Kamran A. Saraf, Anesthesia Resident, Department of Anesthesiology, Perioperative Medicine, and Pain Management; Carol Bowen-Wells, Fellow in Surgical Oncology, DeWitt Daughtry Family Department of Surgery; Eli Avisar, Assistant Professor of Surgery, DeWitt Daughtry Family Department of Surgery, Leonard M. Miller School of Medicine, University of Miami, Miami, FL
Kamran A. Saraf, MD
and Pain Management
Carol Bowen-Wells, MD
Fellow in Surgical
DeWitt Daughtry Family
Department of Surgery
Eli Avisar, MD
Assistant Professor of
DeWitt Daughtry Family
Department of Surgery
Leonard M. Miller School
University of Miami
Introduction: Hibernomas are benign soft-tissue tumors comprised of remnants of brown fat. They often present as painless, slow-growing masses. Fewer than 200 of these lesions have been reported in the world literature, most of which were centrally located or found in the very proximal shoulder and thigh.
Results and discussion: A rare case of a peripherally located hibernoma is reported. This paper also discusses the radiographic findings, pathologic features, and management of hibernomas.
Conclusion: Treatment of hibernomas consists of surgical resection, and recurrence is rare.
Hibernomas are benign soft-tissue tumors composed of remnants of brown fat. They are painless, hypervascular, slow-growing masses that appear clinically similar to malignancies such as low-grade liposarcoma. A review of the literature revealed about 200 reported cases of hibernoma worldwide. Most of the described cases were centrally located on the body. We present the clinical, radiographic, and pathologic features of a hibernoma that was peripherally located in the elbow region.
A 42-year-old white man was referred to our institution for evaluation of an enlarging soft-tissue mass on his distal left upper arm, above the elbow. The patient had noticed the mass 9 months earlier, and it had gradually increased in size (Figure 1). On physical examination, a 12 x 8-cm mass was palpated on the dorsal aspect of his distal upper arm. The mass was firm, rubbery, and slightly tender to palpation. No overlying warmth or erythema was noted, and no lymphadenopathy was found. The patient had no neurological deficit but reported experiencing a painful sensation along the dorsum of his lower left arm.
Magnetic resonance imaging (MRI) of the upper extremity demonstrated a well-defined mass in the region of the brachioradialis muscle, displacing the radial nerve laterally. The mass did not invade the surrounding structures. It exhibited high signal intensity, consistent with a lipomatous tumor (Figure 2).
Open biopsy was undertaken, and pathology findings were consistent with a hibernoma. Surgical resection was performed under general anesthesia, yielding a 9.5 x 6 x 4.5-cm, rubbery, encapsulated mass. The mass had grown inside the brachioradialis muscle and displaced the radial nerve laterally. It was excised from the surrounding tissue without difficulty. Cut serial sections revealed soft, yellow, homogeneous, glistening surfaces (Figure 3). No hemorrhage or necrosis was identified. Histology confirmed tumor characteristics consistent with a hibernoma, showing large multivacuolated cells, scant cytoplasm, and central nuclei (Figure 4).
Hibernomas are rare tumors, which Merkel originally termed "pseudolipoma" in 1906.1 In 1914, Gery referred to them as "hibernomas" due to the tumor's similarity to the brown fat of hibernating animals.2 Brown fat is a source of nonshivering thermogenesis in animals due to its rich vascular supply and abundant mitochondria.3 Brown fat is also present in infants and gradually decreases over time, beginning at about 8 weeks.4 Common sites of origin for brown fat include the intrascapular area, axilla, neck, and mediastinum. These are areas of fetal distribution and sites of residual brown fat in adults.5 One review found the highest incidence of hibernomas to be in the thigh.6 Although hibernomas are usually seen in the third to fifth decades of life, they can occur at any age. Reports of sex predilection vary.5
Hibernomas are firm, mobile, and slow-growing, sometimes mimicking lipomas. They are usually painless, and symptoms tend to be compressive.7 Hibernomas have low metastatic potential. There are no reported cases of recurrence following complete local excision.6 Localized warmth over the mass may be apparent due to the tumor's hypervascularity.3,8 Core-needle biopsy is not recommended because of the risk of significant bleeding.7,9
On computed tomography scanning, hibernomas are heterogeneously hypodense because of their lipid and fibrovascular content. On MRI, they appear bright on T1-weighted images and have moderate signal intensity on T2-weighted images.10 Their high vascular content on MRI commonly results in the diagnosis of liposarcoma.11
Grossly, hibernomas are soft and well-encapsulated, with a lobular appearance and pigmentation that is brown-to-yellow.12 Internally, their architecture is homogeneous, with fatty tissue interspersed among multiple fine septations. No areas of hemorrhage or necrosis are present.
Histologically, hibernomas are well-vascularized and have a lobular growth pattern. They are typically comprised of three cell types: multivacuolated cells with visibly eosinophilic cytoplasm and small central nuclei; univacuolated adipocytes; and smaller granular eosinophilic cells containing little lipid.13 The lack of mitotic figures and pleomorphism characterizes hibernomas as benign tumors and helps to distinguish them from low-grade liposarcomas.14 On electron microscopy, there is a relatively high concentration of mitochondria and a paucity of organelles.
Although these tumors are considered benign, they may grow to significant sizes and compromise surrounding structures. Surgical excision is the treatment of choice for these lesions.
Hibernomas are rare benign lipomatous tumors that can become large masses over time and compress surrounding structures. They are usually located centrally; peripherally occurring hibernomas, such as our patient's, are rare. Their clinical and radiologic appearance is commonly mistaken for malignant liposarcomas. A tissue diagnosis is essential to ensure appropriate treatment, which is complete excision of the hibernoma.
The authors have no relationship with any commercial entity that might represent a conflict of interest with the content of this article and attest that the data meet the requirements for informed consent and for the Institutional Review Boards.