ESR1 and breast cancer: This study assessed the proportion of estrogen receptor-positive hypometabolic tumours, clinical factors influencing [ 18 F]FDG uptake, and the prognostic impact.<h4>Methods</h4>Baseline [ 18 F]FDG PET/computed tomography (CT) and [ 18 F]FDG PET/MRI exams of female patients diagnosed with estrogen receptor-positive locally advanced (cT3-4N0 or cT1-4N+), metastatic, or recurrent breast cancer between 2013-2022 were retrospectively collected.