Fluorodeoxyglucose uptake in absence of CT abnormality on PET-CT: What is it?

Figure 8 Focal brown fat uptake. A 50-year-old woman with history of metastatic breast cancer had fluorodeoxyglucose positron emission tomography-computer tomography (PET-CT) for restaging. Axial PET image shows a focus in the left supraclavicular region (arrow). On the integrated CT, there is no node or lesion except for fat tissue in this location. The uptake is from the brown fat.