| Abstract|| |
Hibernoma is a rare, benign soft tissue tumor described in <200 case reports/case series. It is slow-growing, painless, and commonly mistaken for lipoma or liposarcoma. Histopathological diagnosis is must for confirmation. Total excision is the treatment of choice. We present a rare case of hibernoma of thigh, which was initially misdiagnosed as atypical lipomatous tumor/well-differentiated liposarcoma on imaging, which turned out to be a hibernoma on histopathological examination.
Keywords: Brown fat, hibernoma, lipomatous tumour
|How to cite this article:|
Patil SD, Sheik AR, Tewari V, Mutreja D. Hibernoma: A missed diagnosis!!. Indian J Pathol Microbiol 2019;62:461-3
| Introduction|| |
Hibernoma is a rare, benign soft tissue tumor originating from brown fat. It is a slow-growing, painless tumor that accounts for 1.6% of benign lipomatous tumors and 1.1% of all adipocytic tumors. Grey coined the term “hibernoma” because of its resemblance to brown fat of hibernating animals., Furlong et al. have reported 170 cases of hibernoma in an Armed Forces Institute of Pathology study. This article describes a rare case of hibernoma thigh.
| Case History|| |
A 39-year-old lady presented with complaints of a slowly growing painless swelling on anterior aspect of right thigh. There was no history of trauma or similar swelling elsewhere. The patient does not have any comorbidities or any other significant past history. Physical examination revealed a solitary, round, soft, nontender mass, located on anterior aspect of thigh. Laboratory examination was normal. (Hb 13.2 g/dl, total leukocyte count 10,700/mm3, DLC-N60L35E03M02, fasting blood sugar level of 89 mg/dl, serum creatinine of 0.9 mg/dl). Ultrasonography (USG) revealed a well-circumscribed hyperechoic lesion in the anterior compartment of thigh. Magnetic resonance imaging (MRI) of thigh [Figure 1] revealed a well-circumscribed oval lesion within the vastus intermedius muscle of thigh. It showed fat signal on noncontrast images and heterogenous postcontrast enhancement. There were no areas of hemorrhage or necrosis or prominent vascular channels within this lesion. No infiltration into the surrounding muscle, bone, or vascular structures seen. On the basis of MRI findings, a diagnosis of atypical lipomatous tumor/well-differentiated liposarcoma (WDLPS) was made. Further patient underwent incisional biopsy. Histopathology from the studied sections revealed encapsulated, lobulated, soft and greasy mass with yellowish to patchy red-brown cut surface. Microscopy [Figure 2] revealed hibernoma cells as large multivacuolated fat cells with eccentric nucleus and abundant granular to vacuolated cytoplasm. The patient underwent complete resection of the tumor and remained symptom free during 2-year follow-up with no signs of recurrence.
|Figure 1: T2 coronal, T1 axial, T1 fat saturated pre- and postcontrast MRI of thigh showing a heterogeneously enhancing hyperintense oval lesion in vastus medialis muscle|
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|Figure 2: Hematoxylin and eosin stained sections of tumor (10×, 40× views): hibernoma cells – large cell with eccentric nuclei and abundant granular cytoplasm|
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| Discussion|| |
Hibernoma is a rare adipocytic tumor and forms approximately 1.1% of all adipocytic tumors. Its occurrence in third to fourth decade of life contrasts with other fatty tissue tumors such as liposarcomas that occur in fourth to seventh decade.,, Literature review reveals a slight female preponderance.,, Hibernoma originates from brown fat. Brown fat forms 5% of body weight in fetus and neonates. Progressively, it decreases over a period of time and in adults it persists in neck, mediastinum, axilla, periadrenal, and perirenal areas., The most common reported sites of hibernoma are interscapular region, axilla, neck, thigh, mediastinum, and retroperitoneum.,, The other rarely reported sites are scalp, buttock, popliteal fossa, scrotum, intracranial, intraspinal, and periureteric.,, Atypical lipomatous tumors are commonly seen in extremities, whereas WDLPSs are seen in retroperitoneum and mediastinum. Hibernomas are usually asymptomatic and present as slow-growing painless mass. Large tumors may be symptomatic due to pressure effect on adjacent organs. Majority of the hibernomas are subcutaneous and only 10% are intramuscular. They do not have malignant potential. They are usually mistaken for other fat containing tumors on clinical examination and imaging.
Initial imaging workup includes USG followed by computed tomography (CT) or MRI, 18-F-fluorodeoxyglucose position emission tomography (18F-FDG-PET), and angiography. USG shows a well-circumscribed hyperechoic mass. Doppler imaging may show hypervascularity within the tumor. CT scan shows a lobulated well-defined fat-attenuating mass with multiple septations and variable enhancement. MRI shows an encapsulated, well-defined fat-containing mass. Presence of T1 hyperintense signal which is slightly less than the subcutaneous fat and incomplete fat suppression on fat saturated sequences may be a useful clue. They show diffuse postcontrast enhancement as compared to lipomas which do not show any enhancement. Presence of thickened (>2 mm) irregular septa and nonfatty elements within the tumor can help to differentiate WDLPS from lipomas and hibernomas. 18F-FDG-PET helps to differentiate hibernoma from liposarcoma, as hibernomas have increased uptake. Angiography may help in differentiating hypervascular hibernoma from hypovascular tumors such as lipoma. Thus imaging cannot reliably diagnose hibernoma, and histopathological confirmation is essential as was in our case.
Macroscopically, hibernomas are well-circumscribed, encapsulated, lobulated soft greasy mass of usually 5–10 cm size. Cut surface reveals yellow to red brown appearance with rare areas of hemorrhage. Microscopic sections reveal encapsulated tumor comprising large multivacuolated cells and univacuolated cells. Multivacuolated cells have eccentric vesicular nucleus and characteristically fine granular, vacuolated cytoplasm. Abundant capillaries with lack of mitosis or atypia is a usual feature. Presence of atypia and lipoblasts helps to differentiate liposarcoma from lipoma and hibernoma. These adipocytic tumors can be reliably diagnosed with the help of molecular markers such as MDM-2, CDK-4, and p-16 [Table 1].
Hibernoma is a rare benign brown fat tumor, which is usually misdiagnosed clinically and radiologically as lipoma or liposarcoma. Thus, hibernoma should always be considered as a differential for these lesions.
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Abdul R Sheik
Department of Radiology, CHAF, Bangalore, Karnataka
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]