Lai Peng Chan 1 R. Gee 2 Ciaran Keogh 2 Peter L. Munk 2 Received September 16, 2002; accepted after revision pril 29, 2003. 1 Department of Diagnostic Radiology, Singapore General Hospital, Outram Rd., 169608 Singapore. 2 Department of Radiology, Vancouver General Hospital, University of ritish Columbia, 899 W. 12th ve., Vancouver, C, V5Z 1M9 Canada. ddress correspondence to P. L. Munk. JR 2003;181:955 959 0361 803X/03/1814 955 merican Roentgen Ray Society Original Report Imaging Features of Fat Necrosis OJECTIVE. We describe five cases of biopsy-proven fat necrosis and review the imaging with an emphasis on MRI. CONCLUSION. MRI showed the classically reported finding of central globular high signal intensity with a low-signal-intensity rim in only two of the five cases. We noted peripheral contrast enhancement, which to our knowledge has not been reported previously, in three of our five cases. Other patterns such as amorphous, cloudlike stranding and a serpentine appearance were also encountered. In reviewing the literature, we found that the MRI appearance of fat necrosis is more varied than previously thought. F at necrosis infrequently presents as a soft-tissue mass. When it does so, it is usually situated over osseous protuberances [1]. Clinically the patient may be asymptomatic or may present with pain, skin induration, ecchymosis, skin retraction, or skin thickening. variety of causes of fat necrosis have been reported, including trauma, collagen vascular disease, myeloproliferative disorders, and complications of pancreatic disorders such as in disseminated fat necrosis [2]. Even in cases in which there is a traumatic cause, a history of injury is often not present because of bias in patient recall. In this report, we present a spectrum of appearances of fat necrosis. We found that the criteria established by previous authors are incomplete in characterizing the appearance of fat necrosis and some of the lesions we saw did not fit into the categories described. Materials and Methods We used the radiology database at our institution to find records of patients with a suspected diagnosis of fat necrosis who had undergone either CT or MRI from 1996 to 2001. Six patients were found, five of whom had histologic correlation and were included in this study. CT scans were obtained on a HiSpeed scanner (General Electric Medical Systems, Milwaukee, WI), and MRI was performed on a 1.5-T system (Signa, General Electric Medical Systems). Imaging parameters for MRI were as follows: T1-weighted sequences (TR range/te range, 500 800/14 16), T2- weighted sequences (2400 4200/34 105), and inversion recovery sequences (TR range/te, 2500 3500/20 msec; inversion time, 150 msec). The study group comprised four men and one woman. The patients ranged in age from 50 to 79 years. ll the patients had been referred to our institution for the investigation of a palpable mass because our institution is a tertiary referral center for musculoskeletal tumors. Of the five patients, two patients had fat necrosis in the subcutaneous fat, two had fat necrosis in a lipoma, and one patient had areas of fat necrosis in an atypical lipoma. ll these cases had histologic confirmation by either core needle biopsy or surgical resection. The excised specimens were not available for review or mapping correlation with the images, but the pathology reports were reviewed for descriptive comments characterizing the nonadipose areas. Results The CT appearance of fat necrosis in the extremities has been poorly described. In our first two patients, we found that subcutaneous fat necrosis can appear as a well-defined hypodense mass with rim enhancement in the subcutaneous plane (Fig. 1) or as a globular mass with central fat density (Fig. 2). In these two patients, histologic confirmation was by core needle biopsy. The specimens JR:181, October 2003 955
