Inclusion Criteria:
1. Male or female patients aged ≥19 years, regardless of race/ethnicity
2. Patients newly diagnosed with invasive breast cancer within 90 days prior to screening, with documented histopathological confirmation.
3. Patients diagnosed with histologically confirmed estrogen receptor-positive, histologic grade 1 or 2 primary breast cancer
4. Patients with clinical stage IIIA-IIIC disease according to the 8th edition of the American Joint Committee on Cancer (AJCC) TNM staging system
5. Patients who have undergone, or are scheduled to undergo chest computed tomography (CT), abdominal CT, and bone scintigraphy within 90 days prior to screening before the 18F-FES PET/CT scan
6. Patients with an Eastern Cooperative Oncology Group (ECOG) performance status (PS) score of ≤2
Exclusion Criteria:
* The subject or the subject's legally authorized representative has not signed the informed consent form
* Subjects who are currently receiving systemic therapy for breast cancer (e.g., neoadjuvant chemotherapy, hormonal therapy)
* Subjects with a history of another invasive malignancy within the past 2 years (except for non-melanoma skin cancer)
* Subjects diagnosed with distant metastases on chest CT, abdominal CT, or bone scintigraphy performed within 90 days prior to screening or before the 18F-FES PET/CT scan
* Subjects for whom neither biopsy nor ≥6 months of follow-up imaging has been performed for lesions suspicious for distant metastases
* Pregnant or breastfeeding women. The following cases are considered not at risk of pregnancy: (1) physiologically postmenopausal (no menses for ≥2 years), (2) surgically sterile (history of bilateral oophorectomy or hysterectomy), (3) women of childbearing potential with a documented negative serum or urine pregnancy test within 24 hours prior to 18F-FES administration, and who agree to use effective contraception during the trial.
* Subjects with serious and/or uncontrolled and/or unstable medical conditions (e.g., congestive heart failure, acute myocardial infarction, severe pulmonary disease, chronic kidney disease, or chronic liver disease)
* Subjects who are relatives or students of the investigator, or otherwise in a dependent relationship with the investigator
* Subjects who, in the judgment of the investigator, are unable to provide complete data for the clinical trial due to personal circumstances or other reasons
