Examine | Case Reports: Reassess for Certainty

Misinterpreted BAT uptake can cause false-positive FDG-PET

In this installment of Examine | Case Reports, we look at the case of a false-positive FDG-PET in a 12-year-old girl due to brown adipose tissue (BAT) uptake in the abdomen. This case report was originally published in the October issue of the Journal of Radiology Case Reports. The full version – including interactive features, discussion, and references – can be accessed at www.radiologycases.com.

FDG uptake in the metabolically active 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. This case report describes 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 an unusual location. It underscores the importance of a high degree 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.

A 12-year-old female, diagnosed to have Ewing’s sarcoma of the left suprapubic ramus, was referred for whole-body FDG-PET for a metastatic survey and disease evaluation following her surgery, chemotherapy, and local external radiotherapy. The baseline FDG-PET (Fig. 1 upper panel) showed intense FDG uptake in bilateral neck and paraspinal regions. There was, however, no clinicoradiological evidence of disease in the neck to account for such avid uptake.

This, along with the bilaterally symmetrical uptake pattern, indicated that the hypermetabolism in all probability was due to the BAT. Solitary intense focus was also noted in the left side of the abdomen posteriorly, which gave an initial impression of left adrenal involvement. Strenuous efforts were undertaken to elucidate the abovementioned uptake because the rest of the whole body survey was unremarkable.

Correlative anatomic imaging modalities (USG and CT of the abdomen) did not reveal any evidence of adrenal metastasis. A repeat intravenous diazepam primed FDG-PET (Fig. 1 lower panel) was carried out to settle the BAT uptake. The uptake, however, persisted in the repeat scan, indicating the BAT uptake to be “diazepam resistant.” Hence, it was concluded that the uptake was due to uptake in the metabolically active infradiaphragmatic brown fat. The patient did not have any evidence of disease even at 24 months’ follow up.

Recognizing FDG uptake in infradiaphragmatic brown fat is important while interpreting PET study, and the uptake may not be always bilaterally symmetrical. Benzodiazepine premedication is not 100-percent efficacious in abolishing the aforementioned activity.

– This case report was originally published in the October issue of the Journal of Radiology Case Reports and can be viewed in full, including interactive features, at www.radiologycases.com.