Unilateral Brown Fat FDG Uptake after Childhood Sympathectomy Mimicking Malignancy Resolved by Hybrid PET/MR Image Fusion.

Physiologic 18 F-fluorodeoxyglucose (FDG) uptake in brown adipose tissue (BAT) is a recognized source of false-positive findings on positron emission tomography (PET) scans, typically presenting as symmetric, multifocal activity in fat-density tissue. We report a case of a 30-year-old woman with left-sided cervical swelling and unilateral, left-sided FDG uptake mimicking malignancy, but without corresponding computed tomography abnormalities. The patient's history included right-sided mediastinal ganglioneuroma resection in childhood, resulting in Horner syndrome. Hybrid PET/magnetic resonance (MR) imaging with image fusion definitively localized the FDG activity to left-sided BAT, confirming the tissue origin and linking the unilaterality to sympathetic denervation from prior right stellate ganglion disruption. This case illustrates that hybrid PET/MR can resolve diagnostic uncertainty in atypical BAT presentations and highlights the importance of recognizing altered BAT metabolism after sympathetic injury to avoid misinterpretation of PET findings.