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1171

Review

Masses Larry

of the Anterior

R. Brown1

and Gregory

Mediastinum:

This review describes

state-of-the-

CT and MR imaging of the anterior mediastinum. After a discussion of CT and MR imaging and indications for their use, art

normal

and abnormal

CT and MR findings

tinum are reviewed. Abnormalities neoplasms, cysts, and mediastinal as thymolipomas,

goiters,

cysts,

in the anterior

medias-

include benign and malignant

stance, in some cases, MR imaging can distinguish fibrosis from viable tumor. The ability to obtain sagittal and coronal images with MR imaging can add information on extent and localization

of disease.

We review the efficacy of CT and MR imaging evaluation of the anterior mediastinum.

in the mid 1970s, CT has emerged

as the

Indications

CT is used most often to evaluate if a definite radiography.

of the mediastinum diagnostic is present

the anterior

or possible abnormality Usually the abnormality

mediastinum

is detected is either diffuse

or a focal bulge or mass.

on chest widening

CT improves

accuracy in determining whether a real abnormality in the mediastinum. CT also often can show the

main technique used to evaluate the anterior mediastinum after plain film studies. A major advantage of CT over plain films is better contrast discrimination, which permits distinction between solid, fatty, cystic, calcified, and vascular structures. Also, cross-sectional imaging eliminates the problem of superimposition of mediastinal structures. Thus, CT is better

exact nature and extent of masses. A second common indication for CT is to search

at localizing

and mediastinal parathyroid parathormone production. MR imaging is usually

and revealing

diastinal abnormalities better at differentiating

can preclude More

the exact

extent

of anterior

me-

than is the plain film. CT is usually benign from malignant processes and

the use of more invasive diagnostic

recently,

MR imaging

has been found

procedures. useful,

usually

as an adjunct to CT, for clarifying problems encountered with CT or for examining patients who cannot tolerate IV administration of contrast distinguish between Received 1

March

material.

some

15, 1991;

Both authors: Department

MR

tissues

accepted

imaging

can,

at times,

that CT cannot.

after revision

For in-

anterior

mediastinal

December

abnormality

but is not shown on chest include thymoma in patients carcinoid

in patients

with

that

is clinically

radiographs. Examples with myasthenia gravis, ectopic

corticotropin

adenoma reserved

in patients for clarifying

for an

suspected

of this thymic

production,

with ectopic problems

encountered on CT or to examine patients who cannot tolerate IV administration of contrast material. The flexible orientation of the plane of section allows MR to image mediastinal

masses

in virtually

any projection,

by CT. The disadvantages include decreased spatial

an advantage

not shared

of MR imaging of the mediastinum resolution compared with that of

July 23, 1991.

of Diagnostic Radiology, Mayo Clinic and Mayo Foundation,

200 First St. SW.,

L. A. Brown. AJR 157:1171-1180,

in the

thyroid. Several masses such and lymphangiomas often do

not require removal and now can be diagnosed with reasonable accuracy when imaging results are combined with clinical history. Detection, diagnosis, staging, and follow-up of malignant anterior mediastinal masses are important and have been improved with CT and MR imaging.

Since its advent

CT and MR Imaging

L. Aughenbaugh

CT and MR imaging allow earlier diagnosis and more specific characterization of anterior mediastinal masses than is possible

with plain film radiographs.

Article

1991 0361-803X/91/1576-1

171 © American

Roentgen

Ray Society

Rochester,

MN 55905.

Address reprint requests to

BROWN

1172

CT and the inability

to detect

pathologic

AND

calcification

accu-

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rately. Motion artifact, a problem in early MR imaging of the mediastinum, has been reduced considerably with ECG-gated sequences.

Technique Our

CT

thickness gland

procedure

is to obtain

at 1 0-mm

to the level

intervals

images

from

of the adrenal

of 1 0-mm

slice

the level of the thyroid

glands

in order

to evaluate

the entire chest, including the mediastinum. Window widths and levels of 400 and 20, respectively, are good for mediastinal imaging.

One hundred

thirty

kilovolts

and 1 1 0 mA with a

38- to 40-cm field of view are used. Targeted with

a 25-cm

field

of view

is occasionally

reconstruction

AUGHENBAUGH

AJR:157, December 1991

contains the thymus, mediastinal fat, lymph nodes, and internal mammary arteries and veins. On axial CT images, the upper anterior mediastinum is triangular, with its apex pointing anteriorly toward the sternum. The apex continues anteriorly as the anterior junction line between the anteromedial aspects of the two lungs. This is a potential space that contains fat in patients more than 40

years old. The anterior junction line ends over the base of the heart, and below this level the anterior mediastinum represents the space between the heart and sternum. The thymus is usually visible in patients younger than 40 years. It is an arrowhead-shaped structure anterior to and molded by the great vessels (Fig. 1). It consists of two lobes, right and left, although the interface between the lobes can be distinguished in only one third of normal glands. Using CT, Baron et al. [3]

done

to improve

imaging of a particular region of interest. Contrast material given IV is often unnecessary

showed thick and dense glands in patients younger than 19 years and progressive loss of thickness and density with

in evaluat-

advancing age. By about age 40 years, the thymus usually is almost completely replaced by fat. Residual glandular tissue is visible only as a slightly grainy appearance of the fat. Mediastinal fat is the most signal-intense, solid tissue seen

ing the anterior mediastinum, because structures are usually separated by enough fat to eliminate confusion. If contrast material is to be used, an unenhanced study should be done first to evaluate high-attenuation features of certain mediastinal abnormalities: calcification of lymph nodes and neo-

plasms, content

milk of calcium in mediastinal in goiters, and high hemoglobin

hemorrhage

diagnosed

[1].

