Hibernoma is an infrequent type of benign soft tissue tumor composed of brown adipocytes that may present in a similar manner to liposarcoma, despite the fact that it has no malignant or metastatic potential.
Hibernomas are usually located in the interscapular region, but may also be encountered in the cervical, thigh or axillary areas. Intra thoracic, intramuscular, abdominal and retroperitoneal lesions have also been described, as well as exceptional locations such as the uterus, breast, popliteal area, cranium, pericardium  or mediastinum. Lesions may be multiple or bilateral, are painless, but may cause symptoms related to their mass effect  or to their secretory activity, as they are known to sometimes produce steroid hormones. They are usually described as slow growing (but fast growth is also possible) and firm or rubbery and are mostly encountered in adults in their thirties, and differential diagnosis with lipomas and liposarcomas must be taken into consideration, given that clinical presentations of the three pathological entities often overlap  . Once the diagnosis of hibernoma is established, the physician does not need to be concerned about malignant transformation or metastasis . However, recurrence is possible, although very rare.
The first step in diagnosing a hibernoma after the clinical suspicion is raised is to perform imaging procedures. A radiography of the affected region will show no bone erosion or calcification. Echography describes hyperechoic tissue, while Doppler imaging highlights the presence of enlarged vessels. Computer tomography scanning describes well vascularized, well circumscribed, heterogeneous, diffusely septated lesions with a possibly incomplete peripheral capsule. Hibernoma signal intensity in magnetic resonance imaging is variable , usually high, but slightly inferior to that of subcutaneous tissue . The degree of homogeneity is variable . This method shows diffuse gadolinium enhancement of an otherwise isointense or mildly hypointense tissue formation. Myxoid hibernomas, being richer in water have higher signal intensity on magnetic resonance imaging scans . Fluorodeoxyglucose positron emission tomography shows important radiotracer capture  due to the fact that hibernomas contain numerous mitochondria and are metabolically very active. Scintigraphy also reveals high radiotracer uptake with 99mTc-tetrofosmin, 123I-meta-iodobenzylguanidine and 99mTc-methoxyisobutylisonitrile administration. If an angiography is considered to be necessary, it may reveal, besides intense vascularization, the presence of internal arterio-venous shunting  which is important information prior to excision.
If clinical and imaging differential diagnosis cannot exclude a malignancy, excision is imperative. Analysis of the surgical specimen reveals a light brown to gray mass of tissue surrounded by a thin capsule . Microscopic examination reveals high -number of mature, multivacuolated or uni vacuolar adipocytes intersecting with brown fat, intense vascularization, and few adipocyte atypia. Tumors may, however, be more or less well differentiated. Immunohistochemical analysis shows a positive cluster of differentiation (CD) 68 and negative CD 34.
In cases where cytogenic analysis reveals the presence of a 11q13 translocation, liposarcoma is improbable . Rearrangements in the 11q13-21 region can be detected using metaphase fluorescent in situ hybridization.