect of
the trachea and along the posterior heart border. The posterior
mediastinum is defined by a line that is 1 cm posterior to the
anterior edge of the vertebral bodies. The middle mediastinal
compartment lies between the anterior and posterior mediastinum.
This division of mediastinal compartments is also used in
Lines, stripes, and interfaces
Normal anatomic structures of the mediastinum may be altered by
mediastinal disease. This alteration of normal anatomy may alert
the radiologist to the presence of a mediastinal mass. Thus,
familiarity with the appearance of normal mediastinal structures on
chest radiography is a crucial part of identifying an abnormality
The anterior and posterior junction lines are normal mediastinal
structures that are not evident in all patients. However, when
present, these lines can be helpful in determining an abnormality.
The close apposition of the lungs and pleura anterior to the
ascending aorta and right ventricular outflow tract forms the
anterior junction line, which courses obliquely from the upper
right to lower left behind the retromanubrial region toward the
Similarly, the posterior junction line is formed by the close
apposition of the lungs and pleura posterior to the esophagus and
anterior to the spine, coursing vertically from the lung apices to
the top of the aortic arch.
These lines should be smooth and concave. Although no abnormality
may be assumed in the absence of these landmarks, when either is
displaced or bulging, it is an indicator of a possible mediastinal
The right paratracheal stripe appears as a thin, soft tissue
density structure located between the tracheal air column and the
adjacent right lung.
This can be followed as it passes vertically from the undersurface
of the clavicles to the azygos arch. It should be smooth and range
in width from 1 to 4 mm.
The azygoesophageal recess is an interface formed by the
apposition of the right lung and pleura and the right lateral
margin of the azygos vein and esophagus. This descends vertically
from the undersurface of the azygos arch to the diaphragm, with a
gentle convexity curving toward the left. The interface should be
smooth without focal bulges or extreme deviation from the
The left paraspinal interface is formed where the left lung and
pleura abut the left lateral margin of the vertebral bodies and
paravertebral soft tissues. Although not always seen in its
entirety, it should be smooth as it courses vertically from the
level of the aortic knob to the diaphragm and have a uniform
distance from the lateral margin of the spine.
Conventional radiographic signs
In addition to altering normal structures, the presence of
mediastinal malignancies can result in radiographic signs or
patterns that aid in detection and localization. Many of these
classic signs and patterns were originally described by Felson
and continue to be beneficial in interpreting chest
When a mass abuts a normal mediastinal structure of similar
radiodensity, the margins of the 2 structures will be obliterated.
This sign is referred to as the "silhouette sign," although there
is actually desilhouetting of the normal structure by the mass.
This apparent loss of the margin of the normal structure can be
used to localize a mediastinal mass to the same compartment as the
The hilar overlay sign (Figure 3) is another sign described by
that is especially useful in distinguishing an anterior mediastinal
mass from a prominent cardiac silhouette. If the bifurcation of the
main pulmonary artery is >1 cm medial to the lateral border of
the cardiac silhouette, it is strongly suggestive of a mediastinal
mass. If the pulmonary artery arises from the lateral heart border,
this favors an enlarged heart. In other words, because the
pulmonary arteries arise from the heart, when the heart enlarges,
then pulmonary arteries must move laterally with the heart border.
An anterior mediastinal mass that appears as an enlarged cardiac
silhouette will not cause displacement of the pulmonary
The hilar convergence sign is used to distinguish between a
prominent hilum and an enlarged pulmonary artery.
If the pulmonary arteries converge into the lateral border of a
hilar mass, the mass represents an enlarged pulmonary artery
(Figure 4). If the convergence appears behind the abnormality or
arises from the heart, a mediastinal mass is more likely. In other
words, because the pulmonary artery branches arise from the main
pulmonary artery trunk, an enlarged pulmonary artery will have
branches that arise from its outer margin (the vessels converge
toward the main pulmonary artery). A hilar mass may have the
appearance of an enlarged pulmonary artery, but the vessels will
not arise from the margin; instead they will seem to pass through
the margins as they converge on the true artery (Figure 5).
Finally, the cervicothoracic sign (Figure 6) is used to
determine the location of a mediastinal lesion in the upper chest.
The uppermost border of the anterior mediastinum ends at the level
of the clavicles. However, the medial and posterior mediastinum
extends above the clavicles. A mediastinal mass that projects
superior to the level of the clavicles must therefore be located
either within the middle or posterior mediastinum. Furthermore, the
more cephalad the mass extends, the more posterior the location.
