Mediastinal masses: A system for diagnosis based on computed tomography

Mediastinal masses: A system for diagnosis based on computed tomography

CT: THE JOURNAL OF COMPUTED TOMOGRAPHY 1986;l O:ll-21 11 GlMEDIASTINAL MASSES: A SYSTEM FOR DIAGNOSIS BASED ON COMPUTED TOMOGRAPHY DAVID S. FEI...

6MB Sizes 0 Downloads 65 Views

CT: THE JOURNAL

OF COMPUTED

TOMOGRAPHY

1986;l O:ll-21

11

GlMEDIASTINAL MASSES: A SYSTEM FOR DIAGNOSIS BASED ON COMPUTED TOMOGRAPHY DAVID

S.

FEIGIN,

MD,

AND

ERIC

A revised system for mediastinal moss differentiation has been developed and used at the Veterans Administration Medical Center, San Diego, and University Hospital of the University of California, San Diego. Contrast medium infusion or bolus computed tomography of the entire mediastinum is obtained, except when the mass is probably a thyroid mass (lz31 scan then obtained first) or an esophageal mass (barium swallow evaluation then obtained first). Next computed tomography density is determined, and only subsequently are masses of similar density subdivided by location. Further differentiation is based on computed tomography imaging parameters. A new algorithm for evaluation of mediastinal masses has resulted from use of this system. The system has been shown to increase specificity of differential diagnosis and to result in increased efficiency of evaluation of patients with mediastinal masses. KEY WORDS:

Computed tomography, mediastinum; Mediastinum, abnormalities; Radiography, mediastinum

The improved accuracy of computed tomography (CT) over other imaging modalities, especially plain film radiography, is now widely recognized and accepted. Both sensitivity and specificity of mediastinal mass imaging has been well documented

From the Department of Radiology, University of California, San Diego, and the Veterans Administration Medical Center, San Diego, California. Address reprint requests to: David S. Feigin, MD, Department of Radiology (V-114], University of California, San Diego, La Jolla, California 92093. Received January 1985. 0 1986 by Elsevier Science Publishing CO., Inc. 2 Vanderbilt Avenue, New York, NY I 0017 0149-936X/86/$3.50

M.

PADUA,

MD

(1, 2). Despite this acceptance and the wide availability of CT in most areas, an early and critical role for CT in the evaluation of mediastinal masses has rarely been emphasized. The traditional algorithm for evaluation of mediastinal masses (3) emphasizes location-anterior, middle, posterior, or superiorparticularly as demonstrated on the lateral plain film or conventional tomogram. This approach often results in an overly lengthy, nonspecific list of possible diagnoses. A more specific, relevant, and useful differential diagnosis can be achieved by use of an approach that emphasizes, as its primary basis, CT density rather than location. In addition, CT allows redefinition and expansion of the traditional “anterior, ” “middle,” and “poterior” categories of location that were based solely upon the lateral roentgenogram and that were often misleading or difficult to apply. The new algorithm for mediastinal masses is thus based upon CT density, rather than location, as the primary criterion. Location and imaging appearance then become secondary criteria to be used along with clinical information in arriving at a differential diagnosis (Table 1). As magnetic resonance imaging (MRI) begins to demonstrate its value in the evaluation of mediastinal masses, its use may replace or augment the role of CT (4-6). At present, however, the efficacy of MRI for this use remains unproved. This communication deliberately emphasizes an approach to differential diagnosis of mediastinal masses rather than detailed discussions of individual abnormalities. Only the most common masses are considered. Detailed analyses of individual entities may be found in the references.

