AJR 2003; 181:955-959
© American Roentgen Ray Society
Imaging Features of Fat Necrosis
Lai Peng Chan1,
R. Gee2,
Ciaran Keogh2 and
Peter L. Munk2
1 Department of Diagnostic Radiology, Singapore General Hospital, Outram Rd.,
169608 Singapore.
2 Department of Radiology, Vancouver General Hospital, University of British
Columbia, 899 W. 12th Ave., Vancouver, BC, V5Z 1M9 Canada.
Received September 16, 2002;
accepted after revision April 29, 2003.
Address correspondence to P. L. Munk.
Abstract
OBJECTIVE. 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.
Introduction
Fat 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. A 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. All 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. All 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. 2A). In these two
patients, histologic confirmation was by core needle biopsy. The specimens
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 T1-weighted images, surrounded by
an irregular hypointense rim (Fig.
2B). The lesion was hyperintense on inversion recovery images
(Fig. 2C).
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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.
Axial contrast-enhanced CT scan of thigh shows hypodense lesion in
subcutaneous tissue with rim enhancement (arrow) and perilesional
stranding (arrowheads).
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Fig. 2A. —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. Axial unenhanced CT scan shows mass in
subcutaneous fat with central fatty density (arrow) and peripheral
globular component (arrowhead) with density similar to muscle
density.
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Fig. 2B. —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. Sagittal T2-weighted image (TR/TE, 2400/90)
shows lesion has central high signal intensity and low-signal-intensity rim
(arrow).
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Fig. 2C. —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. Axial fast STIR image (2500/20) shows
lesion (arrows) is hyperintense on this sequence.
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The most varied appearance of fat necrosis was seen in the lipomas and
atypical lipoma. In the third patient, both the CT scan
(Fig. 3A) and MRI
(Fig. 3B) 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.
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Fig. 3A. —50-year-old man with mass at posterior aspect of thigh. Fat
necrosis was confirmed after core biopsy. Axial unenhanced CT scan shows
well-defined lesion in biceps femoris muscle that has density similar to that
of subcutaneous fat, consistent with intramuscular lipoma. Amorphous cloudlike
areas of stranding (arrows) can be seen at lateral aspect of
lesion.
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Fig. 3B. —50-year-old man with mass at posterior aspect of thigh. Fat
necrosis was confirmed after core biopsy. Axial 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).
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In the fourth patient, MRI showed a serpiginous lesion
(Fig. 4A) 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.
4B). 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.
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Fig. 4A. —73-year-old man who presented with mass in anterolateral
thigh. After resection, mass was found to be lipoma with area of fat necrosis.
Axial 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).
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Fig. 4B. —73-year-old man who presented with mass in anterolateral
thigh. After resection, mass was found to be lipoma with area of fat necrosis.
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).
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MRI in the fifth patient showed ill-defined nodules in a fat-containing
lesion that was eventually resected and found to be a low-grade 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. 5A) and could be hypo-
or hyperintense on T2-weighted images
(Fig. 5B). After 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.
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Fig. 5A. —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. Axial 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.
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Fig. 5B. —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. Axial fat-saturated T2-weighted fast spin-echo image
(2000/100) shows that nodules can appear hypointense (arrows) or
hyperintense (arrowheads).
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Fig. 5C. —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. 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).
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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,
4B, and
5C).
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Fig. 2D. —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. Sagittal fat-suppressed T1-weighted
gadolinium-enhanced image (500/16) shows lesion is hypointense with irregular
rim enhancement (arrows).
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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 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. 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.
Although 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 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 cloud-like 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. Biopsy 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 cross-sectional 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.
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Abstract
Introduction