cysts, high iodine content in recent

Some

lesions,

however,

if IV contrast

material

is given.

are

more

easily

sufficient for vascular structures to be identified easily, for enhancement of a mass to be evaluated, and for the various mediastinal structures to be distinguished confidently. The recent availability of programmable, high-volume, low-flow-

rate injectors has permitted a more uniform and higher volume of contrast material to be delivered. This, combined with rapid or dynamic scanning with conventional scanners or use of ultrafast CT, permits excellent visualization of the entire mediastinum in a much shorter time.

The most common

findings

technique

for MR imaging

uses spin-

sequences. Ti -weighted images are obtained fat from nonfatty structures. In our practice,

on these images are normal, the examination

terminated.

If an abnormality

are obtained

to characterize

is present,

T2-weighted

tissues further.

to if

can be images

To decrease

flow

artifacts in vessels, we use a spatial presaturation technique. Ordered phase encoding is used to decrease respiratory artifacts, and cardiac gating is used to decrease cardiac motion artifacts. Although transverse sections are generally

used, coronal or sagittal views are obtained

Normal

when appropriate.

CT and MR Findings

The anterior mediastinum mediastinum lying posterior

is defined as that part of the to the sternum and anterior to

the great vessels and pericardium. The mediastinal pleura in contact with the anteromedial aspect of each lung forms the

lateral boundaries. The anterior mediastinum extends from the diaphragm inferiorly to the thoracic inlet superiorly [2] and

MR images of the mediastinum.

The thymus

is more easily recognized on MR imaging than on CT. shape, size, and signal intensity are age dependent. In dren, the thymus has an intermediate intensity similar to of muscle on Ti -weighted images. As fatty replacement

gresses

Various techniques have been advocated for the administration of contrast material. Optimally, opacification should be

echo pulse distinguish

on Ti -weighted

in childhood,

the thymus

assumes

The chilthat pro-

a higher intensity.

On T2-weighted images, however, the relaxation time of the thymus is similar to that of fat and does not change with age. The thymus appears larger and more discrete on MR imaging than on CT. On T2-weighted images, fluid and some neo-

plasms increase in signal intensity, whereas muscle and, to a lesser extent, fat decrease in signal intensity. The vessels of the mediastinum that contain flowing blood commonly have no evident

signal,

which

tures from mediastinal

Abnormal

allows

vessels

easy distinction

of solid struc-

[4] (Fig. 2).

CT and MR Findings

Differentiation by CT between a large, normal thymus and a thymic mass is sometimes difficult, particularly in younger patients with a large thymus. A thymic mass usually appears

as a solid, oval or rounded density, with the bulk of the mass lateral to the midline. A mass does not conform to the normal contours of the mediastinum (Fig. 3). The presence of lobulation usually indicates a mass rather than a large gland. Obliteration of normal tissue planes indicates an infiltrating or invasive tumor. Occasionally, only one lobe of the thymus is visible and, if large, may be confused with a mass. In this circumstance, the lobe usually retains its elongated shape and drapes around the mediastinum rather than standing apart from mediastinal structures. The thymus undergoes

atrophy and regrowth for malignant

regrowth

disease.

during various phases of chemotherapy In patients

of the volume of the thymus

younger

than

may exceed

35 years,

50% after

chemotherapy or after recovery from an illness. This is called thymic rebound. Sudden rebound of the thymus may be mistaken for a mass [5]. We have seen the thymus enlarge

MASSES

AJR:157, December 1991

OF ANTERIOR

1173

MEDIASTINUM

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Fig. 1.-30-year-old woman with normal thymus. Thin right and left lobes of thymus (arrows) form an arrowhead configuratIon on CT scan.

Fig. 2.-Spin-echo TI-weighted image of normal medlastlnum In an adult High signal intensity of normal thymus (arrows) is due to fatty replacement.

the trachea

to deviate

is almost

always

of thyroid

origin.

Calcification within an upper mediastinal thyroid gland is a common finding. In children, a mass in this location is usually

a cystic hygroma. Most anterior mediastinal masses below the level of the clavicles have a similar appearance on plain films;

Fig. 3.29-year-old man with recent onset of myasthenla gravis and thymoma that was seen to be invasive at surgery. CT shows oval mass (arrow) of left lobe of thymus consistent with diagnosis of thymoma.

however,

resection

with

acromegaly

5), and it can resemble

(Fig. 4) or Graves

a thymic

disease

(Fig.

mass [6].