BASIC

SCHEMATIC

APPROACH

The proposed schema is outlined in Table 1. Most mediastinal masses are visible on standard plain films and, according to recent reports, are even

12

CT: THE JOURNAL

FEIGIN AND PADUA

OF COMPUTED

TOMOGRAPHY

VOL. 10 NO. 1

TABLE 1. Schema for Mediastinal Mass Evaluation

Digital techniques

Suspected metastatic malignancy Evaluation of lymphomas Suspected thymic neoplasms

(optional]

Probable thyroid mass

\

,

COMPUTED TOMOGRAPHY Density Location Other features Shape Edge sharpness Contour Relation to normal structures Effect on normal structures Other abnormalities

DIFFERENTIAL

ULTRASOUND ARTERIOGRAPHY VENOGRAPHY (?)MRI CYST PUNCTURE

more easily visualized using digital chest radiography (7). Overpenetrated frontal views maximize visibility of mediastinal masses and may be helpful, especially in shallow oblique projections, to confirm the presence of a suspected mass. Whenever a mass is strongly suspected or confirmed by such standard radiography, CT should be the next study performed, with the following two exceptions. First, masses widening the superior mediastinum at the thoracic inlet, with tracheal deviation or narrowing or both, usually represent enlarged thyroid glands (Figure 1). Expecially in those cases with clinical evidence of thyroid enlargement in the neck, thyroid scans using lz31 or 1311 should be considered

rather than CT evaluation. If a CT study is performed with contrast medium enhancement before thyroid scanning with iodinated radiopharmaceuticals, the sensitivity of the nuclear study isinhibited by iodination of the thyroid gland from the contrast material, A delay of several weeks is then necessary before optimal sensitivity of thyroid scanning. The second exception to implementation of CT immediately after plain films occurs if the patient admits to dysphagia. Barium evaluation of the esophagus may reveal an intrinsic esophageal lesion that may obviate the need for CT. Even though CT may eventually be indicated for further evaluation of esophageal abnormalities, especially carcinoma

JANUARY

1986

MEDIASTINAL

MASSES

13

FIGURE 1. Mediastinal enlargement, multinodular goiter. (A) Frontal plain film with bilateral widened upper mediastinum and tracheal narrowing. (B) A CT scan at the level of the sternoclavicular junctions shows a large thyroid mass surrounding and narrowing the trachea with displacement of other mediastinal structures laterally and posteriorly. Calcification is present in the right lobe of the thyroid and the overall thyroid density slightly exceeds that of the average muscle.

(8, 9), initial detection is more easily accomplished using standard barium evaluation. In some circumstances, CT of the mediastinum is indicated even in the presence of normal plain films, including digital imaging and overpenetrated oblique radiography. The proven increased sensitivity of CT in the detection of enlarged mediastinal lymph nodes and abnormalities of the thymus has (10-12).Therebeen especially well documented fore, CT is often indicated in clinical circumstances

such as evaluation of the mediastinum for involvement in malignancies, especially bronchogenic carcinoma; evaluation of lymphomas, especially Hodgkin’s disease; and evaluation of patients with myasthenia gravis or red cell hypoplasia for thymic abnormalities. The density of the mass as demonstrated by CT is the primary criterion for differential diagnosis in this system. Specific location is considered only for subcategorization of specific densities. Principal

CT: THE JOURNAL OF COMPUTED

FEIGIN AND PADUA

14

density categories are fat, water or fluid, soft tissue or clotted blood, flowing blood (vessels), or mineral material (usually calcium). Density determination does not necessarily require quantitative measurement by use of narrow windows or other special manipulations of the CT display. Comparison with normal chest wall structures, especially muscles and subcutaneous fat, is usually sufficient to detect principal density. Fluid densities may be difficult to differentiate from soft tissue densities but are often clearly denser than fat and less dense than soft tissue muscles (Figure 1). Insufficient chest wall fat may inhibit comparison in very thin individuals. Following determination of density and location, other CT characteristics of mediastinal masses should next be considered in improving specificity of differential diagnoses (Table 1).Such characteristics include shape, edge sharpness, contour, relation to normal structures, effect on adjacent structures [especially displacement), and the presence of other abnormalities, including those visible in the lungs and abdomen as well as those remote from the mass in the mediastinum. Clinical information, including the age and sex of the patient, is often as useful as any imaging parameter in achieving a logical and useful differential diagnosis list.

