Diagnostic Performance of [18F]FES PET/CT for Axillary LN Metastasis in ER-positive Breast Cancer

Inclusion Criteria:

* Male or female subjects aged 19 years or older regardless of race/ethnicity.
* Subjects with histologically confirmed moderate-to-strong estrogen receptor-positive invasive breast cancer within 90 days prior to \[18F\]FES PET/CT imaging
* Subjects whose primary tumor of cT1-3 according to the American Joint Committee on Cancer (AJCC) 8th tumor staging system
* Subjects with suspected or confirmed axillary lymph node metastasis clinically or in imaging test (ultrasound)
* Subjects who scheduled to undergo sentinel node biopsy or axillary lymph node dissection within 90 days of \[18F\]FES PET/CT imaging
* Subjects whose Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 2 points or less

Exclusion Criteria:

* Subject or the subject's legally acceptable representative does not provide written informed consent form
* Subjects with confirmed or suspected distant metastases.
* Previous history of ipsilateral axillary lymph node dissection, sentinel lymph node surgery, or lymph node dissection biopsy.
* Patients who are scheduled for or have undergone chemotherapy, radiotherapy, antihormone therapy, targeted therapy, or immunotherapy between \[18F\]FES PET/CT and pathological diagnosis
* Subjects who are pregnant or lactating. Exclusion of the possibility of pregnancy is made by one of the following: 1) Physiologically menopausal (menstruation has stopped for more than 2 years), 2) Surgically infertility (with a history of bilateral oophorectomy or hysterectomy), 3) In the case of subjects with a possibility of pregnancy, negative serum or urine pregnancy test before administration of \[18F\]FES has to be negative within 24 hours, and the subjects are instructed to use contraception during her participation in this study.
* Subject has concurrent severe and/or uncontrolled and/or unstable medical condition other than cancer (e.g., congestive heart failure, acute myocardial infarction, severe lung disease, chronic kidney disease or chronic liver disease).
* Subject is a relative or student of the investigator or otherwise in a dependent relationship
* Subject has already participated in this study
* Subject not being able to provide intact data for this study due to personal circumstances or other reasons in the judgment of the investigator