is a Resident in the Department of Radiology, Brigham and Women's
is an Assistant Professor of Radiology at Harvard Medical School
and a Radiologist at Brigham and Women's Hospital, Boston,
The mediastinum is defined as the area bounded by the two lungs
laterally, the diaphragm inferiorly, the thoracic inlet superiorly,
the sternum anteriorly, and the vertebral column posteriorly. It is
divided primarily into anterior, middle, and posterior compartments
by the most commonly used classification scheme.
The anterior compartment (pre-vascular space) includes all
structures posterior to the sternum and anterior to the
pericardium, aorta, and brachiocephalic vessels. This area includes
lymph nodes, internal mammary vessels, the thymus, and fat. The
middle mediastinum is largely a vascular compartment and includes
the pericardium and its contents, the ascending and transverse
aortas, the superior vena cava (SVC), the brachiocephalic vessels,
the trachea, the phrenic nerve, the upper portion of the vagus, and
the lymph nodes. The posterior compartment (post-vascular space) is
bounded anteriorly by the pericardium and contains lymph nodes,
vertebral bodies, and the contents of spinal canal. This space also
includes the esophagus, the azygos and hemiazygos veins, the
descending aorta, and the intercostal and autonomic nerves. The
assessment of masses relies heavily, but not exclusively, on the
ability to define an "anatomic" location for lesions. This review
will focus on the nonvascular entities that commonly present on
radiologic imaging. Primary tumors of the mediastinum are rare,
comprising about 3% of chest tumors.
The following discussion addresses the most common lesions, rather
than an exhaustive cataloguing of the wide variety of conditions
that may present as mediastinal masses.
Workup of mediastinal masses includes chest radiographs (CXR),
computed tomography (CT), and magnetic resonance imaging (MRI). CT
is better than a CXR in characterizing mediastinal masses as to
fat, fluid, or calcium content, as well as localizing origin and
extent of lesions and identifying associated abnormalities.
The primary role of MR in the evaluation of the mediastinum is
problem solving, as it shows some superiority in characterizing
various tissues. T2- and T1-weighted fat-saturation sequences are
most useful for this purpose.
Anterior mediastinum/vascular space
More than 35,000 cases of lymphoma are diagnosed annually. Both
Hodgkin's disease (HD) and non-Hodgkin's lymphoma (NHL) affect the
mediastinum, but HD represents 25% to 30% of all cases of lymphoma
and is the most common entity to present as an intrathoracic
The age distribution of HD is bimodal with peak incidence at early
adulthood and after age 50.
At the time of presentation, 85% of patients have chest
and 90% of patients have lymph node enlargement in the mediastinum,
but only 25% of all cases of disease is limited to the chest.
Of the HD subtypes, the nodular sclerosing type most commonly
affects the anterior mediastinum.
Non-Hodgkin lymphoma is histopathologically distinct from HD, with
intrathoracic involvement in 40% of patients at presentation. Only
10% have disease limited to the chest.
Of the NHL subtypes, the large B-cell lymphoma and the
lymphoblastic lymphoma most frequently affect the anterior
Lymphoma may present as nodal enlargement seen as singularly
enlarged nodes or as a conglomerate mass of nodes. On CT, the mass
may be homogeneous or heterogeneous depending on the presence of
calcification, hemorrhage, necrosis, or cystic components
(figure 1). On MR, lymphoma appears as homogenous low-signal on
T1-weighted images and as homogenous high-signal or intermixed
areas of low and high intensity on T2-weighted images.
Thymoma and other thymic neoplasms
Thymomas are the most common primary tumor type in the anterior
There appears to be no sex predilection for thymomas, but most
cases occur in patients who are older than 20 years of age.
There is a common association with myasthenia gravis (MG). Up to
50% of patients with thymoma have MG, but only 10% to 15% of
patients with MG have thymomas,
and some patients have been found to have attenuation of symptoms
following resection of the thymoma.
Half of patients with thymomas are asymptomatic.
