A 26-year-old woman with multiple endocrine neoplasia type 1 (MEN1) was referred for positron emission tomography-computed tomography (PET-CT) to evaluate a new pancreatic mass. Axial CT (A) and fused PET-CT (B) images show an ovoid hypermetabolic low-density lesion in the subcutaneous fat of the left thigh, abutting the posterior muscular compartment. Subsequent ultrasound shows the lesion to be hyperechoic (C, arrow). Differential considerations included hibernoma, liposarcoma, atypical lipoma, and metastases. Ultrasound-guided biopsy revealed a hibernoma.
Hibernomas are rare, benign brown adipose-containing tumors typically in intramuscular fatty regions. The 4 types, from most to least common, are: typical, myxoid, lipoma-like, and spindle cell.1 Approximately 30% are located in the thigh, followed by the shoulder and back. Mutations involving the chromosome band 11q13 are linked to hibernomas and MEN1 syndrome, with several case reports demonstrating a possible link.2
Hibernomas typically have attenuation between fat and muscle with varying degrees of enhancement.2 On ultrasound, hibernomas are usually well-delineated and hyperechoic, similar to lipomas. They are intensely fluorine 18 fluorodeoxyglucose (FDG)-avid and have signal intensity similar to fat on MR. Imaging is not diagnostic. Percutaneous biopsy can be helpful, but surgical excision is usually recommended.
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Tillman BG, Johnston MJ. At the Viewbox: Hibernoma. J Am Osteopath Coll Radiol. 2016;5(3):27.