Breast cancer patients often receive systemic drugs as the initial curative treatment of their disease. The initial systemic therapy leads to a variable degree of tumor shrinkage. At present, there is no evidence-based consensus among breast surgeons on the optimal extent of resection after considerable tumor shrinkage and whether resection margin should be designed according to the tumor extent before treatment or the extent after treatment. This trial will help determine the optimal extent of resection in tumors that exhibits a significant change in volume after primary systemic therapy.