[hibernus, Latin = winter]

= rare benign slow-growing soft-tissue tumor composed of brown fat arising in regions in which vestiges of fetal brown fat persist

Histo: admixture of multivacuolar adipocytes (containing multiple granular fine cytoplasmatic vacuoles) + brown fat cells interspersed with univacuolar adipocytes

Age: 20–40 years: M <F

Location: thigh, buttock, scapular region, trunk, neck, mediastinum, chest wall, perirenal, breast, scalp, periureteric region

• painless firm slow-growing mass
• warm overlying skin ← metabolic mitochondrial activity with relatively increased vascularity

Mean size: 9.4 (range, 1–24) cm

• well-demarcated mass with attenuation and signal intensity between that of subcutaneous fat and muscle
• varying degrees of internal contrast enhancement
• NO calcifications

US:

• echogenic mass with well- / ill-defined border
• increased flow in large surface vessels

CT:

• well-defined hypoattenuating lesion with intratumoral septa
• enhancement of septa ± entire mass

MR:

• usually slightly hypointense relative to subcutaneous fat on T1WI
• variable intensity on T2WI
• hyperintense to subcutaneous fat on STIR
• variable contrast enhancement of septations
• rarely isointense to subcutaneous fat on all sequences (if multivacuolated adipocytes constitute <70% of mass)

PET:

• very high standardized uptake values (SUV): 1.9 – 26.7