Clinical Research Directory

Inclusion Criteria: * Male or female participants aged 19 years or older, regardless of race or ethnicity. * Histologically confirmed invasive breast cancer that is ER-positive and HER2-negative by immunohistochemistry. * Scheduled to initiate first-line systemic endocrine therapy with an aromatase inhibitor in combination with a CDK4/6 inhibitor for metastatic breast cancer within 60 days of screening. * Have undergone or be scheduled to undergo \[18F\]FDG PET/CT before initiation of endocrine therapy combined with a CDK4/6 inhibitor. * Have measurable or clinically evaluable metastatic disease on standard-of-care imaging. * Eastern Cooperative Oncology Group (ECOG) performance status of 0-2. Exclusion Criteria: * Failure of the participant or the participant's legally authorized representative to provide written informed consent. * History of another invasive malignancy within the previous 2 years, except for non-melanoma skin cancer. * Absence of lesions with significant uptake on \[18F\]FDG PET/CT, or inability to quantitatively evaluate major lesions because of lesion location (e.g., brain or other sites with high physiologic uptake), severe motion artifacts, reconstruction errors, or other technical limitations. * Metastatic disease confined exclusively to the liver on imaging. * Previous systemic treatment for metastatic breast cancer, including endocrine therapy, targeted therapy, or anticancer chemotherapy including antibody-drug conjugates (ADCs). * Pregnancy or breastfeeding. Participants are considered not to be of childbearing potential if they are physiologically postmenopausal (amenorrhea for at least 2 years) or surgically sterile (documented bilateral oophorectomy or hysterectomy). Participants of childbearing potential must have a negative serum or urine pregnancy test within 24 hours before \[18F\]FES administration and must use contraception during the study period. * Serious, uncontrolled, and/or unstable medical conditions, including but not limited to congestive heart failure, acute myocardial infarction, severe pulmonary disease, chronic kidney disease, or chronic liver disease. * Being a relative or student of an investigator or otherwise having a dependent relationship with an investigator. * Any personal circumstances or other conditions that, in the investigator's judgment, are likely to prevent complete collection of study data.