Hibernoma - Intense uptake seen on Tc-99m tetrofosmin and FDG positron emission tomographic scanning

A 65-year-old man with several subcutaneous masses (clinically diagnosed as lipomata), underwent Tc-99m tetrofosmin scanning for chest pain. One of these masses (in the right thigh) was incidentally noted to take up tetrofosmin. Computed tomography (CT) and magnetic resonance imaging (MRI) demonstrated a fat-containing lesion, and F-18 fluorodeoxyglucose (FDG) positron emission tomography (PET or C-PET) showed active uptake. Biopsy showed the lesion to be a hibernoma. Hibernoma is a benign but metabolically active tumor of brown fat origin. Brown fat is a major heat producer in the newborn and contains high levels of mitochondria. This high metabolic rate probably explains the uptake of Tc-99m tetrofosmin and F-18 FDG. Nevertheless, without a biopsy, the imaging studies could not exclude liposarcoma.