With MR imaging, most masses of the mediastinum are identified by their morphology, although fluid and fatty masses often have features that allow more specific tissue characterization [4]. Abnormalities of the middle mediastinum, such as an ascending aortic aneurysm or a pericardial cyst, can project anteriorly and simulate an anterior mediastinal mass. A mass

in a cardiophrenic angle, such as a fat pad, enlarged lymph node, hernia of the foramen of Morgagni, or pericardial cyst, lies anteriorly, but these actually originate in the middle mediastinum.

Discussion

of these

abnormalities

is beyond

the

such

as pleural

or biopsy

in some

cases.

The distinguishing

thymic

epithelial

cells

mixed

with

cytologic results and no extension outside Cytologically, malignant thymomas are thymic carcinomas than as malignant carcinomas have a worse prognosis than

Mediastinal

Abnormalities

On plain films of the chest in an adult, any anterior mediastinal mass that involves the upper mediastinum and causes

features

lymphocytes.

the thoracic cavity). better described as thymomas. Thymic thymomas [7].

The mean age at diagnosis is 52 years. The frequency in men and women is approximately equal. Almost half these patients have myasthenia gravis. An additional iO% have other paraneoplastic phenomena, including hypogammaglob-

ulinemia

and anemia gravis

due to erythrocyte

not associated

with

thymoma

hypoplasia. is usually

ated with thymic lymphoid follicular hyperplasia, or may not cause enlargement of the thymus. Anterior

nodules,

Thymomas may be encapsulated (with intact fibrous capsules), invasive (with benign-appearing cytologic characteristics but infiltrative growth), or metastasizing (with seeding of pleural surfaces and pulmonary parenchyma but with benign

thenia

scope of this review.

findings

of several of these masses are described here. Thymoma.-A thymoma is composed of benign-appearing neoplastic

in patients

associated

pleural effusions, pulmonary nodules, or infiltration into the adjacent lung parenchyma can help in difterential diagnosis. Clinical findings also can help in differentiating one lesion from another. Thymoma is the most common neoplasm of the anterior mediastinum in adults. The description of its features on plain films of the chest will apply to the other tumors discussed later. Once a mass is discovered on plain films of the chest, CT or MR imaging may show certain features that are unique to a specific lesion and often enable the radiologist to make a definitive diagnosis. This eliminates the need for

Myasassoci-

which may In general,

symmetric, diffuse thymic enlargement in patients with myasthenia gravis indicates hyperplasia. However, 25-50% of patients with myasthenia gravis and histologic hyperplasia

will show a normal-sized Chest

radiographs

gland on CT.

of patients

with thymoma

show

a mass

BROWN

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1174

FIg. 4.-52-year-old man with acromegaly. A, CT scan shows enlarged thymus gland or thymoma (arrows). B, 4 months later, after resection of pituitary adenoma, thymus expected appearance for patient’s age.

Fig. 6.-Posteroantenor chest radiograph shows enlargement and slight lobulation of right cardiac margin (arrows) due to large right anterior mediastlnal thymoma.

projecting These

can be extremely

small,

(arrows)

subtle,

of cases.

oval opacities

(17% of cases) [8]. Large masses projecting to the right or left of the cardiac margin are easily mistaken for the heart (Fig. 6). The presence of subtle lobulation and increased density of the heart in that area along with a normal cardiac configuration on the opposite side helps to identify the abnormality as a mass rather than an abnormal cardiac contour.

Large thymomas

AUGHENBAUGH

has

returned

AJR:157, December1991

to its

Fig. 5.-24-year-old woman with Graves disease and fullness of anterior mediastinum on chest radiograph. CT scan shows symmetrically enlarged thymus with thickened right and left lobes (arrows) related to Graves disease.

Fig. 7.-58-year-old man examined because of nodule seen on lateral chest A, CT scan at mid chest level shows it is a pleural nodule (arrow). B, At a slightly lower level, CT scan shows small unsuspected invasive Diagnosis: thymoma with pleural metastasis.

over some portion of the hilum in 50-80%

masses

AND

that project to both sides of the mediastinum

usually are malignant [9]. Calcification occurs in 7% of cases and can be coarse, dense, and irregular or ringlike. Calcification can occur in invasive or encapsulated tumors. Nearly one fourth of thymomas in one series were not detected initially on the posteroanterior radiograph [8]. When thymomas were missed, the average delay in final diagnosis

radiograph. thymoma

was 41 months. This delay often allows extensive infiltration of these tumors. Metastases from

(arrows).

growth and a thymoma

usually involve the pleura, so that the presence of a pleural metastasis accompanying an anterior mediastinal mass makes thymoma likely (Fig. 7). Smaller tumors can be detected by CT, which allows an accurate search for thymoma in patients with myasthenia gravis (Fig. 8). CT is helpful in judging the extent of tumor,

but in some cases fibrous adherence of tumor without invasion may be confused with infiltration of tumor in the mediastinum. In general, preservation of fat planes around the tumor margins is strong evidence that the thymoma is not invasive. If the fat planes are completely obliterated, the tumor usually is invasive. When the fat planes are only partially preserved, only about half the tumors are invasive [10]. Attenuation is

AJR:157,

MASSES

December1991

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Fig. 8.-36-year-old woman with gravis and normal findings on chest CT scan shows a 3-cm encapsulated (arrow) in anterior mediastinum.

