31 years old male with swelling at lateral aspect of right upper thigh. No nidus to suggest AVM. Lesion is made predominantly of fat.
A hibernoma is an uncommon benign fatty tumor that arises from the vestiges of fetal brown fat and is named “hibernoma” because of its resemblance to the brown fat in hibernating animals.1
Hibernomas usually occur between the ages of 20 and 40 and have a slight female prevalence.1
They grow slowly and usually present with painless enlargement. Symptoms related to the compression of adjacent structures rarely develop.2 There are no reports of metastases or malignant transformation.1
Hibernomas are typically fatty, hypervascular lesions that are grossly similar to lipoma. They are well-defined, encapsulated, and mobile masses. Their color varies from tan to red brown, depending on the amount of intracellular lipid. The diameter usually ranges from 5 to 10 cm, but they may reach up to 20 cm, particularly in the retroperitoneum.
Upon microscopic examination, the tumors are characterized by cells of various degrees of differentiation. Multivacuolar adipocytes and brown fat cells with granular eosinophilic cytoplasm are interspersed with univacuolar adipocytes. Hypervascularity combined with abundant mitochondria give hibernomas their color.1-4
Morphologic variants of hibernoma in descending order of frequency are1-4 :
• tyical ( most common )
• lipoma-like, and
• spindle cell variants.
They are usually seen in locations where normal brown adipose tissue is present, such as the thigh, shoulder, back, neck, chest, arm (in descending order of frequency), and rarely in the retroperitoneum.1-2
Recently, the thigh has been described as a very common location.
Among the diagnostic procedures, CT, MR imaging can provide helpful information.
On CT, hibemomas have attenuation that varies between that of fat and skeletal muscle. They are well circumscribed and may have a peripheral soft tissue attenuation capsule. They enhance slightly and may contain prominent enhancing vessel.2
Although they present as brown fat, the imaging characteristics on T1- and T2-weighted images demonstrate high signal intensity but slightly less than that of the subcutaneous fat. On MR imaging, flow voids can be identified.1-3-5
On fat suppression sequences, there may be incomplete fat suppression because of the nature and amount of lipids.1-2
The treatment of hibernomas consists of complete surgical resection and local recurrence does not occur.
General imaging considerations include
• resolving haematoma
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