Discussion
Hibernoma is a rare, benign tumor which consists of cells that histologically resemble brown fat cells. It was first reported by Merkel in 1906 and then named by Grey as “hibernoma” in 1914, because of its histological similarity to the brown fat of hibernating animals [2]. Unlike typical adipose tissue, brown fat has abundant vascularity to generate heat [3]. Brown fat usually disappears after 8 weeks of life, and it is slowly replaced by white fat [4].
Hibernoma accounts for only 1% of all benign lipomatous tumors and are usually asymptomatic unless it compresses nearby structures with bulky mass. It is most commonly identified in the thigh, but it can also occur anywhere with scattered brown fat, such as bone, retroperitoneum, scalp, neck, axilla, shoulder, thorax, breast, stomach, etc. [2]. Its prevalence peaks in the fourth and fifth decades of life, and gender predominance is controversial among articles [3, 4].
Due to its low incidence, it is sometimes misdiagnosed as well-differentiated liposarcomas, or myxoid liposarcomas. On computed tomography (CT) scans, hibernomas sometimes present as well circumscribed, hypodense lesions with linear septations, which indicates high vascularity [3]. On MRI, hibernomas can be isointense or relatively hypointense to the surrounding fat on both T1 and T2, and diffuse heterogenous enhancement can be identified due to hypervascularity of the mass. On F-18 FDG PET/CT scan, hibernomas show uptake because of metabolically active brown fat tissue. However, none of images listed above are pathognomic, and this also hinders the differentiation of hibernoma with sarcomas [2].
Therefore, the pathologic diagnosis should be performed before initial treatment to exclude malignancy. Fine needle aspiration is suggested rather than core needle biopsy because there is a possibility of hemorrhage due to high vascularity. On the other hand, some prefer core needle biopsy because sufficient tissue can increase diagnostic yield [2].
As described in this case, large multivaculolated brown fat cells, single central or small eccentric nuclei, large amount of granular cytoplasm, fibrous septae, branching capillaries can be seen in microscopic examination of hibernoma [2].
Furlong et al. suggested four main histological variants of hibernoma: typical, spindle, lipoma-like, and myxoid type. Typical hibernoma and lipoma-like variant are most commonly occurred in thigh, whereas the myxoid and spindle cell type are frequently found in the head and neck region [1]. Typical hibernomas, which is characterized with eosinophilic, pale, and myxoid cells, accounts for about 82% for all cases. The second most common type is the myxoid variant, which has a loose basophilic matrix, accounting for 9% for total cases. Lipoma-like type and spindle cell type are followed, each accounting for 7% and 2% of all cases. Most surgical pathologists do not subclassify aforementioned subtypes because all of them have similar prognosis [2].
Because of its multiple pathological variants and the remaining possibility of misdiagnosis regardless of using clinical, imaging, and pathologic information, some researchers suggested additional chromosomal evaluation to differentiate hibernoma [1].
To date, there is no reported case of malignant transformation or metastasis of hibernoma, so theoretically asymptomatic hibernoma doesn’t need treatment [5]. However, due to diagnostic uncertainty of modalities as mentioned above, the treatment of choice is en bloc resection of the mass. En bloc removal is important to prevent recurrence, and also bleeding due to its high vascularity. According to previous articles, almost none of hibernoma patients had recurrence of disease and none died due to the complications of hibernoma with en bloc resection. Some exceptions with recurrences were possibly due to positive resection margins rather than a true recurrence [2].
Since hibernoma is well known for no recurrence after complete removal, the recurrence after the thoracic surgery in this case is also likely to be a result of remnant tumor after the first surgery. Considering the mass extension from thoracic cage into infra-thyroid level at the initial CT, the recurrence might have not been occurred if the primary surgery was planned as co-operation with thoracic surgeon and head and neck surgeon. In addition, even though vocal fold palsy did not occur in this patient, since it was difficult to identify recurrent laryngeal nerve at the second surgery due to the adhesion caused by the first surgery, co-operation should have been preferred as the initial plan for this patient. Even though hibernoma is a benign tumor, this case emphasizes the importance of complete removal of mass to prevent recurrence.
Conclusion
We present a successful removal of recurred hibernoma in neck without any complication. This is a first case report of cervical hibernoma and its management in Korea.