V ii8i CT of Fatty Thoracic Masses -.-. Fig. 1.-Mediastinal lipomatosis. A, CT scan shows abundant fat throughout superior mediastinum. Fat is homogeneous and similar in attenuation to subcutaneous fat. Arrow = residual thymic tissue or mediastinal lymph node, v = right innominate vein, A = aortic arch. B, In a different patient, CT scan shows lipomatous hypertrophy of interatrial septum. RA = right atrium, LA = left atrium, arrow = bronchogenic carcinoma. 1 Pictorial Harvey S. Glazer,1 Mark R. Wick,2 Dixie J. Anderson,1 Janice W. Semenkovich,1 Paul L. Molina,1 Marilyn J. Siegel,1 and Stuart S. SageI1 A variety of masses within the mediastinum, lung, pleura, or chest wall may contain areas of fat attenuation. In some cases, the mass is entirely fat (e.g., mediastinal lipomatosis), whereas in others, fat is only one component of the lesion (e.g., teratoma). In this report, the gamut of fatty thoracic masses that can be seen on CT is illustrated. Mediastinum Mediastinal Lipomatosis Essay Mediastinal lipomatosis is a benign entity in which increased collections of unencapsulated, histologically normal fat are present in the mediastinum. It is most commonly seen with Fig. 2.-Morgagni s hernia. CT scan at level of heart shows a large Morgagni s hernia contaming large bowel and omental fat. Focal eventration of diaphragm can be differentiated from a Morgagni s hernia by the intact diaphragm in the former entity. Received May 7, 1992; accepted after revision June 1 8, 1992. 1 Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO 631 1 0. Address reprint requests to H. S. Glazer. 2 Department of Pathology, Washington University Schcol of Medicine, St. Louis, MO 631 10. AJR 159:1 181-1 187, December 1992 0361-803X/92/1 596-1 1 81 American Roentgen Ray Society
1182 GLAZER ET AL. AJR:159, December 1992 obesity, administration of corticosteroids, or Cushing s syndrome. However, in some cases, no such predisposing factors are present. The fat accumulates most commonly in the upper anterior mediastinum but also may be seen in the cardiophrenic angles (pericardial fat pads), paraspinal areas, or even in the interatrial or atrioventricular grooves (Fig. 1). Associated extrapleural fat also may be present. Usually no compression of adjacent structures occurs. The fat is homogeneous and similar in attenuation to subcutaneous fat (-80 to -120 H). If the fat is inhomogeneous, other causes, such as neoplastic infiltration, mediastinitis, or prior surgery or irradiation need to be considered. Small foci of residual thymic tissue in the anterior mediastinum, however, should not be interpreted as representing infiltrated fat. Herniation of Abdominal Fat : A hernia through the foramen of Morgagni usually occurs in the right cardiophrenic angle and commonly contains omental fat (Fig. 2). Detection of fine linear opacities within the fat, representing the omental vessels, may be helpful in distinguishing this entity from a pericardial fat pad. Omental fat also can extend into the mediastinum through the esophageal hiatus (Fig. 3). Fat herniation through the foramen of Bochdalek or through acquired diaphragmatic defects is more commonly seen posteriorly in the left hemithorax than in the right hemithorax (Fig. 4). Neoplasms Lipomas are uncommon mediastinal tumors that may be seen in any part of the mediastinum. They are well defined, may be encapsulated, and usually do not compress adjacent structures unless they are large (Figs. 5 and 6). Although generally homogeneous, lipomas may contain thin fibrous septa. If the mass is inhomogeneous, contains areas of softtissue attenuation, or is poorly defined, an alternative diagnosis should be considered (e.g., liposacroma; Fig. 7). Benign mature teratomas are the most common mediastinal Fig. 3.-Hiatal hernia. A and B, CT scans show herniation of omenturn (arrows) in association with a hiatal hernia (H). E = esophagus. Fig. 4.-Bochdalek s hernia. Fig. 5.-Mediastinal lipoma. CT scan shows A and B, CT scans show intraabdorninal fat and top of left kidney (K) extending through a posterior homogeneous fatty mass (M) displacing trachea defect (arrowheads) in left hemidiaphragm. (T) and esophagus (E) laterally. (Case courtesy of A. Hammerman, St. Louis, MO.)
