This study is focused on the value of 18F-FES-PET in predicting the Fulvestrant efficacy in hormone receptor-positive breast cancer. The imaging agent involved in this study is 16α-[18F]-fluoro-17β-estradiol (18F-FES).

This study is an open, multi-center and prospective clinical study, It is the first time investigators are examining this imaging agent in Chinese patients with breast cancer.

The Chinese FDA has approved 18F-FES as an investigational tracer in a selective few research studies. The Chinese FDA has not approved this tracer for any use outside of research. In this research study, the investigators are investigating the use of a PET tracer called 18F-FES in patients with breast cancer.

The purpose is to take images that can measure the amount of a protein called Estrogen Receptor, or 'ER' in breast tumors. The expression level of ER is tightly correlated with the Fulvestrant efficacy in patients with breast cancer. The investigators will take the pictures of the participant's residual tumor lesions using a PET/CT scanner with 18F-FES used as the tracer before and after the first cycle of Fulvestrant treatment. The investigators are trying to clarify the relationship between the clinical Fulvestrant efficacy and the amount change of 18F-FES uptake by breast tumor around the first cycle of Fulvestrant treatment.

This could help them to predict the efficacy of endocrine therapy by monitoring the dynamic levels of ER in advance, and potentially help guide treatment of breast tumors in the future.

18F-FES was initially developed to image the primary breast tumor. In previous studies of 18F-FES conducted in humans, there were no adverse events (there have been no problems). While this is not the first time 18F-FES has been used in humans, it is the first study in which the investigators study its predictive value in the treatment efficacy in breast cancer.

Inclusion Criteria:

• Be at least 18 years or older (required for legal consent)
• Have Eastern Cooperative Oncology Group (ECOG) performance status ≤2 (Karnofsky ≥60%, see Appendix A)
• Pathologically diagnosed metastatic breast cancer
• Fulvestrant used as the first-line treatment
• Have primary breast tumor that is at least 2 cm in diameter as measured in any dimension on either mammogram, ultrasound, or breast MRI
• Have the ability to understand and the willingness to sign a written informed consent document.

Exclusion Criteria:

General Exclusion Criteria

• Previously treated metastatic breast cancer
• Concurrent malignancy of any type
• Impaired elimination (as defined as having problems with urination)
• Participation in a research study/studies involving radiation exposure within the past 12 months
• Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, major kidney or liver disease, or psychiatric illness/social situations that would limit compliance with study requirements
• Pregnancy
• Currently lactating (either breast feeding or breast pumping)

General PET/CT Safety Exclusion Criteria:

• Electrical implants such as cardiac pacemakers or perfusion pumps
• Ferromagnetic implants such as aneurysm clips, surgical clips, prostheses, artificial hearts, valves with steel parts, metal fragments, shrapnel, metallic tattoos anywhere on the body, tattoos near the eye, steel implants, ferromagnetic objects such as jewelry or metal clips in clothing
• Pre-existing medical conditions including a likelihood of developing seizures or claustrophobic reactions, and any greater than normal potential for cardiac arrest
• Inability to lie comfortably on a bed inside a PET camera for 90 minutes as assessed by physical examination and medical history (e.g. back pain, arthritis)
• Pregnancy: A negative serum pregnancy test is required on the day of the PET procedure for female subjects of child-bearing age.