DIFFERENTIAL

DIAGNOSIS

The differential diagnosis of the most common mediastinal masses is summarized in Table 2. The ta-

TABLE 2. Differential Diagnosis of Common Mediastinal Location Anterior

Anterior diaphragm Middle

Diaphragm Superior

Fat Lipoma

Water/fluid (Cystic masses; soft tissue)

Pericardial Lipoma

Omental hernia

cyst

Cysts-foregut, respiratory, enteric, neurenteric Pancreatic pseudocyst Lymphangioma

Posterior

Diffuse Lipomatosis

Sclerosing mediastinitis

see

TOMOGRAPHY

VOL. lo NO. 1

ble emphasizes that CT density, although generally more specific than location, may be misleading. Many cystic masses, for example, contain sufficiently thick fluid to appear as soft tissue densities, as will be discussed below. Following determination of density, subdivisions by location should include more choices of specific locations than the three or four regions traditionally used. Division of the mediastinum into seven regions (Table 2) increases specificity of differential choices and emphasizes the absence of sharp boundaries between regions. The anterior mediastinal region has its center behind the midportion of the sternum, above the origin of the pulmonary outflow tract, and to the anterior and left of the ascending aorta. The anterior diaphragmatic region is the lower portion of the traditional anterior mediastinum, including and surrounding the lower portion of the cardiopericardial silhouette. The middle mediastinal region remains as traditionally defined by the trachea and posterior border of the heart anteriorly, and the front of the spine posteriorly. The superior region is the entire area above the top of the aortic arch and thus overlaps the upper portion of the posterior and middle regions and the uppermost tip of the anterior region. The middle and posterior diaphragmatic region (or, more simply, the diaphragmatic region) borders the diaphragm behind the heart shadow. The posterior region is lateral to the spine on either side. Finally, a “diffuse” region should be considered for masses that surround sev-

Masses CT Density Soft tissue

Vascular

Mineral

Thymic lesions, teratoid lessions, lymphomas, benign lymph nodes

Anomalous vessels

Benign lymph nodes; teratomas-(teeth, bones; other masses)

Morgagni hernia Esophageal masses, lymph nodes

Hiatal hernia, Bochdalek hernia Thyroid masses, parathyroids Neurogenic tumorsnerve sheath, ganglion, extramedullary hematopoiesis

Benign lymph nodes, vessels

Anomalies, tortuosity Dissection aneurysm

Thyroid Vessels

masses

JANUARY

MEDIASTINAL

1986

MASSES

15

2. Pericardial cysts. (A) Typical frontal film findings of a mass in the right cardiophrenic angle. (B) A CT scan with contrast medium enhancement at the level of the center of the mass shows soft tissue density, indistinguishable from the density of the heart. FIGURE

eral, or all, normal mediastinal structures. Clearly these definitions of regions may be applied to plain films, especially oblique and lateral films, as well as to CT scans. Fat Density

Masses

Lipomas appear as localized, sharp, fat density masses anywhere in the mediastinum. They are most common adjacent to the pleura and, if small,

may not be clearly fat density. Change in shape and appearance between inspiration and expiration, and with changes in patient position, may be helpful using both plain films and CT scans (13). Omental hernias occur when the abdominal mesentery herniates into the chest, usually through the esophageal hiatus. They often resemble hiatal hernias but are usually of fat density and associated with normal barium esophagrams. A CT demonstration may be facilitated in the prone position be-

16

FEIGIN AND PADUA

CT: THE JOURNAL OF COMPUTED

TOMOGRAPHY

VOL. 10 NO. 1

FIGURE 3. Congenital respiratory cyst. (A, B) Plain films show a middle mediastinal mass on the right with a typical appearance of distinct inferior and posterior borders on the lateral film. (C) Contrast mediumenhanced CT scan at the level of the carina shows a water-density mass [question mark) with a location posterior to the main bronchi that is typical of a respiratory or enteric cyst.

cause standard supine imaging may compress the fatty lesion or force it inferiorly into the abdomen precluding demonstration. Lipomatosis of the mediastinum is usually diffuse but may be most obvious in the anterior mediastinum with CT (14). The triviality of the abnor-

mality and its graphic CT demonstration preclude the necessity for further evaluation after CT. It is remarkable that diffuse lipomatosis is often indistinguishable from soft tissue infiltration using plain films alone (15). Many mediastinal masses contain large amounts

JANUARY

1986

MEDIASTINAL

MASSES

17

FIGURE 4. Neurilemoma, benign. (A, B) Frontal and lateral plain films show a round, discrete mass in the posterior superior mediastinum. (C) A CT scan demonstrates widening of a neural foramen and a soft tissue density both inside and outside the spinal canal.

of fat but are heterogeneous with soft tissue components generally visible with CT. Examples include thymomas, thymolipomas, and, rarely, teratomas.