AJR:159, December 1992 CT OF FATTY THORACIC MASSES 1183 Fig. 6.-Esophageal lipoma. CT scan shows homogeneous fatty mass (arrow) in right lateral wall of barium-filled esophagus (E). A submucosal mass was seen on a prior upper gastrointestinal examination. Fig. 9.-Thymolipoma. A, CT scan shows a large, predominantly fatty mass adjacent to right side of heart. Scattered areas of soft-tissue attenuation are seen within mass. More cephalad scans showed extension to level of aortic arch. (Case courtesy of D. Sloan, Herrin, IL.) B, Photomicrograph of histologic section shows a lobule of normal thymic tissue with a Hassall s corpuscle surrounded by mature adipose tissue. germ cell tumor and are usually located within the thymus; rarely, however, these lesions can arise in the posterior mediastinum as well. On CT scans, the admixture of fat, fluid, soft tissue, and calcification suggests the diagnosis, although fat is present in 50% or fewer of cases (Fig. 8). Rarely a fatfluid level may be seen. Malignant germ cell tumors are mostly soft tissue and usually have poorly defined borders. Thymolipomas are composed of mature thymic lobules and fat. Islands of soft-tissue attenuation seen on CT scans are attributable to the presence of admixed thymic tissue and fibrotic stroma (Fig. 9). These lesions are usually seen in children and young adults and usually are quite large. Lipoblastomas, which occur almost always in infants and young children, are usually seen in the extremities or trunk and only rarely involve the mediastinum (Fig. 1 0). They contain Fig. 7.-Liposarcoma. CT scan shows large, relatively inhornogeneous mass in right side of mediastinum. Note that mass has slightly higher attenuation than subcutaneous fat does. Mass extended into right side of neck to involve right recurrent laryngeal nerve, paralyzing right vocal cord. Although no biopsy was done, mass has shown progressive slow growth and presumably is a well-differentiated liposarcoma. Fig. 8.-Benign mature teratoma. CT scan shows a large, complex mass containing fat, multiple cysts, and calcification (arrow). lobules of immature adipose tissue separated by fibrous septa. Miscellaneous Fatty Mediastinal Masses Extravasation of lipid-rich hyperalimentation fluid is a rare cause of a fatty mediastinal mass [1 ]. Adipocytic metaplasia in extramedullary hematopoiesis has been described and is a consequence in some cases of resolution of the causative hemolytic disorders [2] (Fig. 1 1). Central fibrofatty replacement of mediastinal lymph nodes can normally occur as a result of previous reactive inflammatory disease (Fig. 12); more diffuse low-attenuation nodes have been reported in Whipple s disease [3].
1184 GLAZER ET AL. AJR:159, December 1992 Fig. 10.-Lipoblastoma. A and B, CT scans show a large multiseptated fatty mass that infiltrates right side of neck, displaces trachea (T) and esophagus (E), and extends into spinal canal (arrow). (Case courtesy of J. P. Kuhn, Buffalo, NY.) #{149}1 Fig. 12.-Fibrofatty replacement of medias- Fig. 13.-Hamartoma. CT scan of 2-mm-thick tinal lymph node. CT scan of 2-mm-thick section section shows a smooth 1-cm pulmonary nodule shows fat (arrow) within a 1-cm pretracheal containing fat. lymph node. Fig. 1 1.-Extramedullary hematopoiesis associated with hereditary spherocytosis. A, CT scan through lower thorax shows bilatoral paraspinal masses with soft-tissue attenuation. B, CT scan obtained 5 years after splenectomy shows that masses now have attenuation of fat. Diffuse fatty infiltration of liver also has occurred. (Reprinted with permission from Martin et al. [2].) Fig. 14.-Lipoid pneumonia. CT scan at level of superior segment of left lower lobe of lung shows a predominantly fatty attenuation area of lung consolidation. Similar findings were seen in superior segment of right lower lobe and in right middle lobe. Subsequently, patient related a 20-year history of placing olive oil in her nostrils at night to prevent colds.
AJR:159, December 1992 CT OF FATTY THORACIC MASSES 1185 Fig. 15.-Prominent fat collection in hilum of right lung. CT scan at level of right pulmonary artery (RPA) shows a 1.5-cm mass (arrows) that is caused by fat and a few small lymph nodes. Lung Neoplasm Hamartomas account for about 7% of all resected pulmonary nodules. The CT finding that is suggestive of this lesion is a smooth pulmonary nodule 2.5 cm or less in diameter that contains focal collections of fat or fat alternating with areas of calcification [4] (Fig. i 3). Thin-section images (2 mm or less) may be necessary to show fat within the nodule. Conservative follow-up is recommended for lesions that satisfy the CT criteria for hamartoma. Endobronchial hamartomas are less common and are often symptomatic, in contrast to the pulmonary ones. Lipomas are another rare endobronchial fatty neoplasm. Lipoid Pneumonia Areas of fatty attenuation within a pulmonary infiltrate are diagnostic of lipoid pneumonia (Fig. 1 4). Areas of higher attenuation also may be seen as a result of associated inflammation. Hilar Fat Collections Normal collections of fat and lymph nodes can be seen in both hila (Fig. 1 5). Awareness of this normal finding will help avoid confusion with lymphadenopathy. The fatty component may be more apparent on thin-section images. Pleura and Chest Wall Extrapleural Fat Collections of extrapleural fat normally may be seen on CT scans, especially on thin-section scans (Fig. 1 6). This adipose Fig. 16.