OF

ANTERIOR

MEDIASTINUM

1175

myasthenia radiograph. thymoma

Fig. 9.-58-year-old woman with homogeneous anterior mediastinal mass on unenhanced CT scan. Enhanced CT shows thymoma (arrows). MIxed attenuation qualities of mass simulate interfaces. Pathologically, thymoma was found to be Invasive.

An anterior mediastinal mass accompanied by pleural metastasis, myasthenia gravis, anemia, or hypogammaglobulinemia

The presence of an anterior mediastinal mass in patients with clinical signs of Gushing syndrome or other tumors of the MEN I syndrome makes mediastinal carcinoid the most likely diagnosis. Germ cell tumors.-These tumors arise from primitive cells that differentiate into embryonic and extraembryonic structures; the result is a spectrum of benign and malignant neoplasms. Histopathologic variants include teratoma, seminoma, embryonal cell carcinoma, yolk sac tumor, choriocarcinoma, and mixed types. Teratomas account for most mediastinal germ cell tumors. Teratomas can be further subdivided into (1) mature solid, (2) cystic (dermoid cyst), (3) immature, (4) malignant (teratocarcinoma), and (5) mixed. Mature teratomas account for 75% of mediastinal germ cell tumors. They are composed of several tissue elements that

in a patient

abnormally

often more heterogeneous in invasive thymoma after IV administration of contrast material (Fig. 9). Assessment of tumor during surgery is the most reliable means of determining invasiveness. A cystic component is present in one third of cases, but in our experience, a cyst is rarely seen by CT [8].

On MR images, thymomas appear as oval or lobulated masses with a signal intensity on Ti-weighted images similar to that of muscle. Signal intensity increases and approaches that offat on T2-weighted images. Sagittal images are useful, allowing another dimension of the tumor to be evaluated for invasion or adherence. However, CT has proved superior to

MR imaging in the evaluation

of thymomas

spatial

definition

resolution

and thymic

40-60

years old makes

because

in a shorter

the diagnosis

of better time

[1 1].

of thymoma

a strong possibility. Neuroendocrine

tumors.-These

tumors

are usually

corn-

posed of small, uniform round cells. A spectrum of malignancy is found in these tumors, from the well-differentiated mediastinal carcinoids to atypical carcinoids, and finally to the most malignant variety-the oat cell carcinoma of the thymus. Thymic carcinoid, which is thought to arise from the neural crest, is by far the most common neoplasm in this group, but among thymic tumors it is still relatively uncommon. As with other neuroendocnne neoplasms, such as pheochromocytoma, islet cell tumor of the pancreas, medullary carcinoma of the thyroid, and small-cell carcinoma of the bronchus, thymic carcinoid is often hormonally active. In cases of thymic carcinoid, ectopic corticotropic hormone production, which

causes [12].

Cushing

syndrome,

in 34-38%

of patients

and pituitary

may be associated with other adenoma, islet cell tumor of the

adenoma

when it is part of the multiple

endocrine neoplasia (MEN), type I, syndrome. Findings on plain films of the chest are often normal or equivocal [13]. A CT scan is often necessary for diagnosis

because

the development

of two

or three

reported

[1 7]. Few teratomas

are totally solid, although

nearly

all contain a solid component. In our experience, MR imaging has been helpful in detecting cystic, solid, and fatty areas within the tumor, which helps in differential diagnosis (Fig.

12).

Nonfunctioning carcinoids tumors, such as parathyroid

pancreas,

is found

recapitulate

embryonic layers (ectoderm, endoderm, and mesoderm) [14]. They usually contain mostly ectodermal elements such as sebaceous material, hair, calcification, and a large cystic component. They are more frequent in young adults, and the benign type is more common in females. The plain film appearance of teratoma is similar to that of thymoma. On CT, most teratomas contain a prominent cystic cornponent, small dense localized areas of calcification or ossification, and, in up to half, either fat or mixed low-density material with an attenuation value nearer to that of fat than that of water [15, 1 6] (Fig. 1 1). Fat/fluid levels also have been

these tumors

may be quite small and metastasize

early (Fig. 10). Osteoblastic

metastasis

is typical

of carcinoid.

Malignant

germ

cell tumors

usually

and more than half are seminomas.

occur

in young

Seminomas

males,

are radiosen-

sitive and are highly curable, whereas nonseminomatous malignant germ cell tumors are treated with a combination of surgery and chemotherapy and have a less favorable prognosis. On CT, seminomas are large, bulky, uncalcified tumors with relatively homogeneous attenuation and smooth margins. If

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1176

BROWN

A

AND

AUGHENBAUGH

AJR:157, December1991

B

Fig. 10.-A, 28-year-old man with Cushing syndrome due to ectopic corticotropic hormone production by thymic carcinold. CT scan shows calcification within tumor (arrows). (Reprinted with permIssion from Brown et al. [13].) B, 39-year-old woman with Cushing syndrome. MR image shows thymic carcinoid of left anterior medlastinum (arrow).

Fig. 11-22-year-old man with left anterior mediastinal mass. CT scan shows encapsulated, cystic, mixed-attenuation mass (arrows) typical of benign cystic teratoma.