Once an abnormality is detected on a conventional chest
radiograph, cross-sectional imaging plays an important role in the
further characterization of the mediastinal mass. CT and MRI
provide excellent anatomic definition and characterization of
mediastinal masses and, therefore, are the imaging modalities of
choice. More recently, functional imaging, using PET and integrated
PET/CT imaging, has been useful in assessing mediastinal masses.
Tumors of the anterior mediastinum
The most common primary tumor of the anterior mediastinum is
Thymoma usually occurs as an incidental finding in an otherwise
healthy individual but may also present along with a parathymic
syndrome, such as myasthenia gravis, pure red cell aplasia, and
hypogammaglobulinemia. Thymoma is characteristically a
well-defined, round or oval anterior mediastinal mass arising from
one of the thymic lobes (Figure 7). They are typically located just
anterior to the aortic root, although they can occur anywhere from
the neck to the cardiophrenic angle. On CT, thymoma presents as an
encapsulated, well-defined soft tissue mass that may be homogeneous
or heterogeneous depending upon the degree of hemorrhage, necrosis,
or cyst formation. Calcifications are infrequent and may be small,
curvilinear, or punctuate. On MRI, a thymoma may be homogeneous or
heterogeneous in signal, but usually appears isointense to muscle
on T1-weighted imaging and has increased signal on T2-weighted
imaging. Invasive thymomas spread locally by seeding the pleural
space and may even progress to circumferentially encase the lung.
The invasive nature is usually not determined by imaging, but
pleural or pericardial implants can occasionally be detected by CT
and MRI. MRI can also be utilized to reveal involvement of vascular
Thymic carcinoma is an aggressive malignancy with a propensity
to spread through early local invasion to regional lymph nodes and
with distant metastases.
CT shows a large, poorly defined, infiltrative anterior mediastinal
mass that may exhibit cystic changes.
Pleural and pericardial effusions are frequently associated, in
contrast to benign or invasive thymoma, which rarely are associated
with pleural effusions.
Thymic carcinoma can be difficult to differentiate from thymoma in
the absence of distant metastases.
Thymic carcinoid is a rare malignancy arising from the thymic
cells of neural crest origin. Patients often present with endocrine
abnormalities, most commonly Cushing's syndrome or multiple
endocrine neoplasia type 1 (MEN 1) syndrome. CT shows a large,
lobulated anterior mediastinal mass that may have foci of
hemorrhage and necrosis. Calcification may also be present.
Mediastinal germ cell tumors are thought to originate from
primitive germ cells misplaced in the mediastinum during early
The most common extragonadal primary site is the anterior
mediastinum, especially the anterosuperior portion.
Teratomas are the most common mediastinal germ cell tumors.
Mature teratomas, which represent approximately 60% to 70% of
mediastinal germ cell tumors, are usually well-differentiated and
On chest radiography, these are rounded to lobulated, well-defined
anterior mediastinal masses that may extend to one side of the
midline and may attain large sizes.
Calcification and, rarely, identifiable bone or teeth, may be
CT reveals a heterogeneous, predominantly cystic mass with
well-defined margins. An anterior mediastinal mass with foci of
fluid, soft tissue, calcium, and/or fat attenuation is highly
specific for a mature teratoma.
Most lesions of the thyroid are goiters or cysts, and it is not
uncommon for them to be detected on CT. Thyroid lesions are
reported to extend into the mediastinum in 1% to 3% of patients at
the time of thyroidectomy.
The most important feature to distinguish mediastinal thyroid from
other disease is continuity between the cervical and mediastinal
components. While most thyroid lesions extending into the
mediastinum are goiters, radiologists must be aware that malignant
lesions can also present in this manner. The most common
histologies of thyroid carcinomas are papillary, follicular,
medullary, anaplastic, and Hürthle cell.
Of these, medullary carcinoma is the most likely to metastasize to
the mediastinum, while the others tend to spread to the lungs. CT
complements nuclear medicine imaging of thyroid malignancies by
showing anatomical definition of a primary tumor, extent of
mediastinal involvement, and pulmonary metastases. Because of the
high iodine content of thyroid tissue, unenhanced CT imaging of the
thyroid may reveal foci of high attenuation. There may also be
areas of low attenuation due to hemorrhage and cyst formation, and
calcifications may be present. Intravenous contrast is usually not
administered because of interference with radioactive iodine
nuclear medicine studies. However, if contrast is administered,
intense and prolonged enhancement may occur. Radioactive Iodine-123
and -131 scintigraphy may be confirmatory when radioactive uptake
from functioning thyroid tissue is exhibited.