Chan et al. Fig. 1. 79-year-old woman with mass at medial aspect of thigh and no history of trauma. Mass was biopsied and found to be area of fat necrosis. xial contrast-enhanced CT scan of thigh shows hypodense lesion in subcutaneous tissue with rim enhancement (arrow) and perilesional stranding (arrowheads). C D Fig. 2. 73-year-old man with recent diagnosis of carcinoma of prostate and incidental mass in his left buttock. This mass was confirmed to be fat necrosis after core biopsy., xial unenhanced CT scan shows mass in subcutaneous fat with central fatty density (arrow) and peripheral globular component (arrowhead) with density similar to muscle density., Sagittal T2-weighted image (TR/TE, 2400/90) shows lesion has central high signal intensity and low-signal-intensity rim (arrow). C, xial fast STIR image (2500/20) shows lesion (arrows) is hyperintense on this sequence. D, Sagittal fat-suppressed T1-weighted gadolinium-enhanced image (500/16) shows lesion is hypointense with irregular rim enhancement (arrows). 956 showed ghost clusters of nonviable fat cells and clusters of histiocytes, foam cells, and multinucleated giant cells compatible with fat necrosis. On MRI, subcutaneous fat necrosis appeared as a globular lesion with signal intensity similar to that of fat on T1weighted images, surrounded by an irregular hypointense rim (Fig. 2). The lesion was hyperintense on inversion recovery images (Fig. 2C). The most varied appearance of fat necrosis was seen in the lipomas and atypical lipoma. In the third patient, both the CT scan (Fig. 3) and MRI (Fig. 3) showed amorphous, cloudlike stranding in a fat-containing lesion compatible with a lipoma. The core biopsy revealed that much of the adipose tissue was nonviable and that small areas of fat necrosis were present. Some degree of bland spindle cell proliferation was also noted, appearing to represent reactive myofibroblasts. In the fourth patient, MRI showed a serpiginous lesion (Fig. 4) in an encapsulated fat-containing mass. This irregular crenated curvilinear lesion was hypointense on T1- and T2-weighted images and showed irregular rim enhancement after gadolinium administration (Fig. 4). This mass was resected en bloc. The curvilinear lesion was a 5-cm area of fibrous tissue intermixed with adipose tissue, resembling the appearance of fat necrosis in fibrous tissue. The appearance of the rest of the lesion was consistent with a lipoma. MRI in the fifth patient showed ill-defined nodules in a fat-containing lesion that was eventually resected and found to be a lowgrade liposarcoma. Two areas of irregularity were seen at opposite ends of the mass on MRI. These lesions had hypointense rims and hyperintense centers on T1-weighted images (Fig. 5) and could be hypo- or hyperintense on T2-weighted images (Fig. 5). fter en bloc resection, the more inferior part of the lesion corresponded to a 6.5-cm area of hard fibrous tissue, and the more superior area corresponded to three confluent nodules measuring from 1.0 to 1.5 cm. These findings correlated well with the MRI appearance, as shown in Figure 5C; the irregular areas corresponded to areas of fat necrosis, showing foamy macrophages and adjacent fibrosis. Gadolinium enhancement has not, to our knowledge, been reported but was seen in both subcutaneous and deep fat necrosis in three of our patients. Gadolinium enhancement appeared irregular and usually at the periphery of the lesion (Figs. 2D, 4, and 5C). JR:181, October 2003
Imaging Features of Fat Necrosis Fig. 3. 50-year-old man with mass at posterior aspect of thigh. Fat necrosis was confirmed after core biopsy., xial unenhanced CT scan shows well-defined lesion in biceps femoris muscle that has density similar to that of subcutaneous fat, consistent with intramuscular lipoma. morphous cloudlike areas of stranding (arrows) can be seen at lateral aspect of lesion., xial T2-weighted image (TR/TE, 2000/105) shows stranding (arrows) at lateral aspect of lesion, which is hypointense to subcutaneous fat. Lesion otherwise mirrors signal of subcutaneous fat on all sequences (not shown). Discussion Fat necrosis in the extremities is a known entity but is not well documented. It often presents as a lump [1] that requires further workup, as was the case in our series of patients. The interval between a traumatic event and observation of a palpable lump is often prolonged, so the clinical history may be noncontributory or even misleading [3]. Indeed, only one patient in our series recalled an episode of remote trauma. In our series, fat necrosis occurred in the subcutaneous plane in two patients and in an intramuscular lipoma or an intramuscular atypical lipoma in three patients. This finding is not surprising because of the large amount of fat in lipomas and because the lesions were large and superficial in location and could have undergone repeated trauma. The radiologic appearance of fat necrosis in the extremities has not been well described. The mammographic features of fat necrosis, which are generally well known, include a spiculated density and single or Fig. 4. 