Medical Imaging to Re-evaluate Elacestrant Clinical Usage

The goal of this clinical trial is to evaluate whether elacestrant, an oral selective oestrogen receptor degrader, can improve outcomes in patients with oestrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced or metastatic breast cancer without detectable ESR1 mutation after progression on endocrine therapy plus a CDK4/6 inhibitor. The study aims to answer two key questions: * Does \[18F\]-fluor-oestradiol positron emission tomography/computed tomography (18F-FES-PET/CT) result (homogeneous vs. heterogeneous) determine the efficacy of elacestrant in participants with advanced/metastatic ER-positive breast cancer and ESR1-mut-nd? * Does 18F-FES-PET/CT imaging predict patient outcomes, including progression-free survival, overall survival, or tumor response? There is no comparison group in this study. All participants receive elacestrant. Researchers will compare outcomes in people who have different levels of estrogen-receptor heterogeneity on FES-PET/CT imaging. Participants in the study will: * Take elacestrant 345 mg orally once a day, in 28-day treatment cycles (the dose may be lowered to 258 mg if needed due to side effects). * Undergo 18F-FES-PET/CT and 18F-FDG-PET/CT imaging both at the beginning of the study and as part of routine clinical evaluation every 8 weeks * Undergo blood sample collection every 8 weeks * If selected for a sub-study, undergo an additional FES-PET/CT scan 4 weeks after starting treatment