Prospective Cohort for Tumor Bed Boost Radiotherapy in HER2 Positive Breast Cancer

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