73-year-old man who presented with mass in anterolateral thigh. fter resection, mass was found to be lipoma with area of fat necrosis., xial T1-weighted image (TR/TE, 500/16) of distal femur shows large septated intramuscular fat-containing lesion (arrows) with epicenter in vastus lateralis and intermedius muscles, consistent with lipoma. Note central hypointense serpiginous lesion (arrowhead)., Sagittal fat-saturated contrast-enhanced T1-weighted image (500/16) shows homogeneous signal loss in fatty portion of lesion, enhancement of capsule and internal septa (arrows), and rim enhancement of serpiginous portion (arrowheads). JR:181, October 2003 957
Chan et al. Fig. 5. 67-year-old man with enlarging mass in anterior right thigh. This lesion was resected and found to be atypical lipoma with multiple areas of fat necrosis., xial unenhanced T1-weighted image (TR/TE, 500/14) shows well-defined lobulated intramuscular mass of fat signal intensity with internal septations. Two nodules with hypointense rims and hyperintense centers (arrows) are present., xial fat-saturated T2-weighted fast spin-echo image (2000/100) shows that nodules can appear hypointense (arrows) or hyperintense (arrowheads). C, Sagittal fat-saturated contrast-enhanced T1-weighted image (500/14) shows homogeneous signal loss of entire lesion and all nodules exhibiting irregular rim enhancement (arrows). C multiple benign-appearing lipid-filled cysts [4]. Fat necrosis has also been described extensively as a complication of pancreatitis, appearing as masses in fat [2]. Lopez et al. [5] described fat necrosis in the extremities appearing as a small well-defined linear spiculated lesion with either a globular or a laminar component on MRI. 958 Some of the cases in that study had signal intensity similar to that of fat in the globular component. Tsai et al. [3] described the MRI features of subcutaneous fat necrosis as a lack of discrete mass, linear hypointensity on T1-weighted images, and hyper- or hypointensity on T2-weighted images. lthough these studies present interesting findings, we found that histologic correlation was limited. Only one of 12 cases in the study by Lopez et al. and two of 13 cases in the study by Tsai et al. had histologic confirmation of their imaging findings. This limitation was also commented on by Ehara [6]. Canteli et al. [7] described a biopsy-proven case of fat necrosis in which the MRIs JR:181, October 2003
Imaging Features of Fat Necrosis showed multiple high-signal-intensity areas in the subcutaneous fat that were surrounded by hypointense septations, resembling a bunch of grapes. For the second and fifth patients in our series, the appearance of fat necrosis was similar to that described by Canteli et al. and Lopez et al., with a central globular high-signal-intensity area surrounded by fibrous tissue of low signal intensity. The lack of a discrete mass was described by Tsai et al. as a criterion for fat necrosis, but one of our patients had a lesion that was masslike. Two of our cases in which fat necrosis occurred in a lipoma or an atypical lipoma had an unusual appearance. In the third patient, fat necrosis appeared as amorphous cloudlike stranding. In the fourth patient, fat necrosis had an unusual serpentine appearance that, to our knowledge, has not been previously described. We postulate that the hypointense stranding is due to the presence of fibrous tissue and scarring in or adjacent to the area of fat necrosis. Lopez et al. [5] found no contrast enhancement in four of 12 cases that received gadolinium. Three of our five patients were given gadolinium, and all lesions showed rim enhancement. The enhancement is likely to be reactive because of the presence of vascularized fibrous or granulation tissue. Distinguishing fat necrosis in a