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by the American Institute of Ultrasound in Medicine J Ultrasound Med 21:759-765 • 0278-4297 Mature Mediastinal TeratomaSonographic Imaging Patterns and Pathologic CorrelationDepartment of Internal Medicine, Taipei County San-Chung Hospital (T.-T.W.), and Departments of Internal Medicine (H.-C.W., P.-C.Y.), Radiology (Y.-C.C.), Surgery (Y.-C.L.), and Pathology (Y.-L.C.), National Taiwan University Hospital, Taipei, Taiwan, Republic of China. Address correspondence and reprint requests to Hao-Chien Wang, MD, Department of Internal Medicine, National Taiwan University Hospital, 7 Chung-Shan S Rd, Taipei 100, Taiwan, Republic of China.
Objective. To characterize the sonographic patterns of teratomas located within the mediastinum and to correlate them with pathologic findings, because there have been few reports concerning the application of sonography in the diagnosis of mediastinal teratoma. Methods. Over a 9-year period, we carried out an image analysis of the sonographic findings of 28 mediastinal teratomas whose diagnoses were proved surgically. Results. Sonography showed that 18 patients had a complex mass of heterogeneous echogenicity, including regional bright echoes (n = 12), acoustic shadows (n = 8), hyperechoic lines and dots (n = 7), soft tissue septa (n = 9), dermoid plugs (n = 6), and hypoechoic areas (n = 11). This type of mediastinal teratoma revealed, in pathologic findings, varying combinations of fat, sebaceous and mucinous materials, hair, mineralized elements, and multiple compartments. Eight patients had a homogeneous hyperechoic mass, and the 2 remaining patients had floating spherules within a cystic mass. Pathologically, the homogeneous mass mainly contained hair and sebaceous material. The floating spherules were also composed of sebaceous tissue and were above the fluid of the cyst. Conclusions. On the basis of the evaluation of the sonographic and pathologic findings, we described 3 major patterns of mediastinal teratoma: a complex mass of heterogeneous echogenicity, homogeneous high echogenicity within a solid mass, and floating spherules within a cystic mass. Thus, sonography can improve the diagnosis of mediastinal teratoma. However, additional prospective studies are needed to test the sensitivity and specificity of these sonographic patterns in the diagnosis of mediastinal teratoma.
Key Words: mediastinal teratoma • pathologic entities • sonographic pattern • sonography Abbreviations: CT, computed tomography
Most patients with teratomas are usually asymptomatic, so the teratoma is usually discovered by accident when routine radiography is done on plain film.1 Although the presence of fat and calcification within a mass is a clue to the presence of a teratoma, the preoperative diagnosis is often difficult to determine on the basis of conventional radiography because of the inability of this method to show the fat or calcification.1 The most common sites of teratomas are the ovaries and mediastinum.2 With the advance of imaging technology, the sonographic and computed tomographic (CT) findings of a cystic teratoma of the ovary have been described well in other studies.3–6 Recent publications reported successful image diagnosis of mediastinal teratomas on the basis of CT7,8; however, there have been few reports of sonography as an alternative method for its diagnosis. Although the sonographic features of mediastinal teratomas have been mentioned in a small series of articles,9–12 to our knowledge, no detailed imaging patterns have been reported. In this study, we reviewed 28 cases of mediastinal teratoma with the purpose of characterizing the sonographic appearances of these teratomas and their correlation with pathologic findings.
