SABR PRIMER - Evaluating Stereotactic Ablative Radiotherapy for Primary and Regional Breast Tumors

Purpose: The purpose of this study is to determine if stereotactic ablative radiotherapy (SABR) can provide effective locoregional control for patients with metastatic breast cancer. As secondary objectives, the investigators will assess the overall survival, progression-free survival and time to switch of next line of systemic therapy. The investigators will also assess radiation-related adverse events using CTCAE v5.0, with a specific focus on dermatitis, lymphedema and brachial plexopathy. Finally, as an exploratory objective, the investigators aim to correlate toxicities and outcomes with peripheral blood biomarkers and circulating tumor DNA to potentially help predict responses in future patients receiving combined therapy. Primary Objective: • Assess median, 1-year and 2-year locoregional control Secondary Objectives: * Assess median, 1-year and 2-year overall survival * Assess median, 1-year and 2-year progression-free survival * Assess median time to systemic therapy switch * Assess radiation-related adverse events using CTCAE v5.0 Correlative Objectives: • Correlate outcomes with peripheral blood-derived parameters Research Design The trial is designed as a single-arm interventional study investigating stage IV breast cancer with persistent or progressive locoregional disease despite the use of standard systemic therapy A retrospective study will be conducted to create a matched cohort to compare locoregional control rates with patients who received conventional fractionated radiotherapy.