Primary cardiac neoplasms are rare, with a reported prevalence
of only 0.001% to 0.03% of patients in an autopsy series.
The most common primary cardiac tumor is myxoma, which comprises
approximately half of all cases.
Cardiac sarcoma is the second most common primary cardiac neoplasm.
Although still relatively uncommon, cardiac involvement by
metastatic disease is estimated to be 100 to 1000 times more common
than a primary cardiac neoplasm.
Cardiac metastasis is typically associated with widely disseminated
disease. The most frequently implicated primary neoplasms are lung,
breast, lymphoma, and melanoma.
Echocardiography has traditionally been the primary imaging
modality for intracardiac disease. However, the morphology and
location of a cardiac tumor can be adequately shown by CT. With the
development of multichannel CT and cardiac-gated CT, imaging of the
heart with CT is a rapidly emerging technology that may prove
useful in imaging cardiac tumors. In addition, CT is superior to
echocardiography in the evaluation of the pericardium, great
vessels, and other mediastinal structures, which allows for the
assessment of disease extent.
Cardiac-gated MRI has a well-established role in the evaluation of
a cardiac mass
and is currently the imaging modality of choice for cardiac tumors.
The advantages of MRI include a wide field of view, high contrast
and spatial resolution, and multiplanar imaging. These features
provide accurate localization of a mass, including its anatomic
relationship to the cardiac chambers (Figure 8). MRI also allows
determination of involvement of myocardium, pericardium, or other
adjacent mediastinal structures. Gadolinium contrast administration
may show differential enhancement of a tumor in comparison to the
surrounding normal myocardium.
Lymphoma is another malignancy that is well known to affect the
anterior mediastinum, although it may be present in any of the 3
mediastinal compartments. This entity will be discussed within the
Tumors of the middle mediastinum
Most middle mediastinal tumors are mainly lymphatic in origin,
including lymphoma and metastatic tumors, especially in the lung
and breast. Also, esophageal cancer can occur as a primary
malignancy of the middle mediastinum (Figure 9). However, both
lymphoma and metastatic disease may be found in any of the
Lymphoma is one of the most common mediastinal neoplasms. The 2
major classifications, Hodgkin's disease (HD) and non-Hodgkin's
lymphoma (NHL), both may affect the mediastinum. Typically, HD
involves the mediastinum, whereas NHL involves the lungs; but with
extensive or disseminated disease, there may be an overlap between
both regions. The majority of patients with HD have an abnormal
chest radiograph, while less than half of patients with NHL have
abnormalities on chest radiography.
On CT, both mediastinal HD and NHL may present as multiple, rounded
soft tissue masses that correspond to individual lymph nodes or as
a bulky soft tissue mass due to coalescence of lymphadenopathy.
Homogeneous soft tissue attenuation is typical, although
heterogeneity with areas of low attenuation may be present, which
represent necrosis, hemorrhage, or cyst formation. Calcification is
rare in untreated patients. CT may reveal associated mediastinal
infiltration and displacement, compression, or invasion of the
airway and/or vascular structures, such as the superior vena cava
(SVC), the pericardium, and the heart. Involvement of the pleura,
lung, and chest wall may also be seen.
Accurate staging of lymphoma can result in improvement in patient
outcomes, as disease extent is an important factor in determining
appropriate therapy. Contrast-enhanced CT is currently the standard
imaging procedure for the staging and restaging of lymphoma. Lymph
node involvement by CT is defined by size criteria, with 10 mm
measured in the short-axis plane considered the upper limit of
normal. This is problematic, as small lymph nodes harboring active
disease may be overlooked on CT, while benign, inflammatory
lymphadenopathy may be misdiagnosed as lymphoma. Several studies
have shown the value of 18-fluorodeoxyglucose PET (FDG-18-PET) in
the baseline evaluation of lymphoma
; and FDG-PET has also been shown to be superior to CT for the
staging of lymphoma.
