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

Inclusion Criteria:

• Female patients with age minimum 19
• Pathological confirmation of HER2+ invasive breast cancer
• Eastern Cooperative Oncology Group performance status 0-2
• Informed consent of the participant

Exclusion Criteria:

• Pathological confirmation of ductal carcinoma in situ of the breast
• Previous history of radiation therapy to ipsilateral breast

Any HER2+ invasive breast cancer patient with eligible performance except previous history of radiation therapy to ipsilateral breast