The purpose of this study is to evaluate clinical outcomes and adverse events of salvage treatment for locoregional recurrence of breast cancer. The main questions it aims to answer are: * Clinical outcomes after salvage treatment for locoregional recurrence * Adverse events and quality of life after salvage treatment for locoregional recurrence * Patient characteristics and treatment specifics which are related to the clinical outcomes and/or adverse events * Molecular signature associated with treatment resistance Participants will be assessed by multi-dimensional methods during and after radiation therapy: * Assessment for the disease status (disease-free or recurrence) including physical and radiologic examination * Assessment for the adverse events according to CTCAE version 5.0 * Assessment for the molecular signature using residual tissue after pathologic diagnosis * Assessment for the quality of life using questionnaires (BREAST-Q)