The purpose of this study is to analyze treatment outcomes related to tumor bed boost of postoperative radiation therapy in patient with HER2+ breast cancer who underwent breast conserving surgery. The main questions it aims to answer are: * 7-year ipsilateral breast tumor recurrence * 7-year disease-free survival * 7-year locoregional recurrence * 7-year overall survival * Adverse events of radiation therapy Participants will be assessed by multi-dimensional methods 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