We conducted a retrospective review of 28 cases of surgically proved benign mediastinal teratoma at National Taiwan University Hospital between 1990 and 1998. All cases had been evaluated with sonography before being selected for review. We analyzed each patient's clinical history, pathologic findings, and radiographic images. We also took into account other factors, including age, sex, and symptoms at admission to the hospital. The surgical and pathologic reports were reviewed to determine lesion location, size, and gross and pathologic features. Chest sonography and thoracic CT scans were evaluated in every case. Sonographic features, including size, location, internal echogenicity, septa, and areas of calcification, were all recorded. The size was defined as the greatest dimension of the visible area. The location within the mediastinum was determined by the criteria of Fraser et al.2 A cystic lesion was defined as an encapsulated tumor with a thin wall but with no echogenic material. A polycystic lesion indicated septum formations within a cyst. A solid mass was characterized as a dense echoic pattern without a cystic component. A complex structure of internal elements appeared as densely echogenic material with or without an acoustic shadow or hyperechoic dots within a cystic mass. A dermoid plug was described as an outgrowth from the inner surface of a cyst containing hair and other atypical tissues.13 All studies were performed with a real-time electronic scanner (SSD-2000; Aloka Co, Ltd, Tokyo, Japan) with 2- to 5-MHz linear and convex transducers. The possible locations of mediastinal teratomas were first determined from the chest radiography and CT, after which the patients were examined by transducer scanning through the intercostal space over the whole chest with special attention to the area where the lesion was located. The examinations were performed with the patients in the supine or lateral decubitus position. The acoustic window normally used was the parasternal area of the anterior chest wall. The direction of the examination was sagittal and intercostal. Patients were asked to hold their breath temporarily to minimize interference from movement of the chest wall while the lesion was exposed. The lesion was localized and recorded with real-time gray scale sonography. Chest CT was also performed for correlation with the sonographic findings.
Clinical Appearances Among the 28 patients, we took 94 pictures, which were recorded in our computer system by 7 sonographers. The group consisted of 18 male and 10 female patients 9 to 41 years old (mean age, 34 years). Eleven patients (39.0%) were younger than 30 years at the time of the investigation. Fifteen patients (53.6%) had symptoms, including chest pain (n = 8), cough (n = 4), dyspnea (n = 2), and hemoptysis (n = 1). Thirteen patients (46.4%) were asymptomatic. All patients underwent surgical resection, and the teratomas all proved pathologically to be benign lesions, located within the anterior mediastinum. The tumor size ranged from 3 to 15 cm in its longest dimension. The data were categorized under 3 subtypes (Table 1  ).
Sonographic Features and Pathologic Correlation Type I: Complex Mass of Heterogeneous Echogenicity Eighteen patients (64.3%) had a complex mass of mixed echogenicity with or without an acoustic shadow. An example is shown in Figure 1 . All of the patients had 2 or more of the sonographic features. Of these, 12 had regional bright echoes on the transthoracic sonograms. Seven had hyperechoic lines and dots. Acoustic shadows were obtained in 8 cases. Soft tissue septa were visualized in 9 masses. Dermoid plugs were found in 6 cases. Eleven patients had hypoechoic regions (good through-transmission).
Pathologically, solid components were identified in all type I teratomas and contained a wide range of element types. Cheesy, mucinous material and sebaceous tissue were noted in 17 lesions (94.4%); hair was found in 11 lesions (61.1%); and mineralized elements consisting of calcification (n = 8), bone (n = 8), cartilage (n = 5), or teeth (n = 3) were found in 14 lesions. Septa were noted in 9 cases.
Type II: Homogeneous High Echogenicity Within a Solid Mass
Type III: Floating Spherules Within a Cystic Mass Two mediastinal sonograms revealed highly echogenic mobile spherules of variable size within a huge cyst. An example is shown in Figure 3 . One case of type III mediastinal teratoma was reported in our previous publication.12 When the patient changed position from supine to lateral decubitus, these spherules drifted from the anterior side to the mediastinal side of the cyst.12 At pathologic examination, some sebaceous elements were seen within the cystic mass and floating above the watery fluid.12
The sonographic characteristics of all mediastinum teratomas are summarized in Table 2 . The regional bright echo, dermoid plugs, acoustic shadows, and septum formations were more predominant within the type I mediastinum teratomas. Hyperechoic dots or lines were equally distributed among types I and II. Areas that had good through-transmission were found in each type. One case of a fluid (fat)-fluid level was identified in a type III mediastinal teratoma.
Echo-guided needle aspiration biopsy was done in 2 cases of mediastinal teratoma. Grossly, cheesy material was obtained, and some epithelioid and lymphocytic cells were found at cytologic analysis.
