Hibernoma
Hibernomas are rare benign soft-tissue tumors containing brown fat. Clinically, they present as slow-growing, painless soft-tissue masses. Physical examination usually reveals a palpable, solitary, soft, and rubbery mass within the subcutaneous fat, which is freely mobile and not attached to deep layers.
Conventional radiography may show a radiolucent mass without internal mineralization or associated osseous abnormalities. Sonographically, hibernomas are well-circumscribed, solid, hyperechoic masses with increased internal vascular flow. Computed tomography shows internal septations and low attenuation values, between those of fat and muscle. Hibernomas are usually hyperintense to skeletal muscle on T1-weighted magnetic resonance imaging (MRI) but slightly hypointense to subcutaneous fat. On T2-weighted images, high signal intensity similar to that of subcutaneous fat is typical. Hibernomas demonstrate intense fluorine-18 fluorodeoxyglucose uptake on positron emission tomography, because, unlike other adipogenic tumors, they contain abundant mitochondria and are highly metabolically active.
Complete surgical excision is the treatment of choice, and is considered curative.
Hibernomas demonstrate moderate uptake on bone scintigraphy blood pool images and mild uptake on delayed images.4 Positron emission tomography (PET) is useful in differentiating hibernomas from other fat-containing lesions.9 Hibernomas demonstrate intense fluorine-18 fluorodeoxyglucose uptake because, unlike other adipogenic tumors, hibernomas contain abundant mitochondria and are highly metabolically active.28
Treatment and Prognosis
Complete surgical excision is the treatment of choice; given the behavior of the benign tumor, marginal complete excision is considered curative.5 Intralesional excision may be the only option for large tumors that are near nerves or vessels. However, intralesional excision may result in continued growth and local recurrence.
At surgery, these tumors usually are encapsulated and/or adherent to skeletal muscle or bone, without invasion, and easily separated from surrounding soft tissues.29 No specific surgical considerations are required beyond standard oncological principles, including careful dissection of adjacent nerves and vessels, and hemostasis. Hibernomas have the potential for significant bleeding during surgical excision. In this setting, embolization becomes a consideration, given the identification of large intratumoral vessels and the benign course of these lesions.