Other parathymic manifestations associated with thymoma include
a pure red-cell aplasia and hypogammaglobulinemia. Although most
thymomas are benign and contained by fibrous capsule, a third of
thymomas invade through their capsule even though they remain
These are termed invasive thymomas. There can be transdiaphragmatic
extension, vascular invasion, or mediastinal fat invasion by these
lesions and they may even involve the pleural surface mimicking a
Approximately 30% to 35% of thymomas are malignant.
On CT, thymomas may demonstrate homogenous or heterogeneous
attenuation depending on presence of hemorrhage, necrosis, or
but are usually more solid in appearance than the normal thymus.
They can appear as round, oval, or lobulated masses (figure 2).
Calcification is seen in as many as 25% of cases and is usually
coarse or irregular (figure 3). On MRI, thymomas can have
intermediate- to high-signal intensity on T1-weighted images and
high signal on T2-weighted images (figure 4). Contrast
administration may help define vascular invasion; however, MRI does
not offer a significant advantage over CT in the evaluation of
Cytologically malignant thymic neoplasms are termed thymic
carcinoma and are distinctly separate from invasive thymoma.
Although radiographically similar in their aggressive breaching of
boundaries, the invasive thymoma does not demonstrate the cellular
atypia of a carcinoma. The most common histologic subtypes of
malignant thymic tumors are squamous cell and
Other thymic masses in the mediastinum include thymic cysts
(figure 5), which may be congenital (due to a patent
thymopharyngeal duct) or acquired due either to post inflammatory
conditions or to association with a neoplasm.
Thymolipomas are soft encapsulated masses containing mature
adipose, thymic epithelial cells, and lymphoid tissue.
Substernal goiter accounts for 10% of all mediastinal masses.
Approximately 75% of goiters extend into the anterior mediastinum
while 20% to 25% involve the posterior mediastinum.
CT shows these lesions to be contiguous with the thyroid in the
neck, and demonstrates calcification and areas of cystic change
(figure 6). Precontrast attenuation value may be high due to the
presence of iodine,
and there may be enhancement with intravenous contrast.
The mediastinum serves as the most common site of extragonadal
involvement of germ-cell tumors in adult in the third decade.
Neoplasms in this category include benign and malignant teratomas,
seminomas, and non-seminomatous malignant germ-cell tumors (such as
endodermal sinus tumors, embryonal carcinomas, mixed germ-cell
tumors, and choriocarcinomas). Of these, teratomas are the most
common neoplasms to present in the mediastinum.
The characteristic pathologic and radiographic feature of a
teratoma is the presence of one or more components from the three
embryonic germ-cell layers, hair, calcium, or fat.
Plain radiography usually reveals a large, well-circumscribed
mass causing displacement of adjacent structures. Calcification is
seen in up to 80% of these lesions.
On CT, a benign teratoma is usually cystic and may have fat up to
90% of the time whereas a malignant teratoma is usually more solid
and contains fat only 40% of the time
(figure 7). Fat-fluid levels, considered highly specific for the
diagnosis of mature mediastinal teratoma, are uncommon
(figure 8). CT findings of heterogeneity of the internal components
of the mass and changes in the adjacent lung parenchyma, pleura, or
pericardium may indicate tumor rupture.
Seminomas are most common primary malignant tumor of the
mediastinum and are often metastatic.
They are associated with low levels of beta HCG in contrast to the
non-seminomatous tumors that are associated with higher levels of
They are usually large and lobular on CT and relatively homogenous
in their attenuation.
The non-seminomatous tumors are highly aggressive invasive tumors
that are not usually resectable because of their local invasiveness
and metastatic tendencies.
On CT, they are usually large, lobulated, heterogeneous masses with
areas of hemorrhage and necrosis.
Middle mediastinum/vascular space
These are congenital malformations of the pulmonary and
gastrointestinal tracts caused by abnormal budding of the
bronchopulmonary foregut. These anomalies comprise 10% to 15% of
primary mediastinal lesions. Bronchogenic cysts account for 54% to
63% of such anomalies.
The cyst wall is usually lined by pseudostratified epithelium and
they contain mucoid or serous material, hemorrhage, or pus.
They typically present as masses in the paraesophageal and
tracheobronchial axis and may compress adjacent structures.
