Brown Fat Activity
Unlike normal fat, which stores energy, brown fat is thought to release heat in response to cold exposure or anxiety. It is most abundant in the neck, supraclavicular regions, axillae, mediastinum, paraspinal regions and suprarenal areas.
Unfortunately, brown fat very frequently demonstrates extremely intense metabolic activity on PET/CT scans, which can easily mask underlying FDG-avid malignancy. When present, it most commonly affects the neck and supraclavicular regions, symmetric and bilateral in distribution.
While its very characteristic distribution and bilaterality make brown fat activity an easily recognized finding, extreme care must be made to assure that this benign uptake is not masking co-existing FDG-avid malignancy in these same regions.
This requires a slow and methodical comparison of the each axial PET image with its corresponding CT image, to be assured that each and every area of intense uptake clearly corresponds to fat and only fat.
When marked brown fat activity is present, we typically add a brief disclaimer to the beginning of our reports:
“NOTE: Extensive brown fat activity involving the neck, supraclavicular regions, axilla, mediastinum and paraspinal regions is noted. This can obscure small FDG-avid lymph nodes, if present.”
Occasionally, only a small solitary focus of brown fat will be present. As long as this solitary focus of FDG uptake clearly corresponds to fat (often within the mediastinum), the diagnosis is confirmed:
“A subcentimeter focus of intense uptake projects in the prevascular space. As this corresponds to fat on the co-registered CT images, this finding is most consistent with a solitary focus of brown fat activity.”
Prevention:
For patients who have demonstrated extensive brown fat activity on a scan, some advocate preventative measures at the time of a follow up study.
Recommendations include placing warm blankets on the neck and shoulders after injection of FDG and/or P.O. Propranolol (20 mg) or Diazepam (10 mg) 1 hour prior to injection.
