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.