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Hibernoma is a rare tumor is that is thought to arise from remnants of fetal adipose tissue.

The average age of patients with hibernoma is 38 years. Slightly more men than women are affected.
Patients with hibernoma present with a mass.
Hibernoma is not visible on x-ray.
Complete surgical removal of hibernoma is curative.
Complete Information on this Tumor
Introduction and Definition: 

This rare, benign tumor is thought to arise from remnants of a fetal adipose tissue, or brown fat. It is also known as Lipoma of embryonic fat or Fetal lipoma. For this reason, it may occur where fetal or brown adipose tissue persists in adult life, such as the interscapular region. In actuality, the most common location for this tumor is in areas where brown fat is absent, suggesting that this tumor does not in fact always arise from remnants of fetal fat. Brown fat is present in hibernating animals such as bears. Most fat is a white fat.

Incidence and Demographics: 
This benign tumor comprises approximately 1.6% of all benign lipomatous tumors. Both men and women can be affected, from age to to age 75 years. The peak age at diagnosis is 38 years. Approximately 3/5 of patients are men. The most common anatomic location in one series was the thigh -50 cases, followed by the shoulder -20 cases, back-17 cases, neck-16 cases, chest-11 cases, arm-11 cases, and abdominal cavity-10 cases. (Am J Surg Pathol. 2001 Jun;25(6):809-14.)
Symptoms and Presentation: 

This tumor is typically asymptomatic and presents as a fleshy painless mass, with size at presentation ranging from 1 to 24 cm, with an average size of 9.3 cm. The tumor may be present from one month to 10 years, or may be an incidental finding.

X-Ray Appearance and Advanced Imaging Findings: 
Radiographic findings are variable. Lipomas and hybernomas may be seen as low density areas on plain radiographs, but MRI is a better imaging modality.
Laboratory Findings: 
no relevant laboratory findings
Differential Diagnosis: 
lipoma, low-grade liposarcoma, soft tissue sarcoma
Preferred Biopsy Technique for this Tumor: 
Histopathology findings: 
Several variants of hibernoma have been defined. The subtypes include typical, myxoid, spindle cell, and lipoma-like. Typical type cases account for 85% of the tumors.
Treatment Options for this Tumor: 
Complete surgical removal with a marginal margin, and 100% negative margin is sufficient. Wide margin is not required. Intra-regional margins have been reported to lead to recurrence.Recurrence is rare and metastasis has not been reported.
Preferred Margin for this Tumor: 
Outcomes of Treatment and Prognosis: 
After complete removal, recurrence is not expected and patients are expected to do well. Incomplete removal may result in persistence or regrowth of the inadequately resected portion. Metastasis has not been reported and is not expected.
Special and Unusual Features: 
A brown fat, also called brown adipose tissue, is present in newborn humans and in hibernating mammals. In babies and hibernating mammals, the function of brown fat is to generate body heat. In human babies, brown fat is located on the back, along the upper half of the spine and around the shoulders. Brown fat is highly specialized for heat production. Mitochondria contained within the cells of brown fat contain iron and give the fat a brown appearance. In addition, Brown fat contains more capillaries and uses more oxygen than normal white fat.
Suggested Reading and Reference: 
The International Journal of Biochemistry & Cell Biology Volume 30, Issue 1, 19 March 1998, Pages 7-11