A

C

B

Fig. 12.-fl-year-old woman with large anterior mediastinal teratoma. A, Posteroanterior chest radiograph shows massive right anterior medlastinal mass containing curvilinear calcification in inferior medial wall (arrows). B and C, TI-weighted MR images in axial (B) and coronal (C) projections show large mass Is predominantly high intensity, characteristic of fat. Note lower Intensity soft-tissue elements within Its structure.

cystic changes are present, they occupy 25% or less of the tumor. Nonseminomatous malignant germ cell tumors more commonly contain low-density cystic or necrotic areas, or both, with septated architecture that frequently occupy more

usually have Hodgkin disease, nodular sclerosing type. Childhood mediastinal lymphoma is most commonly of the nonHodgkin type. Nodular sclerosing Hodgkin disease is the most frequent form of lymphoma in the anterior mediastinum and

than half the tumor. Calcification may be present [1 8, 19]. Fat planes are obliterated, indicating invasion. After treatment,

is most

malignant germ cell tumors tend to undergo necrosis and cyst formation. Residual masses may remain for years without tumor activity. Some masses may even enlarge as a result of benign factors, such as an enlarging benign teratoma component or necrosis when tumor markers indicate no activity

mediastinum

[20, 21]. Lymphomas.-Adults

with

primary

mediastinal

lymphoma

common

A primary

in women

mediastinal has much

type of anterior

[14].

lymphoma the same

mediastinal

confined

to the anterior

appearance

as any other

mass on plain films. Secondary

signs are common in Hodgkin disease but uncommon in other masses; these include pleural effusion and erosion of the sternum. Calcification is rare but does occur [22]. Harms and Brown (Harms G, Brown LR, presented at the annual meeting of the Society of Thoracic Radiology, February 1987) studied

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AJA:157,

MASSES

December1991

OF ANTERIOR

the CT appearance of primary intrathoracic lymphoma in 95 patients before treatment and found CT to be helpful in the diagnosis. All but 1 4% of lymphomatous masses (that is, separate lesions 4.0 cm or larger) had associated adenopathy in some other region of the chest. Additionally, many of the lesions that appeared as masses on plain films were shown

by CT to have discrete,

well-defined

indicating matted lymph presumed to be necrosis (Fig. 1 4). Others have

nodes (Fig. 1 3). Low-density areas occurred in about half the patients described associated thick-walled

cysts.

Lung invasion

was present

interfaces

within

them,

in one third of these pa-

tients: visible tumor grew along the bronchovascular sheaths. Although patients with other diseases such as sarcoidosis, metastasis, or infection can have adenopathy in the anterior mediastinum, they seldom have the large bulky adenopathy found in lymphoma. An anterior mediastinal mass that consists of a conglomeration of multiple lymph nodes or is associated with detectable adenopathy elsewhere should strongly suggest the diagnosis of lymphoma. Areas of low density that suggest necrosis, the presence of discrete thick-walled cysts within the mass, and lung invasion by tumor are additional helpful findings. After adequate chemotherapy or radiation therapy for lymphoma, a residual mediastinal mass often is present and most often does not indicate active disease [23, 24]. The residual

mass may be cystic or solid. MR imaging

has shown promise

in the differentiation of active from inactive residual masses. In one study involving mediastinal Hodgkin disease, the initial size of the mass seen on T2-weighted images paralleled the size of the residual mass. Inactive treated lymphomas had

significantly

lower

signal

intensity

on T2-weighted

images

than did active lymphomas [25, 26]. Thymic enlargement may be present during staging. If the thymus is involved with lymphoma, accompanying mediastinal

Fig. 13.-17-year-old

girl with nodular

scle-

rosing Hodgkin disease limited to anterior mediastinum. Contrast-enhanced CT scan shows multiple discrete rounded opacities within mass, indicating massive adenopathy. This differs from thymoma shown in Fig. 9 in that interfaces in lymphomatous mass are due to multiple nodes that are present on scans before and after contrast enhancement.

Fig. 14.-21-year-old lymphoma CT scan character

MEDIASTINUM

1177

adenopathy usually is present near the thymus. If smooth thymic enlargement is seen without accompanying adenopathy, an alternative diagnosis such as a large normal gland

is more

likely

treatment

be mistaken Thymic

(Fig. 1 5). The thymus

as a result

of thymic

for recurrence cysts.-These

rebound,

often

enlarges

and this should

after not

[27]. uncommon

cysts

are

found

any-

where along the embryologic course of the thymus. Krech et al. [28] divided thymic cysts into three groups: inflammatory, congenital, and neoplastic. Jaramillo et al. [29] described a fourth type, which is thymic cyst after thoracotomy. Most thymic cysts are probably congenital and derive from a persistently patent thymopharyngeal duct. They represent 1 % of mediastinal masses [1 4]. They are symptomatic in 40% of

cases [30]. Anterosuperior

mediastinal

cysts are most often

found in children and are elongated, often extending from the neck. Anterior mediastinal cysts are not molded by the vascular structures and are usually round or ovoid. They may undergo hemorrhage and calcification. Plain films show a nonspecific mediastinal mass, but CT reveals a thin-walled cystic structure in the mediastinum that has the density of water (Fig. 1 6). We have not found these to be associated with a soft-tissue component. Hemorrhage can cause a density greater than that of water on CT. Occasionally, one or more ringlike calcified structures are present

as a result of previous

hemorrhage.