-Abundant extrapleural fat. CT scan of 2-mm-thick section shows abundant extrapleural fat (arrows). Extrapleural fat is frequently misinterpreted as pleural thickening on chest radiographs. Arrowheads = intercostal muscles. Fig. 17.-Old tuberculous empyema. CT scan shows abundant extrapleural fat (arrows) underlying an old calcified tuberculous empyema. tissue, which is located between the parietal pleura and endothoracic fascia, is most commonly seen over the posterolateral aspect of the fourth to eighth ribs. Extrapleural collections are larger in patients who are obese or are taking corticosteroids. Fat in the extrapleural space also is commonly thicker adjacent to areas of chronic pleural disease (Fig. 17). The apical cap seen in patients who have had pulmonary tuberculosis results primarily from thickening of the extrapleural fat in this location [5] (Fig. i 8). Extrapleural fat may be displaced medially as a result of extrapleural tumor, edema, or hemorrhage [6]. lntrafissural Fat Small collections of fat can occasionally be seen within the major pulmonary fissures, representing extensions of extrapleural or mediastinal fat (Fig. 19). These collections are most commonly present in the caudal aspect of the major fissure. Lipomatosis Excessive accumulation of subcutaneous fat is usually caused by obesity or corticosteroids. Lipomatosis can be seen in association with collections of fat within the mediastinum or extrapleural space. Diffuse lipomatosis is a rare condition in which an overgrowth of mature fat (without nuclear atypia) infiltrates the muscles of an extremity or, less commonly, the trunk [7]. Neoplasm Extrapleural or chest wall lipomas resemble mediastinal lipomas, appearing as well-defined fatty masses (Fig. 20). In some cases the lipoma is transmural, with both intrathoracic and extrathoracic components. Although a well-differentiated
1186 GLAZER ET AL. AJA:159, December 1992 Fig. 18.-Apical opacity. Fig. 19.-Intrafissural fat. CT scan near top of A, Posteroanterior chest radiograph in patient with history of tuberculosis shows extensive left left hemidiaphragm shows a large band of fat apical capping with associated volume loss in left upper lobe. Sternotomy and surgical clips are (arrows) in base of left major fissure. from a prior coronary artery bypass procedure. B, CT scan at level of top of aortic arch (A) shows that capping is caused by extrapleural fat. Fig. 20.-Chest wall lipoma. CT scan shows a large relatively homogeneous mass posterior to left scapula. Linear opacities within mass presumably represent fibrous septa. Fig. 21.-Metastatic liposarcoma. CT scan shows a large complex mass in left hemithorax that contains fat, fluid, and soft tissue. Patient had previously had a liposarcoma in his lower extremity resected. Arrow = calcified lymph node from prior healed granulomatous disease. Fig. 22.-Surgical flaps. A, CT scan shows a mostiy fat-attenuation myocutaneous flap (F) in anterior aspect of left hemithorax. Enlarged mediastinal lymph nodes also are seen. Patient had prior mastectomy of left breast and resection of chest wall. B, CT scan in a patient who had prior bilateral lung transplantation with bronchial omentopexy shows two pedicles of omental flap extending around heart.
AJR:159, December 1992 CT OF FATTY THORACIC MASSES 1187 liposarcoma may be indistinguishable from a lipoma, in most cases of the former, areas of soft-tissue attenuation are present (Fig. 21). Elastrofibromas are benign lesions thought to be caused by reactive hyperplasia from mechanical stimulation. They are usually located in the subscapular area and contain a mixture of collagen, fibroblasts, and mature fat. The mass is unencapsulated, so on CT scans it may mimic an infiltrating malignant neoplasm (e.g., liposarcoma) [8]. Surgical Flaps Myocutaneous flaps have been used to repair chest wall defects. Fatty replacement of the muscular portion of such a flap can be seen as a mostly fatty mass on CT scans (Fig. 22A). Intrapleural components of the omental flaps used to wrap the bronchial anastomoses in lung transplantations are most frequently seen in the paraspinal region or cardiophrenic angles (Fig. 22B). Miscellaneous REFERENCES 1. Cobb RJ, Mendelson DS. CT findings in mediastinal extravasation of hyperalimentation fluid. J Comput Assist Tomogr 1987;1 1 :158-159 2. Martin J, Palacio A, Petit J, Martin C. Fatty transformation of thoracic extramedullary hematopolesis following splenectomy: CT features. J ComputAssist Tomogr 1990:14:477-478 3. Samuels T, Hamilton P, Shaw P. Whipple disease of the mediastinum. AJR 1990:154:1187-1188 4. Siegelman 55, Khouri NF, Scott WW, et al. Pulmonary hamartoma: CT findings. Radiology 1986:160:313-317 5. im J-G, Webb WA, Han MC, Park JH. Apical opacity associated with pulmonary tuberculosis: high-resolution CT findings. Radiology 1991: 1 78: 727-731 6. Hammerman AM, Susman N, Strzembosz, Kaiser LA. The extrapleural fat sign: CT characteristics. J Comput Assist Tomogr 1990:14:345-347 7. Coode PE, McGuinness FE, Rawas MM, Griffith GG. Diffuse lipomatosis involving the thoracic and abdominal wall: CT features. J Comput Assist Tomogr 1991:15:341-343 8. Berthoty DP, Shulman HS, Miller HAB. Elastofibroma: chest wall pseudotumor. Radiology 1986:160:341-342