Radiologic Pathologic Correlation of Intraosseous Lipomas. Tim Propeck 1, Mary Anne Bullard 1, John Lin 1, Kei Doi 2, William Martel 1

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1 Downloaded from by on 04/10/18 from IP address opyright RRS. For personal use only; all rights reserved Radiologic Pathologic orrelation of Intraosseous Lipomas Tim Propeck 1, Mary nne ullard 1, John Lin 1, Kei Doi 2, William Martel 1 I ntraosseous lipomas are considered rare benign bone lesions, but with the advent of MR imaging more of these lesions are being recognized. Using Milgram s histopathologic and radiologic classification [1], we divided intraosseous lipomas into three categories on the basis of their imaging characteristics. The appearance of these lesions on radiographs, T scans, and MR images can vary as a result of their degree of involution and necrosis. Radiographically, these lesions Pictorial Essay may mimic other entities such as fibrous dysplasia, aneurysmal bone cysts, simple cysts, bone infarcts, and chondroid tumors. lacksin et al. [2] showed that visualizing fat within these lesions aids in diagnosis. MR imaging and T can be diagnostic whereas Fig year-old woman with stage 1 intraosseous lipoma confirmed histologically with needle aspiration., nteroposterior radiograph of right hip reveals large lucent lesion (arrow) with well-defined sclerotic border involving right femoral head and neck., oronal T1-weighted MR image (500/10 [TR/TE]) of pelvis shows homogeneous high-signal-intensity area (arrow) in proximal right femur, corresponding to lesion seen on. High-signal area is isointense to subcutaneous fat., oronal short inversion time inversion-recovery MR image (3500/68; 135-msec inversion time) shows homogeneous low signal intensity in right femoral head (arrow) and neck in region of lesion, consistent with uniform saturation of fat in lesion. Received January 21, 2000; accepted after revision February 23, Presented at the annual meeting of the merican Roentgen Ray Society, Washington, D, May Department of Radiology, University of Michigan Hospitals, 1500 E. Medical enter Dr., nn rbor, MI ddress correspondence to J. Lin. 2 Tollgate Radiology, 215 TollGate Rd., Warwick, RI JR 2000;175: X/00/ merican Roentgen Ray Society JR:175, September

2 Propeck et al. Downloaded from by on 04/10/18 from IP address opyright RRS. For personal use only; all rights reserved radiographs can be ambiguous, especially if there is necrosis within the lesion. D Fig year-old woman with stage 2 intraosseous lipoma confirmed histologically with curettage. and, nteroposterior () and lateral () radiographs of tibia show ovoid lucent lesion in anteromedial proximal tibia with thin sclerotic border (large arrow) and central calcification (small arrow, )., xial T scan reveals lesion of primarily fat attenuation (open arrow) in medial proximal tibia with well-defined sclerotic margin (small solid arrow) and internal calcification (large solid arrows). D, oronal proton density weighted MR image (4000/23 [TR/TE]) shows lesion with signal intensity isointense to subcutaneous fat (small thick arrow) in medial proximal tibia with low signal rim (large thick arrow) corresponding to sclerotic margin and irregular internal low-signal-intensity area (thin arrow) corresponding to dystrophic calcification. E, oronal T2-weighted fast spin-echo MR image (4000/115) with fat suppression shows low signal intensity in lesion consistent with suppression of fat (arrow). linical Features Intraosseous lipomas account for approximately 0.1% of bone tumors. ommon sites include the intertrochanteric and subtrochanteric regions of the femur and calcaneus. They can also occur in the flat bones, pelvis, and other locations. The largest case study of intraosseous lipomas (66 cases) was performed by Milgram [3]. In that study, lesions in 25 patients were incidental findings, whereas in 14 patients there was only minor aching in the region of the lesion, which may have been unrelated to the intraosseous lipoma [3]. The other 27 patients described pain referable to the lesion; in several of these patients, the pain was attributed to a pathologic fracture through the lesion. In Milgram s study, no cases were known to recur after surgery; however, in a subsequent report malignant transformation was described [4]. Histopathology To recognize the imaging features of intraosseous lipomas, it is important to understand the histopathology. Milgram [1] divided intraosseous lipomas into three types: stage 1, solid tumors of viable lipocytes; stage 2, transitional cases with partial fat necrosis and focal calcification but also regions of viable lipocytes; and stage 3, advanced cases in which fat cells have died with variable degree of cyst formation, calcification, and reactive new bone formation. The progression from stage 1 to stage 3 is caused by ischemia and infarction within the lesion and may be related to the rigid honeycomb structure of bone and fat cell expansion and multiplication. These lesions may also cause resorption and expansion of bone, whereas bone infarcts do not cause expansion of the bone [1]. Radiographic Features The radiographic features of intraosseous lipomas often parallel those of the histologic E 674 JR:175, September 2000

