|Classification and external resources|
|ICD-10||D17 (ILDS D17.950)|
Signs and symptoms
Patients present with a slow-growing, painless, solitary mass, usually of the subcutaneous tissues. It is much less frequently noted in the intramuscular tissue. It is not uncommon for symptoms to be present for years.  
In general, imaging studies show a well-defined, heterogeneous mass, usually showing a mass which is hypointense to subcutaneous fat on magnetic resonance T1-weight images. Serpentine, thin, low signal bands (septations or vessels) are often seen throughout the tumor.
From a macroscopic perspective, there is a well-defined, encapsulated or circumscribed mass, showing a soft, yellow tan to deep brown mass. The size ranges from 1 to 27 cm, although the mean is about 10 cm. 
The tumors histologically resemble brown fat. There are four histologic types recognized, but one is the most frequently seen (typical). There is a background of rich vascularity.
- Lobular type: Variable degrees of differentiation of uniform, round to oval cells with granular eosinophilic cells with prominent borders, alternating with coarsely multivacuolated fat cells (pale cells). There are usually small centrally placed nuclei without pleomorphism. The cells have large cytoplasmic lipid droplets interspersed throughout.  
- Myxoid variant: Loose, basophilic matrix, with thick fibrous septa, and foamy histiocytes
- Lipoma-like variant: Univacuolated lipocytes, with only isolated hibernoma cells
- Spindle cell variant: Spindle cell lipoma combined with hibernoma
Oil red O-positive droplets of cytoplasmic lipid can be seen in most cases.
The neoplastic cells are S100 protein positive (approximately 80%), and show membrane and vacuole CD31 immunoreactivity. Uncoupling protein 1 (UCP1), a unique brown fat mitochondrial protein, is also positive.
There are structural rearrangements of 11q13-21, which are considered most characteristic. This alteration can be detected by metaphase fluorescent in situ hybridization (FISH). Interestingly, MEN1 gene (11q13.1) is most frequently deleted, while GARP gene (11q13.5) may also be involved.
The fine needle aspiration smears show small, round, brown fat-like cells, with uniform, small cytoplasmic vacuoles and regular, small, round nuclei. There is usually a rich vascular background of branching capillaries. It is not uncommon to also have mature fat cells.
Complete surgical excision is the treatment of choice, associated with an excellent long term clinical outcome.
The tumor is rare, affecting adults in the 4th decade most commonly. Patients are usually younger than those who present with a lipoma. There is a slight male predominance. Hibernoma are most commonly identified in the head and neck region (shoulders, neck, scapular), followed by thigh, back, chest, abdomen, and arms.
- Furlong, M. A.; Fanburg-Smith, J. C.; Miettinen, M. (2001). "The morphologic spectrum of hibernoma: A clinicopathologic study of 170 cases". The American journal of surgical pathology 25 (6): 809–814. doi:10.1097/00000478-200106000-00014. PMID 11395560.
- Paul, M. A.; Koomen, A. R.; Blok, P. (1989). "Hibernoma, a brown fat tumour". The Netherlands journal of surgery 41 (4): 85–87. PMID 2674772.