Although

CT makes the

diagnosis possible, opinion is still divided on whether or not surgery is required in these cases. MR imaging is helpful in the diagnosis of mediastinal cysts and pseudocysts [31]. On Ti -weighted MR images, cysts are low in signal intensity. On T2-weighted images, cysts are high in signal intensity. If the cyst contains blood, the Ti -weighted signal is higher because of the Ti shortening effect of methemoglobin. Cysts associated with neoplasms either before or after

man with non-Hodgkin involving anterior mediastinum. Initial shows predominantly cystic/necrotic of mass (arrows).

Fig. 15.-26-year-old woman with non-Hodgkin lymphoma who had extensive adenopathy of antenor mediastinum before treatment. After radiation therapy and remission, CT scan shows left lobe of thymus (arrow), which remained enlarged but unchanged during a 12-month interval.

BROWN

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1178

AND

AUGHENBAUGH

AJR:157,

December1991

Fig. 16.-A, 72-year-old woman with anterior mediastinal mass of recent onset. Contrast-enhanced CT scan shows anterior mediastinal mass with attenuation and density similar to that of water, typical of benign thymic cyst. Diagnosis was confirmed surgically. B and C, 42-year-old woman who had thymoma resected 8 years earlier. Chest radiograph (B) shows slowly enlarging mediastinal mass (arrows). scan (C) shows benign cyst (arrows) of right thymic bed. Presumably, this developed postoperatively.

treatment

with

radiation

or chemotherapy

may occur

with

thymoma,

phangioma.

On CT, they usually

are not rare. They

teratoma,

lymphoma,

show

and lym-

a fairly prominent

soft-

tissue component, but in some instances, the cyst predominates with little adjacent neoplastic soft tissue. These may resemble true congenital thymic cysts. Thymollpoma.-These

before

detection

thymic

tissue.

tumors

often

and are composed

They

constitute

2-9%

grow

quite

of lobules of all thymic

large

Occasionally

gravis, Graves globulinemia. Plain

films

they

disease, show

are associated

aplastic

with

anemia,

an anterior

mediastinal

of contrast

displaced mediastinal vessels arranged somewhat concentrically around the mass. We prefer CT to MR imaging in evaluating these lesions because of the specific CT appearance. Ectopicparathyroid adenoma.-The parathyroid glands are derived from the third and fourth branchial pouches. Up to

myasthenia

or hypogammamass,

often

of

with attenuation values approaching those of fat (Fig. i 7). On spin-echo Ti -weighted MR images, the mass has a strong signal intensity because of its fatty composition [32]. Mediastinal thyroid.-Most mediastinal thyroid masses are cervicomediastinal goiters extending from the neck into the goiters

mediastinum.

are situated

anterosuperior goiters

of mediastinal

and 20-25%

posteriorly,

mediastinum.

Multinodular

or adenomatous

are characterized

within

75-80%

anteriorly

by a markedly

torted by multiple nodules sule. Areas of hemorrhage, ation

About

the mass

enlarged

causing

smooth

in the

gland

dis-

and surrounded by a fibrous capcalcification, and cystic degener-

displacement

and often

medias-

narrowing

of

the trachea. Areas of calcification frequently are present. These lesions are often identified on radioisotope scans because

of their uptake

of iodine-i

23.

material.

25% of patients

Most

also show

a capsule,

have more than four normal glands

with

the

[34]. In

patients with persistent hyperparathyroidism after adequate parathyroid surgery, an ectopic adenoma or hyperplastic gland located in the mediastinum must be considered. In one series [34], mediastinal parathyroid tumors occurred in 22% of patients operated on for hyperparathyroidism and 38% of those requiring further surgery for hyperparathyroidism. Eighty-one percent of these tumors were located in the antenor mediastinum, and 1 9% were located along the posterolateral wall of the esophagus or in the posterior mediastinum. Patients with profound hypercalcemia tended to have larger tumors. Although some of these ectopic lesions are located within the thymus, most mediastinal extrathymic parathyroid adenomas can be recognized on CT as small rounded masses,

1-2 cm in diameter,

are common.

On plain films, the mass is in the anterosuperior tinum,

Mediastinal thyroid masses also may be identified by CT scanning. Glazer et al. [33] described diagnostic CT findings in five representative cases: (1) anatomic continuity with the cervical portion of the thyroid gland, (2) focal calcification, (3) relatively high CT number on unenhanced CT scans, (4) increase in CT number after a bolus injection of contrast material, and (5) prolonged enhancement after administration

neoplasms

rather low density and quite large. The tumor may mimic an enlarged heart, enlarged fat pad, or a pericardial cyst. CT, however, usually shows a large bilobulated mediastinal mass

anterosuperior

CT

of fat and

and are most common in adolescents and young adults. Because they are relatively soft, they frequently are asymptomatic.

low

in the upper

portion

of the anterior

mediastinum (Fig. i 8). Small isolated lymph nodes in this area can resemble an adenoma. Functioning mediastinal parathyroid cysts have been reported but are rare [35]. Some authors [34] prefer to use MR imaging to localize mediastinal parathyroid adenomas because no contrast material is required and

AJA:157,

December

MASSES

1991

OF ANTERIOR

MEDIASTINUM

1179

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Fig. 17.-Asymptomatic 76-year-old man in whom anterior mediastinal mass was found mcidentally on chest radiograph. Contrast-enhanced CT scan shows large, homogeneously fatty mass (arrow) typical of thymolipoma. Mass was not resected.

