Asymmetric 18F-FDG Uptake in the Infradiaphragmatic Brown Adipose Tissue (BAT) Mimicking Adrenal Metastasis: A Relatively Rare Site of Brown Fat and a Potential Source for False Positive FDG-PET Study

Abstract

FDG uptake in the metabolically active brown adipose tissue (BAT) is a source of significant concern while interpreting FDG-PET studies. It is also of great interest due to its potential implications for obesity research. In this communication, we describe hitherto unreported asymmetric BAT uptake in the abdomen, persisting after diazepam intervention in the repeat PET study on a separate day. The patient did not have any evidence of disease even at 24 months' follow up. The present case is a useful addition to the current body of literature of false positive FDG-PET due to BAT uptake in unusual location and underscores the importance of high index of suspicion and careful correlation, whenever one comes across an unusual PET finding in a given clinical situation. This assumes important diagnostic value particularly when it coexists in the setting of malignancy where the disease can be falsely upstaged by misinterpretation. The literature relevant to the report is discussed and a schema is suggested for correct interpretation.

Keywords: Brown Fat; FDG-PET; adrenal metastasis.

Figure 1

Whole body FDG-PET imaging was performed sixty minutes after the intravenous injection of 0.14mCi/kg of FDG. Emission and transmission data was acquired with successive overlapping axial frames. Two dimensional acquisition technique was adopted which allows use of a lower dose of radiotracer. The ordered-subsets expectation maximization (OSEM) method was used to reconstruct the PET images. The baseline FDG PET (upper panel) showed intense FDG uptake in bilateral neck and paraspinal regions. Solitary intense focus was also noted in the left side of the abdomen posteriorly (arrow). The uptake persisted in the repeat scan after diazepam intervention (middle panel, lower panel magnification), indicating the BAT uptake to be “diazepam resistant”.

Figure 2

Typical pattern of FDG uptake in the brown adipose tissue: avid bilateral and symmetric, intense, more often multifocal than linear and distributed over neck, supraclavicular and paravertebral region.