Hibernoma

A hibernoma is a benign neoplasm of vestigial brown fat. They were first described under the name 'pseudolipoma' by the German physician H. Merkel in 1906 and the term hibernoma was proposed by the French anatomist Louis Gery in 1914 because of its resemblance to brown fat in hibernating animals.

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. Benign neoplasm with brown fat is noted.

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.

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). MEN1 gene (11q13.1) is most frequently deleted, while GARP gene (11q13.5) may also be involved.