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.
Entire Body System
We present an unusual case of supraclavicular benign hibernoma in a 12-year-old girl who presented with chest discomfort, night sweats, shortness of breath, fatigue, and pruritus. [ncbi.nlm.nih.gov]
A 44-year-old patient was evaluated in the outpatient clinic: he reported abdominal pain for the last 5 months, associated with vomiting and sporadic sweating. Antihypertensive treatment was initiated, which provided a partial response. [elsevier.es]
She reported no trauma to the area and exhibited no systemic signs of weight loss, fatigue, fever, chills, or night sweats. Physical examination revealed a soft nontender right medial thigh mass measuring 18 20 cm. [journals.lww.com]
CASE REPORT A 42-year-old man with no past medical history, was admitted for a 6-month history of a right chest pain without fever, chills, night sweats or weight loss. Physical examination was normal. [latunisiemedicale.com]
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.
- Mediastinal Mass
Further investigation revealed a left upper mediastinal mass next to the aortic arch. Upon surgical exploration a soft encapsulated mass was identified. Pathology was consistent with hibernoma. [ncbi.nlm.nih.gov]
In the first instance, this is an isolated posterior mediastinal mass. On CXR, the differential diagnosis is broad. [cureus.com]
- Soft Tissue Calcification
It was well marginated and showed no soft tissue calcification or invasion of nearby structures. There was no enhancement with intravenous contrast. [healio.com]
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