Abstract:

A 17 year-old male presented to clinic for evaluation of a large, 8 x 8 cm subcutaneous mass of the right upper arm. The lesion was asymptomatic but had been progressively enlarging for the previous 4 years. A lipoma was clinically suspected and decision was made to proceed with surgical excision. During intraoperative dissection a freely-mobile but friable, red-brown fleshy mass was identified within the subcutaneous tissues and submitted to pathology. Multiple sections revealed a well circumscribed tumor composed of multivacuolated round to polygonal adipocytes with central, small nuclei surrounded by normal adipocytes, consistent with hibernoma. Hibernomas are benign neoplasms of vestigial remnants of brown adipose tissue which is thought to allow for non-shivering thermogenesis in babies and hibernating animals. They account for roughly 1% of lipomatous tumors with only 5% of cases occurring in the pediatric population (mean age of diagnosis = 38). Notably, there are four distinct variants of hibernoma: typical hibernomas, lipoma-like hibernomas, myxoid variant, and a spindle cell variant. It is important to keep hibernoma and its morphologic variants on your list of differential diagnoses for lipomatous tumors to avoid misdiagnosis as an atypical lipoma, well-differentiated liposarcoma, or myxoid liposarcoma. To the best of our knowledge, our case represents the first report of a hibernoma found on the upper extremity of a pediatric patient. This case also serves as a reminder for clinicians and pathologists to consider this rare, yet benign tumor in patients of all ages with suspected lipomatous lesions.

Financial Disclosure:
No current or relevant financial relationships exist.

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Kayla Penny