3 Radiologic Pathologic orrelation of Intraosseous Lipomas Downloaded from by on 04/10/18 from IP address opyright RRS. For personal use only; all rights reserved D Fig year-old man with stage 2-3 intraosseous lipoma confirmed histologically with curettage. and, nteroposterior () and lateral () radiographs of knee reveal subchondral lucent lesion with sclerotic margin (large black arrow) and internal calcifications (small back arrow) in lateral femoral condyle. There is joint space narrowing in lateral compartment consistent with degenerative change (white arrow)., T scan shows lesion with well-defined sclerotic margin (short arrow), peripherally located dystrophic calcifications (long arrow), and central area of soft-tissue density (arrowhead) corresponding to fat necrosis. D, oronal T1-weighted MR image (815/20 [TR/TE]) reveals lesion with surrounding low-signal-intensity rim (long thick black arrow) consistent with sclerosis and lowsignal-intensity peripheral area (thin black arrow) corresponding to calcification. Lesion has primarily high signal intensity on T1-weighted MR image (white arrow) consistent with fat and low-signal-intensity central area (short thick black arrow) consistent with fat necrosis. E, oronal T2-weighted fast spin-echo MR image (3630/96) with fat suppression shows suppression of fat in intraosseous lipoma (large arrowhead). gain identified are low-signal-intensity rim (long arrow) and peripheral area of low signal intensity (short arrow) consistent with calcification. Note high signal intensity in center (small arrowhead) consistent with fat necrosis. stage of the lesion. The stage 1 lesions are lucent and represent viable, nonnecrotic fat with resorption of bony trabeculae (Fig. 1). Stage 2 lesions have lucent areas, which consist of viable fat and radiodense areas that consist of fat necrosis and dystrophic calcification (Figs. 2 and 3). Stage 2 lesions can be expansile. Stage 3 lesions reflect resorption of normal bone, but they are more radiodense than stage 1 or 2 lesions. The radiodensity is a result of calcification and extensive fat necrosis [1] (Figs. 4 and 5). Stage 3 lesions also have thick sclerotic borders, presumably related to involution of these lesions. Radiologically, the differential diagnosis of intraosseous lipomas includes fibrous dysplasia, aneurysmal bone cysts, simple bone cysts, bone infarcts, chondroid tumors, and liposclerosing myxofibrous tumors [5, 6]. T Features On T, stage 1 intraosseous lipomas exhibit resorption of bone trabeculae in the lesion and bone expansion. The area of lucency seen on the radiograph corresponds to fat attenuation visible on T. Stage 2 lesions have areas of fat attenuation and patchy areas of increased density corresponding to calcification and fat necrosis (Figs. 2 and 3). Stage 3 intraosseous lipomas are the most E difficult to diagnose because of the reactive ossification, calcification, fat necrosis, and cyst formation caused by necrosis of the fat component. If the lesion has a peripheral rim of discernible fat (Figs. 4 and 5), it will help to eliminate other conditions generally considered in the differential diagnosis. Lesions that exhibit resorption of trabecular bone and predominantly central calcification instead of peripheral calcification are more likely to be stage 3 intraosseous lipomas than bone infarcts; the latter are nonexpansile lesions that have a peripheral serpentine thin rim of sclerosis and are not associated with trabecular resorption [7]. JR:175, September

4 Propeck et al. Downloaded from by on 04/10/18 from IP address opyright RRS. For personal use only; all rights reserved D E F G H Fig year-old man with stage 3 intraosseous lipoma confirmed histologically with curettage., nteroposterior radiograph of tibia shows large round lucent lesion in proximal tibia with surrounding rim of sclerosis (black arrow) and amorphous central areas of increased density (white arrow) representing calcifications., Lateral radiograph of tibia shows large round lucent lesion in proximal tibia with curvilinear sclerotic density (arrow) in posterior portion of lesion., T scan shows lesion with surrounding sclerotic rim (thin black arrow) and immediately adjacent low attenuation inner rim (thick black arrow) consistent with fat. urvilinear calcification (arrowhead ) is visible, corresponding to calcifications seen on lateral radiograph. Lesion primarily has soft-tissue attenuation (white arrow). D, xial T1-weighted MR image (600/14 [TR/TE]) shows peripheral rim of high signal (black arrow) corresponding to fat and intermediate signal intensity in central portion of lesion (white arrow) consistent with extensive fat necrosis. E, oronal proton density weighted MR image (3800/18) shows high-signal-intensity peripheral rim (black arrow) and intermediate-signal-intensity (white arrow) center consistent with peripheral rim of fat and central fat necrosis. Note low-signal-intensity band in center of lesion consistent with calcification (arrowhead). F, oronal proton density weighted MR image (3000/21) with fat saturation shows low-signal-intensity rim consistent with suppression of peripherally located fat (short arrow). entral portion of lesion shows relatively high signal intensity (long arrow) consistent with fat necrosis. Low-signal-intensity band (arrowhead) is seen in central portion of lesion consistent with calcification. G, urettage specimen from peripheral portion of tibial lesion shows lipocytes (long arrow) with fat necrosis (short arrow). (H and E, 100) H, Specimen of central portion of tibial lesion has foci of dystrophic mineralization (long arrow) within extensive background of fat necrosis (short arrow). (H and E, 100) 676 JR:175, September 2000

