Stereotactic Body Radiation Therapy and FES PET/CT Imaging for the Treatment of Oligoprogressive Estrogen Receptor Positive Metastatic Breast Cancer

This phase II trial tests how well stereotactic body radiation therapy (SBRT) works in treating patients with estrogen receptor positive (ER +) breast cancer that has spread from where it first started to other places in the body (metastatic) and has limited disease progression (oligoprogression). Currently, the standard of care for breast cancer patients with oligoprogressive disease is to change systemic therapy when progression occurs. Radiation therapy uses high energy x-rays, particles, or radioactive seeds to kill cancer cells and shrink tumors. SBRT is a type of external radiation therapy that uses special equipment to position a patient and precisely deliver radiation to tumors in the body (except the brain). The total dose of radiation is divided into smaller doses (fractions) given over several days. This type of radiation therapy helps spare normal tissue and has been shown to improve survival. SBRT may kill more tumor cells and allow patients with oligoprogressive ER + metastatic breast cancer to continue taking current systemic treatment. This trial also tests how well ER targeted positron emission tomography (PET)/ computed tomography (CT) imaging, using FES, works in identifying progressive disease in patients with ER + metastatic breast cancer. FES, a radiolabeled substance, binds to estrogen receptors and gives off radiation that can be detected by a PET scan. The PET scan, an established imaging technique that utilizes small amounts of radioactivity attached to very minimal amounts of tracer, FES, forms an image that shows where tumor cells with estrogen receptors can be found in the body. CT images use x-rays to provide an exact outline of organs. FES PET/CT may improve identification of progressive disease in patients with ER + metastatic breast cancer.

PRIMARY OBJECTIVE: I. To determine whether using SBRT to treat oligoprogressive lesions allows ER+ breast cancer patients to continue on their current systemic therapy for at least 24 weeks post SBRT treatment. SECONDARY OBJECTIVES: I. To evaluate adverse events (Common Terminology Criteria in Adverse Events \[CTCAE\] and Patient-Reported Outcomes PRO-CTCAE) in patients who receive SBRT. II. To assess whether F-18 16 alpha-fluoroestradiol (FES)-PET increases the number of lesions found prior to SBRT. III. To determine the impact of SBRT on patient quality of life using the European Organization for the Research and Treatment of Cancer (EORTC) Quality of Life Questionnaire (QLQ)-C30 and QLQ-BR45. IV. To estimate time to the next line of systemic therapy in patients who receive SBRT for oligoprogression. V. To estimate progression-free survival time in patients who receive SBRT for oligoprogression. EXPLORATORY OBJECTIVES: I. To assess whether FES-PET increases the number of lesions found after SBRT. II. To describe circulating tumor deoxyribonucleic acid (ctDNA) levels over time after SBRT treatment. III. To identify potential predictors of outcomes to treatment with SBRT. OUTLINE: Patients currently taking selective estrogen receptor modulators (SERMs)/selective estrogen receptor degraders (SERDs) immediately undergo 3 or 5 treatment fractions of SBRT within 3 weeks in the absence of unacceptable toxicity or evidence of \> 4 sites of disease progression. Patients not currently taking SERMs/SERDs first receive F-FES intravenously (IV) and undergo PET/CT scans at baseline. After baseline FES PET/CT, patients with ≤ 4 sites of progressive disease then undergo 3 or 5 treatment fractions of SBRT within 3 weeks in the absence of unacceptable toxicity or evidence of \> 4 sites of disease progression. All patients undergo FES PET/CT at 12 and 24 weeks. Patients with stable disease (SD) after 12 or 24 week FES PET/CT may continue standard systemic therapy. Patients with ≤ 4 sites of progressive disease after 12 or 24 week FES PET/CT may receive SBRT to additional sites in the absence of unacceptable toxicity or evidence of \> 4 sites of disease progression. All patients also undergo CT, PET/CT, or bone scans, and blood samples collection during screening and on study. After completion of study intervention, patients are followed up at 30 days.