Another useful application of FDG-PET is the assessment of disease
response after treatment (restaging), where active disease can be
differentiated from fibrotic tissue in residual masses identified
There is also data that establishes the ability of FDG-PET to
determine a prognosis. A negative PET scan obtained after treatment
has been shown to predict a low risk of relapse and longer
progression-free survival compared with positive PET results.
Esophageal carcinomas most commonly involve the distal esophagus
and are of adenocarcinoma histology, usually in the setting of
pre-existing Barrett's esophagus. These aggressive malignancies are
often associated with a dismal prognosis, with surgery alone
resulting in a survival rate of only 6% to 40% at 9 to 24 months.
Multimodality therapy has been shown to improve survival in a
certain subset of patients
; therefore, accurate staging is essential to guide treatment
planning and identify patients who may benefit from multimodality
therapy. Traditional noninvasive staging modalities include CT,
endoscopic ultrasonography (EUS), and FDG-PET. Endoscopic
ultrasonography is useful for the evaluation of the depth of
primary tumor penetration within the wall and invasion of
periesophageal tissues. Endoscopic ultrasonography has a reported
accuracy for determining T and N staging of 85% and 75%,
respectively, and a sensitivity ranging from 85% to 95% and 70% to
However, staging by EUS is not possible in all patients with
esophageal cancer because of esophageal stenosis or obstruction by
tumor. Endoscopic ultrasonography is also limited for staging
distant metastases. In addition to evaluation of the primary tumor,
compared with EUS, CT has an additional benefit in the evaluation
of mediastinal invasion, spread to lymph nodes, and distant
metastases. Metastatic disease typically involves distant lymph
nodes (usually abdominal), liver, and lung. Assessment of the
primary tumor with CT is limited without contrast distension of the
esophagus, but may be useful in assessing response to therapy.
It has been shown that FDG-PET is more effective than CT in
detecting distant metastases.
This is an important advantage, as the identification of patients
with distant metastatic disease avoids unnecessary surgery and
ineffective radical therapies that may be associated with high
morbidity in patients with incurable disease. In fact, the
recognition of local or distant metastases with FDG-PET has been
shown to be highly predictive of survival.
Recently, FDG-PET has also been shown to be an accurate test to
predict long-term survival after preoperative chemoradiation
therapy (CRT), with a post-CRT FDG-PET standard uptake value (SUV)
of ≥4 associated with a significantly worse 2-year survival rate
than those with an SUV of <4.
This could assist in customizing therapeutic regimens that minimize
futile treatment associated with high morbidity while maximizing
In non-small-cell lung cancer (NSCLC), the presence of
mediastinal adenopathy critically impacts treatment planning and
potential disease resectability. Ipsilateral mediastinal or
subcarinal adenopathy constitutes N2 disease and may be resectable.
Contralateral mediastinal adenopathy, ipsilateral scalene, or
supraclavicular adenopathy indicate N3 disease, which is
nonresectable. Lymph node size on chest CT is the most common
criterion for the identification of abnormal lymph nodes, with a
short-axis nodal diameter of 10 mm, which is generally considered
the upper limit of normal.
Unfortunately, CT may commonly overestimate or underestimate
disease extent within the mediastinum. Imaging with FDG-PET has
been shown to be useful in the detection of nodal disease. In a
meta-analysis of nodal staging comparing PET and CT, the mean
sensitivity and specificity of PET was 79% and 91%, respectively,
compared with 60% and 77% for CT, respectively.
Similar usefulness of PET has been shown for nodal staging in
patients with breast cancer. Metastatic spread to the mediastinal
lymph nodes commonly occurs in patients with breast cancer.
The failure to recognize mediastinal disease may lead to treatment
failure and/or the development of distant metastatic disease in
patients diagnosed with only locoregional disease by conventional
imaging modalities. In a study of 33 patients with advanced breast
cancer who underwent both FDG-PET and CT as part of their
diagnostic evaluation as well as confirmation by biopsy or
follow-up imaging, the sensitivity, specificity, and accuracy for
identification of nodal disease by FDG-PET was 85%, 90%, and 88%,
respectively, compared with 54%, 85%, and 73%, respectively, by
prospective interpretation with CT.
Integrated PET/CT represents an exciting advance in FDG-PET
imaging, as it offers improved anatomic resolution of FDG-avid
lesions. Integrated PET/CT has been shown to be more accurate than
either PET or CT alone in tumor staging in NSCLC
(Figure 10). In a recent study comparing PET, CT, and fused PET/CT,
the reported accuracy for detecting metastatic mediastinal lymph
nodes was 63% for CT alone, 89% for PET alone, and 93% for PET/CT.