Water or Fluid

Density Masses

Pericardial cysts typically occur along the right heart border, touching both the anterior portion of the diaphragm and the anterior chest wall (16).Although most are clearly cystic by demonstration of

fluid density with CT, some are indistinguishable from soft tissue masses (Figure 2) (17).Ultrasound may be useful in confirming the cystic nature of the lesion. In those cases in which the imaging leaves doubt of the diagnosis, cyst puncture, guided by CT, fluoroscopy or ultrasound, may be used to avoid surgery (18). Congenital foreguts cysts usually occur in the middle mediastinum, especially just posterior to the carina where the respiratory diverticulum buds anteriorly from the foregut in embryonic development.

18

FEIGIN AND PADUA

CT: THE JOURNAL OF COMPUTED

TOMOGRAPHY

VOL. 10 NO. ‘i

FIGURE 5. Anomalous vein, left superior vena cava. (A) Frontal plain film with abnormal vertical density on the left of the mediastinum extending superiorly to the left hilum. (B) A CT scan with contrast mediumenhancement demonstrates a left superior vena cava.

The specific nature of the cyst can only be confirmed by histology. Enteric and neurenteric cysts usually occur in children and young adults with symptoms of chest or back pain. Congenital respiratory cysts are usually asymptomatic and can occur at any age (Figure 3). As with pericardial cysts, soft tissue density is sometimes manifest and ultrasound and cyst puncture may be necessary (19-21). Pancreatic pseudocysts occasionally present as diaphragmatic mediastial masses, usually of fluid density (22). Clinical evidence of pancreatic disease, especially chronic pancreatitis, is usually obtainable. Lymphangiomas are rare fluid collections in the superior mediastinum of children and young adults. They are cystic collections of lymph, generally regarded as the mediastinal counterpart of the cystic hygroma found in the neck of children. Complex mediastinal masses may contain large amounts of fluid. Cystic thymomas and teratomas, for example, are not uncommon (23, 24). Soft Tissue

Density

Masses

Anterior mediastinal soft tissue masses principally include thymic lesions, teratoid lesions (usually teratomas), lymphomas (esecially nodular sclerosing

Hodgkin’s disease), and benign lymph node enlargements. All these entities have been exhaustively discussed in recent reviews (1, 2, 25). Especially well documented is the inability of imaging modalities, particularly CT, to determine whether a thymic lesion is normal thymus, thymic hyperplasia, benign thymoma, or malignant thymoma. Age and clinical information are as useful as imaging in this differential diagnosis. Malignant thymomas, however, are defined by local invasion through their own capsules, rather than by histologic criteria. Such invasion may be visible by CT, even when the plain films are benign in appearance (26). Lymph node enlargement in the anterior mediastinum is usually accompanied by lymph node enlargement in the middle mediastinum. Exceptions include nodular sclerosing Hodgkin’s disease, which may begin in the anterior mediastinum and be confined to that region. The identification of multiple mediastinal masses as lymph nodes is often obvious. Unfortunately it is usually impossible to differentiate benign from malignant lymph nodes unless specific findings, such as calcification, are observed. Thyroid masses are nearly always associated with enlargement of the thyroid gland in the neck (27, 28). The CT appearance is most distinctive, as

JANUARY

1986

MEDIASTINAL

MASSES

19

FIGURE 6. Aneurysmal ascending aorta, Marfan’s disease. (A) Frontal plain film of a 31-year-old man shows a mass on the right side of the mediastinum, widest at the level of the lower portion of the hilum. (B) Contrast medium-enhanced CT scan at the level of the hilum shows massive dilatation of the ascending aorta; a family history of Marfan’s disease was subsequently elicited.

the thyroid gland uniquely surrounds the trachea (Figure 1). The CT density of functioning thyroid tissue may be perceptibly greater than that of soft tissues such as muscles. This is caused by the prevalence of radiodense iodine within functioning glandular tissue. Although arteriography is usually necessary, CT may have a role in the detection of abnormal parathyroid tissue (29). Most posterior mediastinal masses are neurogenie tumors and are usually benign (30). Nerve

sheath tumors appear round, whereas ganglion series tumors are usually elongated vertically. Often CT is very useful in preoperative evaluation of neurogenic tumors because involvement of the spinal canal may be demonstrated as well as or better on CT than on myelography (Figure 4). The masses of extramedullary hematopoiesis are usually also elongated vertically and may be massive in size (31); such masses are often multiple and evidence of marrow hypoplasia is usually clinically obvious.

