Hibernoma
A hibernoma is a benign neoplasm of vestigial brown fat. The term was originally used by the French anatomist 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 brown fat 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 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. Histochemistry. Oil Red O-positive droplets of cytoplasmic lipid can be seen in most cases Immunohistochemistry. 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. Cytogenetics. 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. Cytology. 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. Differential diagnoses. It is important to separate hiberoma from adult rhabdomyoma, a granular cell tumor and a true liposarcoma. Classification. This  lesion has been called a fetal lipoma, 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.