A hibernoma is a

benign neoplasm A benign tumor is a mass of Cell (biology), cells (tumor) that does not Cancer invasion, invade neighboring tissue or Metastasis, metastasize (spread throughout the body). Compared to Cancer, malignant (cancerous) tumors, benign tumors generally ...

of vestigial

brown fat Brown adipose tissue (BAT) or brown fat makes up the adipose organ together with white adipose tissue (or white fat). Brown adipose tissue is found in almost all mammals. Classification of brown fat refers to two distinct cell populations with si ...

. The term was originally used by the French

anatomist Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having its ...

Louis Gery in 1914.

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. Benign neoplasm with

is noted.

Diagnosis

Imaging findings

In general, imaging studies show a well-defined, heterogeneous mass, usually showing a mass which is hypointense to subcutaneous fat on

magnetic resonance Magnetic resonance is a process by which a physical excitation (resonance) is set up via magnetism. This process was used to develop magnetic resonance imaging and Nuclear magnetic resonance spectroscopy technology. It is also being used to ...

T1-weight images. Serpentine, thin, low signal bands (septations or vessels) are often seen throughout the tumor.

Pathology findings

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 Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through adipogenesis. I ...

(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 A lipoma is a benign tumor made of fat tissue. They are generally soft to the touch, movable, and painless. They usually occur just under the skin, but occasionally may be deeper. Most are less than in size. Common locations include upper back, ...

combined with hibernoma

Histochemistry

Oil Red O Oil Red O (Solvent Red 27, Sudan Red 5B, C.I. 26125, C26H24N4O) is a lysochrome (fat-soluble dye) diazo dye used for staining of neutral triglycerides and lipids on frozen sections and some lipoproteins on paraffin sections. It has the appearance o ...

-positive droplets of cytoplasmic lipid can be seen in most cases

Immunohistochemistry

The neoplastic cells are

S100 protein The S100 proteins are a family of low molecular-weight proteins found in vertebrates characterized by two calcium-binding sites that have helix-loop-helix ("EF-hand-type") conformation. At least 21 different S100 proteins are known. They are enc ...

positive (approximately 80%), and show membrane and vacuole

CD31 Platelet endothelial cell adhesion molecule (PECAM-1) also known as cluster of differentiation 31 (CD31) is a protein that in humans is encoded by the ''PECAM1'' gene found on chromosome17q23.3. PECAM-1 plays a key role in removing aged neutrop ...

immunoreactivity.

Uncoupling protein 1 Thermogenin (called uncoupling protein by its discoverers and now known as uncoupling protein 1, or UCP1) is a mitochondrial carrier protein found in brown adipose tissue (BAT). It is used to generate heat by non-shivering thermogenesis, and makes ...

(UCP1), a unique brown fat mitochondrial protein, is also positive.

Cytogenetics

There are structural rearrangements of 11q13-21, which are considered most characteristic. This alteration can be detected by metaphase

fluorescent in situ hybridization Fluorescence ''in situ'' hybridization (FISH) is a molecular cytogenetic technique that uses fluorescent probes that bind to only particular parts of a nucleic acid sequence with a high degree of sequence complementarity. It was developed ...

(FISH).

MEN1 Menin is a protein that in humans is encoded by the ''MEN1'' gene. Menin is a putative tumor suppressor associated with multiple endocrine neoplasia type 1 (MEN-1 syndrome). ''In vitro'' studies have shown that menin is localized to the nucleus, ...

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

Cytology

The

fine needle aspiration Fine-needle aspiration (FNA) is a diagnostic procedure used to investigate lumps or masses. In this technique, a thin (23–25 gauge (0.52 to 0.64 mm outer diameter)), hollow needle is inserted into the mass for sampling of cells that, aft ...

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.

Differential diagnoses

It is important to separate hiberoma from adult

rhabdomyoma A rhabdomyoma is a benign tumor of striated muscle. Rhabdomyomas may be either "cardiac" or "extra cardiac" (occurring outside the heart). Extracardiac forms of rhabdomyoma are sub classified into three distinct types: adult type, fetal type, and g ...

, a

granular cell tumor Granular cell tumor is a tumor that can develop on any skin or mucosal surface, but occurs on the tongue 40% of the time. It is also known as Abrikossoff's tumor, granular cell myoblastoma, granular cell nerve sheath tumor, and granular cell schw ...

and a true

liposarcoma Liposarcomas are the most common subtype of soft tissue sarcomas, accounting for at least 20% of all sarcomas in adults. Soft tissue sarcomas are rare neoplasms with over 150 different histological subtypes or forms. Liposarcomas arise from the pr ...

Classification

This lesion has been called a fetal

, lipoma of embryonic fat or a lipoma of immature fat.

Management

Complete surgical excision is the treatment of choice, associated with an excellent long term clinical outcome.

Epidemiology

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 subcutaneous and muscle tissue of the head and neck region (shoulders, neck, scapular), followed by thigh, back, chest, abdomen, and arms. In rare cases hibernoma may arise in bone tissue, however it is an incidental finding.