20

CT: THE JOURNAL

FEIGIN AND PADUA

OF COMPUTED

TOMOGRAPHY

VOL. lo NO. 1

Diffuse soft tissue density may be the only manifestation of sclerosing mediastinitis, both with plain films and CT (32). In sclerosing mediastinitis, CT is very useful for evaluation of obstruction of veins, arteries, and other mediastinal structures.

4. Axe1 L, Kressel HY, Thickman D, et al.: NMR imaging of the chest at 0.12 T: initial clinical experience with a resistive magnet. AJR 1983;141:1157-62.

Vascular

5. Cohen AM, Creviston S, LiPauma JP, et al.: Nuclear magnetic resonance imaging of the mediastinum and hili: early impression of its efficacy. AJR 1983;141:1163-9.

Density

Abnormalities

The use of contrast medium infusion or bolus injection maximizes the detection of vascular abnormalities with CT. Anomalous vessels, aneurysms, and aneurysmal dilatations are readily differentiated from soft tissue masses, often obviating the need for arteriography or venography (Figure 5) (33, 34). Aortic dissections may be demonstrated by CT with a sensitivity equal to, or even surpassing, arteriography in some series (35). The ability to see the aorta in cross section is an obvious advantage of CT (Figure 6), although MRI may surpass CT in this regard (4, 5). Arteriography is often required to determine the exact extent of a dissection, especially if surgery is contemplated.

Mineral

Density

Abnormalities

The presence of calcification is usually as easily detectable by plain films as by CT. The identification of teeth and bones in teratomas, for example, may greatly simplify diagnosis of an anterior mediastinal mass. The presence of calcification in thymomas and teratomas, however, does not indicate benignancy as it often does when demonstrated in pulmonary masses. Numerous calcified mediastinal malignancies have been reported (3).

CONCLUSION The efficacy of CT as the primary imaging modality for mediastinal masses has been well proved. Its sensitivity and specificity both exceed those of other modalities, especially plain film techniques. Its use is justified in virtually all cases in which mediastinal masses are suspected or demonstrated by other techniques. This focal role for CT becomes especially important in decreasing the use of unnecessary imaging modalities and in decreasing the need and length of hospitalization of patients.

6. Gamsu G, Stark DD, Webb WR, et al.: Magnetic mediastinal masses. of benign imaging 1984;151:709-13.

resonance Radiology

7. Fraser RG, Breatnach E, Barnes GT: Digital radiography of the chest: clinical experience with a prototype unit. Radiology 1983;148:1-5. 8. Picus D, Balfe DM, Koehler RE, et al.: Computed tomography in the staging of esophageal carcinoma. Radiology, 1983; 146:433-B. 9. Thompson WM, Halvorsen RA, Foster WL Jr, et al.: Computed tomography for staging esophageal and gastroesophageal cancer: reevaluation. AJR 1983;141:951-8. 10. Brown LR, Muhm JR, Sheedy PF, et al.: The value of computed tomography in myasthenia gravis. AJR 1983;140:31-5. 11. Kaye AD, Janssen R, Arger PH, et al.: Mediastinal computed tomography in myasthenia gravis. CT 1983;7:273-9. 12. Genereaux GP, Howie JL: Normal mediastinal size and number: CT and anatomic 1984;142:1095-100.

lymph study.

node AJR

13. Chalaoui J, Sylvestre J, Dussault RG, et al.: Thoracic fatty lesions some usual and unusual appearances. J Can Assoc Radiol 1981;32:197-201. 14. Bein ME, Mancuso AA, Mink JH, Hansen GC: Computed tomography in the evaluation of mediastinal lipomatosis. J Comput Assist Tomogr 1978;2:379-83. 15. Viamonte M Jr, Viamonte M: Radiology and pathology CRC Crit Rev Diagn Imaging 1981;93-123. 16. Feigin DS, Fenoglio JJ, McAllister dial cysts: a radiologic-pathologic Radiology 1977;125:15-20.

of fat.

