18F-Fluoroestradiol (18F-FES) is an estrogen receptor-targeting positron emission tomography tracer with high sensitivity and specificity for the detection of estrogen receptor（ER）-positive tumors . 18F-FES has been used as a predictive biomarker to demonstrate estrogen receptor heterogeneity , to evaluate the pharmacokinetics of estrogen receptor-targeted drugs ， to measure residual estrogen receptors during endocrine therapy and to determine biologically optimal doses of novel estrogen receptor-targeted drugs .This study aimes to explore the efficacy of 18F-FES PET/CT in evaluating the expression levels of ER in primary and metastatic breast cancer patients.