Fig. 18.-71-year-old man with persistent hyperparathyroidism after cervical exploration. Small mass (arrow) in anterior mediastinum was identified during surgery as a 1.2-cm ectopic parathyroid adenoma.

artifacts caused by the shoulders rhage into this vascular tumor weighted images show the lesion

are not present. If hemoroccurs, both Ti - and T2to have high signal intensity

Pure hemangiomas of the anterior mediastinum diagnosed by CT if they contain calcified phleboliths.

can

be

[31]. Lymphangioma and hemangioma.-Lymphangiomas and cystic hygromas are tumorlike congenital anomalies consisting of spaces that are lined with endothelial cells and can vary from capillary size to several centimeters in diameter. When these spaces contain chyle, the tumors are classified as lymphangiomas or cystic hygromas, depending on the size of the spaces. Many consider the two as variants of the same anomaly. In our experience [36], purely anterior mediastinal lymphangiomas occur in middle-aged patients, are usually asymptomatic, and have no specific characteristics on radiographs. However, lymphangiomas and cystic hygromas in the anterosupenor portion of the mediastinum can be diagnosed more often. Most of these occur in young patients (average age, 1 1 years), extend from a cervical or supraclavicular location, recur postoperatively, cannot be removed cornpletely surgically because of their proximity to vital structures in the neck, and become infected at some time before surgery. Plain films show elongated masses extending through the thoracic inlet from the neck. The tumor usually causes tracheal deviation. CT scans are diagnostic, showing cervical mediastinal lesions of near water density. The combination of the clinical findings with CT findings makes diagnosis possible. Many lymphangiomas do not require surgery if they do not grow. T2-weighted MR images usually show thin-walled, multiloculated cystic masses of high signal intensity. If the fluid in the mass has a high fat content, Ti -weighted images show relatively high signal intensity [31]. In generalized Iymphangiomatosis (cystic angiomatosis), lymphangiomas can coexist with hemangiomas, and they are difficult to distinguish histologically and with imaging techniques.

Conclusions Whereas many anterior mediastinal masses appear similar on plain films, the use of CT or MR imaging and knowledge of certain important details in the clinical history will allow correct diagnosis in many cases. In a patient 40-60 years old with an anterior mediastinal mass, pleural metastasis, myasthenia gravis, red cell hypoplasia, or hypogammaglobulinemia, thymoma is likely. A similar mass in a patient with Cushing syndrome or MEN I syndrome should be a mediastinal carcinoid. If the mass contains fatty material, dense calcification, cystic areas, or mixed low-attenuation contents, consider teratoma. Lymphomas often present as masses of lymph nodes with adenopathy elsewhere, and lung invasion along the bronchovascular sheaths is not uncommon. Masses purely cystic on CT or MR imaging may be thymic cysts, lymphangiomas, or cystic hygromas, and clinical history and anatomic position are helpful in separating them. On CT, a mass with high attenuation before and after contrast enhancement and arising in anatomic continuity with the lower pole of the thyroid represents a goiter. A mass situated in the anterior mediastinum of a young patient that is purely fatty on CT or MR imaging is usually a thymolipoma. Although some anterior mediastinal masses will not lend themselves to a specific diagnosis, this review emphasizes that a specific diagnosis can be made in many cases, thus aiding in the surgical approach or nonsurgical clinical management.

REFERENCES 1. Glazer

HS, Molina

PL, Siegel

masses on unenhanced 2. Heitzman pathology.

MJ, Sagel

SS. High-attenuation

mediastinal

CT. AJR 1991;156:45-50

ER. The mediastinum: radiologic correlations St. Louis: Mosby, 1977:86-1 23

with anatomy

and

ii 80

3. Baron AL, Lee JKT, Sagel 55, Peterson AR. Computed the normal thymus. Radiology 1982;142: 121 -1 25 4. Spritzer C, Gamsu G, Sostman HD. Magnetic resonance

Downloaded from www.ajronline.org by 190.167.212.148 on 11/06/15 from IP address 190.167.212.148. Copyright ARRS. For personal use only; all rights reserved

5.

6. 7. 8. 9. 10. 11.

12.

13.

14. 15.

16.

17. 18.

19.