HA, Madewell JE: Pericarcorrelation and review.

17. Brunner DR, Whitley NO: A pericardial numbers. AJR 1984;142:279-80.

cyst with high CT

18. Moncada R, Baker M, Salinas M, et al.: Diagnostic role of computed tomography in pericardial heart disease; congenital defects, thickening, neoplasms, and effusions. Am Heart J 1982;263-81. 19. Marvasti MA, Mitchell GE, Burke WA, Meyer JA: Misleading density of mediastinal cysts on computerized tomography. Ann Thoracic Surg 1981;31:167-170. 20. Mendelson DS, Rose JS, Efremidis SC, et al.: Bronchogenic cysts with high CT numbers. AJR 1983;140:463-5. 21. Nakata H, Nakayama raphy of mediastinal Tomogr 1982:6:733-B.

C, Kimoto T, et al.: Computed tomogbronchogenic cysts. J Comput Assist

22. Ball JB Jr, Clark RA: CT of mediastinal lections. Comput Radio1 1982;6:295-300.

pancreatic

23. Dunne MG, Weksberg AP: Thymic cyst: computed phy and ultrasound correlation. CT 1983;7:351-5.

fluid coltomogra-

24. Suzuki M, Takashima T, Itoh H, et al.: Computed tomography of mediastinal teratomas. J Comput Assist Tomogr 1983;7:74-6.

REFERENCES 1. Brown LR, Muhm LR: Computed tomography current perspectives. Chest 1983;83:806-13.

3. Feigin DS: Mediastinal masses. In Eisenberg RL and Amberg JR Critical diagnostic pathways in radiology. Philadelphia: Lippincott, 1981:76-93.

of the thorax:

2. Sones PJ, Torres WE, Colvin RS, et al.: Effectiveness of CT in evaluating intrathoracic masses. AJR 1982;139:469-75.

25. Levitt RG, Husband JE, Glazer HS: CT of primary tumors of the mediastinum. AJR 1984;142:73-8.

germ-cell

26. Zerhouni EA, Scott WW Jr. Baker RR, et al.: Invasive thymomas: diagnosis and evaluation by computed tomography. J Comput Assist Tomogr 1982;6:92-100.

JANUARY

MEDIASTINAL

1986

27. Glazer GM, Axe1 L, Moss AA: CT diagnosis thyroid. AJR 1982;138:495-8.

of mediastinal

28. Silverman PM, Newman GE, Korobkin M, et al.: Computed tomography in the evaluation of thyroid disease. AJR 1984:141:897-902. 29. Krudy AG, Doppman JL, Brennan MF, et al.: The detection of mediastinal parathyroid glands by computed tomography, selective arteriography, and venous sampling. Radiology 1981;140:739-44. 30. Reed JC, Hallet KK, Feigin DS: Neural tumors of the thorax: subject review from the AFIP. Radiology 1978;126:9-17. 31. Dal Pozzo G, Moroni F, Pellegrini GL, Villari N: Computed tomography role in massive thoracic extramedullary haematopoiesis. Europ J Radio1 1982;2:235-7.

MASSES

21

32. Feigin DS, Eggelston JC, Siegelman SS: The multiple rorengen manifestations of sclerosing mediastinitis. Johns Hopkins Med J 1979;144:1-8. 33. Webb WR, Gamsu G, Speckman JM, et al.: CT demonstration of mediastinal aortic arch anomalies. J Comput Assist Tomogr 1982;6:445-51. 34. Webb WR, Gamsu G, Speckman JM, et al.: Computed tomographic demonstration of mediastinal venous anomalies. AJR 1982;139:157-161. 35. Thorsen MK, San Dretto MA, Lawson TL, et al.: Dissecting aortic aneurysm: accuracy of computed tomographic diagnosis. Radiology 1983;148:773-7.