These cysts do not usually communicate with the airway. On CT,
these lesions display water attenuation given their mostly fluid
content (figures 9 and 10). On MR, they demonstrate correspondingly
high T2 signal and possibly high T1 signal when high in
Lymph-node masses are other causes of middle mediastinal masses
and include those from sarcoidosis, Castleman's disease,
bronchogenic carcinoma, and infectious causes such as
Posterior mediastinum/post vascular space
Ninety percent of neurogenic tumors present as posterior
mediastinal masses and represent 75% of posterior mediastinal
These tumors commonly arise either from peripheral nerves, or
sympathetic or parasympathetic ganglia.
Schwannomas and neurofibromas are the most common of the
posterior mediastinal masses and arise from peripheral nerves.
They are benign masses that have similar appearances on CXR
manifesting as lobulated paraspinal masses
(figures 11 and 12). They may deform the adjacent bony structures,
and characteristically grow into the spinal canal in a "dumbbell"
or "hourglass" form.
A plexiform neurofibroma is a subtype of neurofibroma that affects
an entire nerve plexus.
Multiple tumors of neural origin or a single plexiform tumor is
considered pathognomonic for neurofibromatosis.
On CT, schwannomas and neurofibromas, in addition to the
characteristics described above, may enhance homogeneously,
peripherally, or heterogeneously and may contain areas of low
density corresponding to myelin, cystic change, or hemorrhage.
MR is necessary in all cases of suspected neurogenic tumors in the
mediastinum so intraspinal extension may be excluded.
Malignant tumors of nerve sheath origin are the malignant
equivalents of the schwannoma and neurofibroma. They do not,
however, arise from preexisting schwannomas but may arise from a
plexiform neurofibroma. Half of these malignant tumors occur in
patients with neurofibromatosis.
Sympathetic ganglia tumors include ganglioneuroblastomas,
ganglioneuromas, and neuroblastomas. These usually appear as
broad-based, oblong masses adjacent to the spine.
Lateral thoracic meningoceles
Lateral thoracic meningoceles are rare posterior mediastinal
lesions that form as a result of herniation of the meninges through
the intervertebral foramen. They are most common in patients with
neurofibromatosis and are the most common cause of a posterior
mediastinal mass in these patients.
They radiologically manifest as a well-defined paraspinal mass and
may be associated with bony erosion, widening of the neural
foramina, and possibly kyphoscoliosis.
MR can determine the presence of herniation of the subarachnoid
space and signal intensity consistent with CSF
Lesions within the mid- and distal esophagus can present as a
posterior mediastinal mass. Esophageal carcinoma is an important
part of the differential diagnosis, commonly occurring as the
squamous cell type and less commonly as adenocarcinoma. On plain
films, this may be recognized by an abnormal tracheoesophageal
stripe, dilation of the esophagus proximal to the obstructing mass.
Diagnosis is confirmed at endoscopy or by barium swallow. CT may
demonstrate an intraluminal mass, thickening of the esophagus, and
metastases. Benign lesions of the esophagus such as leiomyomas
(figure 14), fibromas, and lipomas may also cause masses in this
mediastinal compartment. They usually involve the lower third of
the esophagus and do not commonly cause proximal esophageal
dilation. Other anomalies that may present in this compartment
include diverticula, hernias, or megaesophagus due to achalasia or
Extramedullary hematopoiesis may also present as a posterior
mediastinal mass. Extramedullary hematopoiesis is seen most
commonly in diseases of ineffective erythropoiesis such as
thalassemia and spherocytosis and can also be seen in sickle cell
anemia. Expansion of the medullary cavity is usually associated
with multiple paraspinal masses
Mediastinal masses present a challenge, as they may include a
vast number of diagnoses, each with complex imaging
characteristics. Rigorous scrutiny of the normal contents of each
compartment helps determine aberrancy. The final diagnosis depends
as much on the clinical information (ie, patient demographics and
symptoms), as the radiologic characteristics. Delineating the
character and extent of a mass enough to guide proper diagnostic
sampling and or therapeutic management remains an important