A 68-year-old woman presented with a painless palpable mass in the left thigh. Magnetic resonance imaging was performed.
Magnetic resonance images of the pelvis obtained at initial presentation to evaluate a clinically palpable mass in the left thigh. Coronal T1-weighted image of the left thigh showing a small subcutaneous fatty mass with ill-defined margins (A). Coronal T2-weighted fat-suppressed image showing homogeneous suppression of fat signal inside the lesion with overlying subcutaneous edema (B).
Answer to Radiologic Case Study
Fat Necrosis Presenting as a Soft Tissue Mass
A 68-year-old woman presented with a painless palpable mass in the left thigh. The patient reported that she had fallen onto her left side 3 weeks before she first felt the mass. The patient was in remission for B-cell lymphoma.
Magnetic resonance images showed a subcutaneous soft tissue mass with ill-defined margins (Figure 1) and intermixed fat, corresponding to a clinically palpable mass. A diagnosis of fat necrosis was suggested. Because of the patient’s history and concern about lymphoma recurrence, the referring clinician ordered an ultrasound-guided needle biopsy. Microscopic examination of the specimen revealed features of fat necrosis characterized by damaged adipocytes with dropped nuclei in a variably sclerotic or fibrotic background with scattered mono-nuclear inflammatory cells and foamy macrophages (Figure 2). There was no evidence of lymphoma. The patient has been symptom free for the past 9 months since the biopsy.
Magnetic resonance images of the pelvis obtained at initial presentation to evaluate a clinically palpable mass in the left thigh. Coronal T1-weighted image of the left thigh showing a small subcutaneous fatty mass (arrow) with ill-defined margins (A). Coronal T2-weighted fat-suppressed image showing homogeneous suppression of fat signal inside the lesion (arrow) with overlying subcutaneous edema (arrowheads) (B).
Biopsy specimen showing characteristic features of fat necrosis, including damaged fat, background sclerosis or fibrosis, and scattered mono-nuclear inflammatory cells and foamy macrophages (hematoxylin-eosin, original magnification x20).
Fat necrosis rarely presents as a soft tissue mass.1 Patients may present with pain, skin induration, ecchymosis, or skin retraction or thickening. The most common etiology of fat necrosis is trauma, although a variety of other causes have been reported, including collagen vascular disease and myeloproliferative disorders. Disseminated fat necrosis may also occur as a complication of pancreatitis.2 As in the current authors’ case, a capsule or pseudocapsule around the fat necrosis is often absent, although encapsulated lesions have been described in the literature.3,4
A well-defined soft tissue mass in the extremities with macroscopic intralesional fat has a short differential that includes fatty tumors such as lipoma, atypical lipomatous tumors, and lipoma variants. It is not unusual to see fat necrosis in lipomas, especially in superficial lesions; this entity could mimic a more aggressive lesion. Unlike these benign entities, myxoid liposarcomas have small amounts of fat that are often difficult to recognize.5 Dedifferentiated liposarcomas typically consist of a low-grade lipomatous tumor juxtaposed with a non-fatty high-grade tumor.6 Additionally, malignant lipomatous tumors such as myxoid and dedifferentiated liposarcomas are typically located deeper beneath the fascia and have well-defined margins and a visible capsule or pseudocapsule.6
Typical magnetic resonance features of fat necrosis include ill-defined margins, soft tissue signal intensity intermixed with fat, and variable enhancement on postcontrast images.1
When a superficial fatty lesion with ill-defined margins is encountered, especially in the presence of recent trauma, a diagnosis of fat necrosis should be considered. Clinical or imaging follow-up should confirm resolution of the fat necrosis without further intervention in most cases.
- Chan LP, Gee R, Keogh C, Munk PL. Imaging features of fat necrosis. AJR Am J Roentgenol. 2003; 181(4):955–959. doi:10.2214/ajr.181.4.1810955 [CrossRef]
- Carasso S, Oren I, Alroy G, Krivoy N. Disseminated fat necrosis with asymptomatic pancreatitis: a case report and review of the literature. Am J Med Sci. 2000; 319(1):68–72. doi:10.1097/00000441-200001000-00007 [CrossRef]
- Lee SA, Chung HW, Cho KJ, et al. Encapsulated fat necrosis mimicking subcutaneous liposarcoma: radiologic findings on MR, PET-CT, and US imaging. Skeletal Radiol. 2013; 42(10):1465–1470. doi:10.1007/s00256-013-1647-y [CrossRef]
- Schmidt-Hermes HJ, Loskant G. Calcified fat necrosis of the female breast [in German]. Med Welt. 1975; 26(24):1179–1180.
- Sundaram M, Baran G, Merenda G, McDonald DJ. Myxoid liposarcoma: magnetic resonance imaging appearances with clinical and histological correlation. Skeletal Radiol. 1990; 19(5):359–362. doi:10.1007/BF00193091 [CrossRef]
- Jelinek JS, Kransdorf MJ, Shmookler BM, Aboulafia AJ, Malawer MM. Liposarcoma of the extremities: MR and CT findings in histologic subtypes. Radiology. 1993; 186(2):455–459. doi:10.1148/radiology.186.2.8421750 [CrossRef]