Computed Tomographic Findings
The sonographic appearance of mediastinal teratomas has been considered highly variable.9,10 The present study was performed to determine the main sonographic patterns of mediastinal teratomas. The most common feature is a complex mass with heterogeneous echogenicity, followed by a homogeneous echogenic mass and floating spherules within a cystic mass. The results of this study show that the major feature of the mediastinal teratoma on sonography is complex heterogeneous echogenicity (type I teratoma). Calcified structures, such as bone and teeth, clumps of hair, and fat in a dermoid plug (or Rokitansky protuberance) surrounded by serous fluid can produce these images.3 The sonographic finding of diffuse hyperechoic lines and dots in a teratoma, as in type II teratoma, has been associated with the presence of hair and watery or sebaceous fluid inside the cyst.3 The sonographic findings in 2 patients were of echogenic floating spherules in an anterior mediastinum cystic mass. Pathologic examination revealed that they consisted of sebaceous materials. These spherules float in the fluid of the cyst and drift when there are changes in posture.12 The sonographic patterns of this report are different from those of previous articles, especially regarding ovarian teratomas. First, the echogenic focus within the ovarian teratoma is often associated with shadowing.5 There may be progressive fading of the ultrasound beam beyond an echogenic mass shown to contain soft tissue, fat, or calcification. In our series, the finding of an acoustic shadow within a mediastinal teratoma was less frequent than in an ovarian teratoma, although mediastinal teratomas were still found pathologically to be composed of high proportions of fat and calcification. A possible explanation for this is that the adjacent air under chest sonography may mask the fat and calcification, which are often associated with an acoustic shadow. Second, 60% to 80% of pelvic teratomas are frequently compartmentalized by thin-tissue septa.4–6 Only 10 (35.7%) of 28 mediastinal teratomas in our series were found to have this sonographically. There are also a few similarities between mediastinal and pelvic teratomas. As a rule, an echogenic mural focus consistent with a dermoid plug in a pelvic mass is considered strong evidence of a teratoma. The dermoid plugs are produced by conglomerates within the lesion of fatty and sebaceous material, hair, soft tissue, calcification, and teeth.4,13 However, these findings, although unique, were not common (6 [21.4%] of 28) in both our series and in pelvic teratoma3,4,6,13 Fat-fluid or fluid-fluid levels have been considered specific but uncommon radiographic findings in pelvic teratomas.3,4,6 This phenomenon was a floating hypoechoic zone of fat that formed an interface with a fluid component beneath it.5,6,14 One case of a fat (fluid)-fluid level was identified sonographically in our cases of mediastinal teratoma. Computed tomography has been recognized as the most effective examination for the diagnosis of mediastinal tumors. When the anterior mediastinal tumor appears inhomogeneous, containing components with soft tissue, water, fat, and calcifications (type I mediastinal teratoma), the diagnosis of teratoma can be easily made on the basis of both modalities. If the tumor appears as a mass with homogeneous density (type II mediastinal teratoma), the differential diagnosis on the basis of CT is somewhat difficult, because teratoma, thymoma, lymphoma, and a pericardial cyst all can have a similar appearance. Our series included 4 cases of type II mediastinal teratoma, which had no conclusive diagnosis after CT examination. When this is the case, sonography may be helpful. These tumors appeared as homogeneous echogenic masses with diffuse hyperechoic dots and lines, and this unique sonographic picture may add information to the diagnosis of teratoma beyond chest radiography and CT. In addition, thymic, bronchogenic, or pericardial cysts can also be differentiated by the presence of a thin wall, because the teratoma is always a thick-walled cystic lesion sonographically.9,10 Type III mediastinal teratomas have a highly unique appearance on chest sonography and CT; however, Wernecke and Diederich10 reported that a thymoma (or thymolipoma) with central necrosis, mediastinal cysts with an inner hemorrhage, and bulky lymphomas (lymph nodes) may be confused with teratomas on mediastinal sonography. The most effective procedure for resolving this problem is a guided biopsy.15–17 In conclusion, we showed 3 major patterns in mediastinal teratoma: a complex mass of heterogeneous echogenicity, homogeneous high echogenicity within a solid mass, and floating spherules within a cystic mass. On the basis of these findings, sonography can improve the diagnosis of mediastinal teratoma. However, additional prospective studies are needed to evaluate the impact of sonography on diagnostic accuracy, diagnostic confidence, the need for percutaneous sampling, and clinical outcome based on these sonographic patterns.
Received January 8, 2002, from the Department of Internal Medicine, Taipei County San-Chung Hospital (T.-T.W.), and Departments of Internal Medicine (H.-C.W., P.-C.Y.), Radiology (Y.-C.C.), Surgery (Y.-C.L.), and Pathology (Y.-L.C.), National Taiwan University Hospital, Taipei, Taiwan, Republic of China.
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Abstract
Introduction
