Atypically Intense Pharmacologically Induced Brown Fat Activation on FDG PET/CT

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  FIGURE 1. A 47-year-old quadriplegic man underwent a PET/CT study with 18F-FDG for evaluation of a lumbar spine soft tissue anomaly seen on MRI. The FDG PET/CT showed intense, multilobular uptake in the supraclavicular region, with additional intense uptake in paraspinal and perinephric fat. Accumulation of FDG in activated brown adipose tissue (BAT) is a well-known phenomenon due to underlying high glucose metabolism for thermogenesis.1 BAT is predominantly located in the cervical, supraclavicular, and paraspinal regions, with some abdominal involvement also reported.2 Although FDG uptake in BAT is sporadic and typically only mildly hypermetabolic as exemplified in the MIP image from another patient (A), our patient’s FDG PET MIP image showed more extensive distribution, including markedly intense FDG uptake in the retroperitoneal and perirenal fat (B).

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  FIGURE 2. On cross-sectional imaging, the extensive FDG uptake localized to subcutaneous fat in the expected locations, including supraclavicular (A and B) and paraspinal regions (C and D).3 In addition, evident on both transverse and coronal orientations (E–H) was substantially increased uptake in the retroperitoneum including perirenal fat, which is rarely reported. Markedly intense perinephric FDG uptake can be seen in other entities such as lymphoma or Erdheim-Chester disease.4,5 However, in the corresponding CT images, the increased FDG uptake was centered on the fatty tissue not renal parenchyma, effectively excluding those possibilities. A chart review prompted by the atypically intense uptake revealed that the patient was receiving mirabegron (Myrbetriq) for bladder dysfunction in the setting of quadriplegia. Mirabegron is a B3-adrenergic antagonist that activates BAT, most likely resulting in this substantial increase in FDG uptake.6 Based on known seasonal variations in FDG uptake in BAT, the expected range for SUVmax in the supraclavicular and paraspinal region is 2 to 10.7 Semiquantitative analysis confirmed the comparatively atypically high SUVmax for our patient, 29.3 for…