BROWN

tomography

AND

AUGHENBAUGH

of

947-951 Foss SD, Kullmann advanced seminoma:

20. imaging

AJR:157,

of the

thorax: techniques, current applications, and future directions. J Thorac Imaging 1989;4(2): 1-18 Choyke PL, Zeman AK, Gootenberg JE, Greenberg JN, Hoffer F, Frank JA. Thymic atrophy and regrowth in response to chemotherapy: CT evaluation. MR 1987;149:269-272 Baron RL, Lee JKT, Sagel SS, Levitt AG. Computed tomography of the abnormal thymus. Radiology 1982;142: 127-134 Lewis JE, Wick MA, Scheithauer BW, Bernatz PE, Taylor WF. Thymoma: a clinicopathologic review. Cancer 1987;60:2727-2743 Brown LA, Muhm JR, Gray JE. Radiographic detection of thymoma. AiR 1980;134:1181 -1188 Ellis K, Gregg HG. Thymomas-roentgen considerations. AiR 1964;91: 105-119 Chen J, Weisbrod GL, Herman SJ. Computed tomography and pathologic correlations of thymic lesions. J Thorac Imaging 1988;3(1):61-65 Batra P, Herrmann C Jr, Mulder D. Mediastinal imaging in myasthenia gravis: correlation of chest radiography, CT, MA, and surgical findings.

and after treatment. 21 . Panicek DM, Toner

AiR 1987;148:515-519 Aosai J, Higa E. Mediastinalendocrine neoplasm, ofprobable thymic origin, related to carcinoid tumor: clinicopathologic study of 8 cases. Cancer 1972;29:1061-1074 Brown LA, Aughenbaugh GL, Wick MA, Baker BA, Salassa AM. Roentgenologic diagnosis of primary corticotropin-produdng carcinoid tumors of the mediastinum. Radiology 1982;142: 143-1 48 Marchevsky AM, Kaneko M. Surgical pathology of the mediastinum. New York: Raven, 1984 Suzuki M, Takashima T, Itoh H, Choutoh 5, Kawamura I, Watanabe V. Computed tomography of mediastinal teratomas. J Comput Assist Tomogr 1983;7:74-76 Brown LA, Muhm JR. Aughenbaugh GL, Lewis BD, Hurt RD. Computed tomography of benign mature teratomas of the mediastinum. J Thorac Imaging 1987;2:66-71 Fulcher AS, Proto AV, Jolles H. Cystic teratoma of the mediastinum: demonstration of fat/fluid level. AiR 1990;154:259-260 Blomli v, Lien HH, Foss#{225}SD, Jacobsen AB, Stenwig AE. Computed tomography in primary non-seminomatous germ cell tumors of the mediastinum. Acta Radiol 1988;29:289-292 Lee KS, Im J-G, Han CH, Han MC, Kim C-W, Kim WS. Malignant primary germ cell tumors of the mediastinum: CT features. AJR 1989;1 53:

27.

22. 23.

24.

25.

26.

28. 29. 30.

31 32.

33. 34. 35. 36.

.

G, Lien HH, Stenwig clinical significance

December1991

AE, Ous S. Chemotherapy of of radiological findings before

Br J Urol 1989;64:530-534 GC, Heelan AT, Bosl GJ. Nonseminomatous

germ

cell tumors: enlarging masses despite chemotherapy. Radiology 1990;175:499-502 Panicek DM, Harty MP, Scicutella CJ, Carsky EW. Calcification in untreated mediastinal lymphoma. Radiology 1988;166:735-736 Thomas F, Cosset JM, Cherel P, Renaudy N, Carde P. Piekarski JD. Thoracic CT-scanning follow-up of residual mediastinal masses after treatmont of Hodgkin’s disease. Radiother Oncol 1988;1 1 : 1 19-1 22 Uematsu M, Kondo M, Tsutsui T, et al. Residual masses on follow-up computed tomography in patients with mediastinal non-Hodgkin’s lymphoma. Clln Radiol 1989;40:244-247 Nyman AS, Aehn SM, Glimelius BLG, Hagberg HE, Hemmingsson AL, SundstrOm CJ. Residual mediastinal masses in Hodgkin disease: prediction of size with MR imaging. Radiology 1989;170:435-440 Webb WA. MR imaging oftreated mediastinal Hodgkin disease. Radiology 1989;170:315-316 Heron CW, Husband JE, Williams MP. Hodgkin disease: CT of the thymus. Radiology 1988;167:647-651 Krech WG, Storey CF, Umiker WC. Thymic cysts: a review of the literature and report of two cases. J Thorac Surg 1954;27:477-493 Jaramillo D, Perez-Atayde A, Griscom NT. Apparent association between thymic cysts and prior thoracotomy. Radiology 1989;172:207-209 Merine DS, Fishman EK, Zerhouni EA. Computed tomography and magnetic resonance imaging diagnosis of thymic cyst. J Comput Tomogr 1988;12:220-222 Molina PL, Siegel MJ, Glazer HS. Thymic masses on MA imaging. MR 1990;155:495-500 Shirkhoda A, Chasen MH, Eftekhari F, Goldman AM, Decaro LF. MR imaging of mediastinal thymolipoma. J Comput Assist Tomogr 1987;1 1:364-365 Glazer GM, Axel L, Moss AA. CT diagnosis of mediastinal thyroid. AiR 1982;138:495-498 Clark OH. Mediastinal parathyroid tumors. Arch Surg 1988;123:10961099 Linos DA, Schoretsanitis G, Carvounis E. Parathyroid cysts of the neck and mediastinum: case report. Acta Chir Scand 1989;155:21 1-216 Brown LA, Intrathoracic

Reiman HM, lymphangioma.

Rosenow EC Ill, Gloviczki PM, Divertie Mayo Clin Proc 1986;61 :882-892

MB.