5 Radiologic Pathologic orrelation of Intraosseous Lipomas Downloaded from by on 04/10/18 from IP address opyright RRS. For personal use only; all rights reserved D Fig year-old man with stage 3 intraosseous lipoma confirmed histologically with curettage. and, nteroposterior () and lateral () radiographs of knee show large round lucent lesion in proximal tibia with sclerotic margin (black arrow) and scattered calcifications (white arrow). and D, T scans with bone () and soft-tissue (D) window settings show lesion in proximal tibia with areas of fat attenuation (white arrow), soft-tissue attenuation (arrowhead), and calcific attenuation (black arrow). E, oronal T1-weighted MR image (720/20 [TR/TE]) shows lesion in proximal tibia with thick rim of high signal intensity (short arrow) corresponding to fat. Lesion also has central globular area of high signal intensity (long arrow) also corresponding to fat. Most of central portion of lesion has intermediate G H (small arrowhead) and low signal intensity (large arrowhead), which are fat necrosis and dystrophic calcification, respectively. F, xial T2-weighted fast spin-echo MR image (2000/70) reveals high-signal-intensity thick rim (short solid arrow) consistent with fat and globular central area of high signal (long solid arrow) consistent with fat. Remainder of central portion of lesion has areas of intermediate (large open arrow) and low signal intensity (small open arrow) consistent with fat necrosis and dystrophic calcification. G, xial T2-weighted fast spin-echo fat-suppressed MR image (2000/90) reveals suppression of thick rim of fat (solid white arrow) as well as central globular area of fat (solid black arrow). These areas appear as low signal intensity on this fat-suppressed sequence. reas of fat necrosis show high signal intensity (open arrow) as compared with intermediate signal intensity on non fat-suppressed sequence. H, urettage specimen from necrotic region shows scattered aggregates of small mature lymphocytes and fibrosis (long arrow). reas of cholesterol cleft formation are thought to be residua of hemorrhage (short arrow). (H and E, 40) E F MR Imaging Features MR imaging reveals viable fat in stage 1 intraosseous lipomas. The fat is isointense to subcutaneous fat on T1-weighted sequences and exhibits low signal intensity with fat suppression on T2-weighted images. thin circumferential rim of low signal intensity on T1- and T2- weighted sequences is typically present demarcating the margin of the fatty lesion consistent with reactive sclerosis surrounding the lesion (Fig. 1). In stage 2 lesions, one can again identify fat and the circumferential rim of decreased sig- JR:175, September

6 Propeck et al. Downloaded from by on 04/10/18 from IP address opyright RRS. For personal use only; all rights reserved nal on T1- and T2-weighted images. Low-signal-intensity areas within the central portion of the lesion on T1- and T2-weighted images are consistent with calcifications (Figs. 2 and 3). Stage 3 lesions show a thin peripheral rim of fat, which can be identified on MR imaging. They also have central calcification and a thick rim of surrounding sclerosis, which have low signal intensity on T1- and T2-weighted sequences. reas of fat necrosis have a variable signal on T1-weighted and increased signal on T2- weighted images (Figs. 4 and 5). onclusion The differential diagnosis of intraosseous lipomas on radiographs is broad and includes chondroid tumors, aneurysmal bone cysts, fibrous dysplasia, bone infarcts, and liposclerosing myxofibrous tumors. Given the variable appearance of intraosseous lipomas in different stages of involution, the radiographic findings can vary from a lucent lesion with a thin sclerotic margin to a radiodense lesion with a thick sclerotic margin. T and MR imaging are more definitive examinations because they can accurately and consistently reveal stage 1 lesions and the characteristic peripheral rim of fat in stage 2 and stage 3 lesions. cknowledgment We thank Lawrence Yao for contributing the case for Figure 2. Refeences 1. Milgram JW. Intraosseous lipomas: radiologic and pathologic manifestation. Radiology 1988; 167: lacksin MF, Ende N, enevenia J. Magnetic resonance imaging of intraosseous lipomas: a radiologic-pathologic correlation. Skeletal Radiol 1995;24: Milgram JW. Intraosseous lipomas. lin Orthop 1988;231: Milgram JW. Malignant transformation in bone lipomas. Skeletal Radiol 1990;19: Shigeru E, Kattapuram SV, Rosenberg E. ase report 619. Skeletal Radiol 1990;19: Kransdorf MJ, Murphey MD, Sweet DE. Liposclerosing myxofibrous tumor: a radiologicpathologic-distinct fibro-osseous lesion of bone with a marked predilection for the intertrochanteric region of the femur. Radiology 1999;212: Williams E, lose PJ, Meaney J, Ritchie D, ogley D, arty T. Intraosseous lipomas. lin Radiol 1993;47: JR:175, September 2000