low-grade well-differentiated liposarcoma from areas of dedifferentiation may be possible. Kransdorf et al. [8] asserted that a feature suggestive of dedifferentiation is a well-defined nonlipomatous mass juxtaposed with a predominantly fatty tumor. In the cases of fat necrosis in lipomatous tumors in our series, none appeared masslike; instead, fat necrosis appeared as amorphous stranding, a serpiginous density, or a central fatty signal with a hypointense rim. It may be more difficult to distinguish between fat necrosis in a lipoma and malignant change. Kransdorf et al. [9] retrospectively compared the imaging features of 35 lipomas and 28 liposarcomas and described features that favor a diagnosis of liposarcoma as a lesion larger than 10 cm, presence of thick septa, presence of either globular or nodular nonadipose areas or masses or of both globular and nodular areas, and decreased percentage of fat composition. In our series, the areas of fat necrosis in a lipoma or an atypical lipoma did not appear globular or nodular; instead, these areas appeared either amorphous or serpentine. In the same study, Kransdorf et al. also found that 11 (31%) of 35 lipomas showed marked nonlipomatous areas. The appearances of the areas were not described in detail, but the surgical and pathology reports showed that they correlated with areas of fat necrosis and associated calcification, fibrosis, and inflammation. iopsy is however still recommended if any questionable areas are seen on MRI. Surgical resection is recommended for large lesions because the likelihood of malignant change is higher than in small lesions. Surgical resection was performed in the last two cases of our series. It may be easier to diagnose superficial fat necrosis using imaging than fat necrosis in nonsuperficial sites and avoid biopsy for several reasons. First, the location of the lesion is subcutaneous. Second, the lesion tends to appear as a globular or laminated mass with a central fatty intensity and a hypointense rim. These findings were seen in some of the biopsy-proven cases described by Lopez et al. [5] and Canteli et al. [7] and also in our cases. We did not encounter any cases similar in appearance to those described by Tsai et al. [3] and did not see any volume loss associated with the lesions. However, the appearances described by Tsai et al. are compatible with scarring and may be seen at a later stage of the disease process. The appearance of fat necrosis on crosssectional imaging seems to vary more than previously described, especially when it is not located in subcutaneous fat. We found the features described by previous authors to encompass only part of the spectrum of imaging appearances of these lesions. For our review of five biopsy-proven cases, we encountered features of fat necrosis that have not been previously described. Study and biopsy of more suspected cases of fat necrosis may be helpful in further characterizing the imaging features of this entity. References 1. Cunningham K, tkinson S, Paes. Subcutaneous fat necrosis with hypercalcemia. Can ssoc Radiol J 1990;41:158 159 2. Carasso S, Oren I, lroy G, Krivoy N. Disseminated fat necrosis with asymptomatic pancreatitis: a case report and review of the literature. m J Med Sci 2000;319:68 72 3. Tsai TS, Evans H, Donnelly LF, isset GS 3rd, Emery KH. Fat necrosis after trauma: a benign cause of palpable lumps in children. JR 1997;169:1623 1626 4. assett LW, Gold RH, Cove HC. Mammographic spectrum of traumatic fat necrosis: the fallibility of pathognomonic signs of carcinoma. JR 1978;130:119 122 5. Lopez J, Saez F, lejandro Larena J, Capelastegui, Martin JI, Canteli. MRI diagnosis and follow-up of subcutaneous fat necrosis. J Magn Reson Imaging 1997;7:929 932 6. Ehara S. MR imaging of fat necrosis. (letter) JR 1998;171:889 7. Canteli, Saez F, de los Rios, lvarez C. Fat necrosis. Skeletal Radiol 1996;25:305 307 8. Kransdorf MJ, Meis JM, Jelinek JS. Dedifferentiated liposarcoma of the extremities: imaging findings in four patients. JR 1993;161:127 130 9. Kransdorf MJ, ancroft LW, Peterson JJ, Murphey MD, Foster WC, Temple HT. Imaging of fatty tumors: distinction of lipoma and well-differentiated liposarcoma. Radiology 2002;224:99 104 JR:181, October 2003 959