Hibernomas

A hibernoma is a rare benign fatty tumor arising from the vestiges (remnant / trace) of fetal brown fat. It is predominantly seen in 20-40 yrs age group with a slight female predilection. It is named so because of resemblance to brown fat in hibernating animals.

Most often the lesion is located in subcutaneous regions of the back (periscapular /interscapular region), neck, axilla, shoulder, thorax, thigh and retroperitoneum. Rarely these are also seen in scalp, breast, peri-ureteric region and scrotum.

Hibernoma usually manifest as slowly growing painless soft-tissue mass. Sometimes patient can be symptomatic due to compression of adjacent neuro-vascular structures, for example sciatica in a posterior thigh lesion.

In ultrasound these usually appear as hyperechoic lesions. Angiography will reveal lesional vascularity with arterio-venous shunting. These can mislead due to increased uptake in 18-FDG-PET.

In CT these lesions show low attenuating lesion, slightly higher attenuating than the subcutaneous fat. Enhancing septae may be seen, with lesion showing mild enhancement. MRI usually demonstrate lesion to be isointense to subcutaneous fat in most cases, with intervening enhancing sepatations. Reduced T1 and T2 signal intensity is also described in few cases. Contrast enhancement need not be present in all cases.

Additionally prominent lesional vessels may be seen in the ultrasound or as T2 flow voids in MRI, with can cause torrential hemorrhage during biopsy, especially in deep seated lesions.

Case courtesy of Dr Matt A. Morgan, Radiopaedia.org. From the case rID: 37589

Above image shows an intermuscular lesion between the gluteus medius and maximus muscles, with intermediate attenuation between fat and muscle. Prominent feeding vessel is seen along the lateral margin of the lesion.

Pre-operatively differentiating from liposarcomas is difficult. Four histological subtypes have been defined: Typical (~80%), Myxoid (8%), Lipoma-like and spindle cell variant. 'Typical' variant has predominantly brown fat. Myxoid variant is seen more often in men with higher water content (increased T2 signal). Lipoma-like lesion is seen more commonly in thigh, more resembling adult fat.


References:
Imaging Findings of a Hibernoma of the Neck
A.C.B.S. da Motta, D.E. Tunkel, W.H. Westra and D.M. Yousem
American Journal of Neuroradiology September 2006, 27 (8) 1658-1659;

CT and MR characteristics of hibernoma: six cases
Dursun, Memduh et al.
Clinical Imaging , Volume 32 , Issue 1 , 42 - 47

From the Archives of the AFIP
Benign Musculoskeletal Lipomatous Lesions
Mark D. Murphey, John F. Carroll, Donald J. Flemming, Thomas L. Pope, Francis H. Gannon, and Mark J. Kransdorf. RadioGraphics 2004 24:5, 1433-1466