Brown Adipose Tissue Mimicking Head and Neck Cancer on PET Scan in a Patient on GLP-1 Drug.

To report a case of a patient undergoing GLP-1 receptor agonist therapy in which increased FDG uptake in brown adipose tissue (BAT) mimicked metastatic head and neck cancer on PET/CT imaging. A 61-year-old female with Class III obesity presented with a right-sided neck mass after significant weight loss following the use of the GLP-1 receptor agonist, Semaglutide. PET/CT revealed FDG uptake in the right level II lymph node and extensive BAT uptake throughout the neck and mediastinum, complicating the diagnosis. Increased FDG uptake in the cervical and supraclavicular BAT regions led to diagnostic confusion, mimicking diffuse regional metastasis. Careful interpretation of PET/CT imaging, with fusion of anatomical and functional data, was essential to differentiate hypermetabolic BAT from malignant disease. Increased BAT FDG uptake, particularly in patients using GLP-1 receptor agonists, can complicate the evaluation of head and neck cancer. Awareness of this interaction is critical to avoid misdiagnosis and overtreatment. Laryngoscope, 135:741-743, 2025.