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.