Stereotactic radiotherapy for oligoprogressive ER-positive breast cancer (AVATAR).

Background: The enhanced knowledge of cancer biology has led to considerable advancement in systemic therapy for advanced breast cancer. Recently, studies showed that cyclin-dependent kinase (CDK) 4/6 inhibitor, when added to endocrine therapy, had improved the outcomes of patients with advanced ER-positive HER2-negative breast cancer. However, the disease often progresses following a period of treatment response.