Sensitivity and specificity for PET/CT was 89% and 94%, for PET 89%
and 89%, and CT 70% and 59%.
Similar results have been reported for the use of PET/CT in the
staging and restaging of lymphoma. A recent study by Schaefer et al
was performed comparing fused PET/CT with contrast-enhanced CT in
60 patients with HD or NHL. The sensitivity of PET/CT and
contrast-enhanced CT was 94% and 88%, and the specificity was 100%
and 88%, respectively. The promising technology of fused PET/CT in
lung carcinoma, esophageal carcinoma, and lymphoma will lead to
further utilization in other mediastinal diseases.
Tumors of the posterior mediastinum
Neurogenic tumors constitute the majority of posterior
mediastinal mass lesions
(Figure 11). Neurogenic tumors are generally categorized as arising
from either peripheral nerves or sympathetic ganglia. Schwannoma,
neurofibroma, and malignant tumor of nerve sheath origin arise from
peripheral nerves. Ganglioneuroma, ganglioneuroblastoma, and
neuroblastoma arise from sympathetic ganglia.
Schwannomas and neurofibromas are the most common mediastinal
The malignant counterpart to these benign tumors is the malignant
tumor of nerve sheath origin (MTNSO). These tumors usually present
as lobulated, spherical paraspinal masses. On cross-sectional
imaging, neurofibromas are usually sharply marginated, homogeneous,
nonencapsulated lesions. On CT, schwannomas are frequently
heterogeneous with areas of low attenuation that correspond to
areas of cystic degeneration, hypocellularity, hemorrhage, lipid,
or myelin. Punctate calcifications may also be detected on CT. Both
types of tumors may display homogeneous, heterogeneous, or
peripheral enhancement following intravenous contrast
administration. Roughly 10% of schwannomas and neurofibromas grow
through the adjacent intervertebral foramen and extend into the
spinal canal with a "dumb-bell" or "hourglass" configuration.
Malignant tumors of nerve sheath origin may invade adjacent chest
wall or mediastinal structures. MRI is indicated in all patients
with a possible neurogenic tumor to ex-clude intraspinal tumor
extension. MRI typically exhibits low-to-intermediate signal
intensity on T1-weighted imaging and may show areas of
intermediate-to-high signal intensity on T2-weighted imaging.
The sympathetic ganglia tumors usually affect children and young
adults and include neuroblastoma, ganglioneuro-blastoma, and
ganglioneuroma. These tumors originate from the nerve cells, as
opposed to the nerve sheaths. Ganglioneuroma and
ganglioneuroblastoma typically occur in the posterior mediastinum.
On CT, ganglioneuroma may present as a well-marginated, elongated,
homogeneous or heterogeneous, paraspinal mass. MRI is useful to
accurately define the presence and degree of intra-spinal
extension. Homogeneous, intermediate signal intensity may be seen
on all sequences. Occasionally, there may be a whorled appearance
on T1-weighted images and heterogeneous high signal intensity on
Neuroblastomas are typically found in young children.
Approximately 50% arise within the adrenal gland, with the
mediastinum being the most common extra-abdominal location. When
mediastinal in location, neuroblastoma is typically an elongated
paraspinal mass. It may displace or invade adjacent structures,
cross the midline, and/or cause extensive skeletal erosion. On
plain film, calcification may be detected. On CT, calcification is
shown in approximately 80% of neuroblastomas. Neuroblastoma is
typically heterogeneous on both CT and MRI. MRI may be preferred
over CT because of its superior soft tissue contrast and its
ability to delineate intraspinal involvement. The lack of ionizing
radiation is another advantage, as neuroblastoma is a disease of
the pediatric population. MRI exhibits heterogeneous low signal on
T1- and high signal on T2weighted imaging. Radionuclide imaging
with iodine metaiodobenzylguanidine is indicated for the evaluation
of primary and metastatic neuroblastoma. Metaiodobenzylguanidine is
an analogue of catecholamine precursors that is relatively specific
for tumors of sympathetic origin.
Mediastinal tumors constitute a wide variety of lesions because
of the diverse anatomic structures within the mediastinum. It is
important for radiologists to understand normal mediastinal
structures, mediastinal compartments, and common mediastinal
malignancies in order to detect and characterize mediastinal