[Treatment Strategy for HER2 Positive and HER2 Low Breast Cancer].

Human epidermal growth factor receptor 2(HER2)positive breast cancer has been regarded as a poor prognosis breast cancer, but since the development of trastuzumab and various anti-HER2 drugs, the prognosis has significantly improved. Also, combining anti-HER2 drugs with chemotherapy as preoperative treatment, shows high ratings of pathological complete response(pCR). In terms of breast cancer, pCR is a prognosis predictive factor. As a result, in perioperative treatment for HER2 positive breast cancer, residual disease-guided approach which means to select postoperative treatment according to the response to preoperative chemotherapy has become the main stream. Furthermore, trastuzumab deruxtecan, which had been covered by insurance only for HER2 positive metastatic or recurrent breast cancer, was newly reimbursed for breast cancer with low HER2 expression at the end of March 2023, and the future movement is attracting attention.