Testing Whether an Imaging Injected Agent Known as [18F] Fluoroestradiol (FES) PET/CT Can Help Predict Who Might Benefit from Hormone (Endocrine) Therapy in People with Newly Diagnosed Breast Cancer that has Spread Outside the Breast

The majority of patients with metastatic breast cancer have estrogen-receptor positive (ER+) disease. While the disease is not curable and requires life-long treatment, patients may survive for many years with advanced disease. Therapy options include cytotoxic chemotherapy and targeted therapy, including endocrine therapy. Selection of drugs to treat stage IV breast cancer focuses on efficacy and also tolerability. Physicians generally choose endocrine-targeted therapies as first-line treatments for patients with ER+ advanced disease due to improved tolerability as well as ease of administration (pills or once monthly subcutaneous injections for endocrine therapy versus intravenous administration for chemotherapy). However, not all patients will respond to first-line endocrine-targeted therapy due to intrinsic endocrine-therapy resistance mechanisms as well as tumor heterogeneity (not all metastatic breast cancer cells will have the same level of estrogen receptor (ER) expression) and there are no current methods in standard practice to inform on either of these issues. FES-PET has shown promise for assessment of heterogeneity of ER expression in individual patients (8,37). This trial will address this need by investigating the utility of [18F]-fluoroestradiol (FES), an ER imaging agent which can assess ER expression across all sites of metastatic disease, to predict response in patients with ER+ metastatic breast cancer.