Preoperative Immunotherapy Combined With Stereotactic Radiation Therapy Boost in the Treatment of HER2-negative Breast Cancer

The goal of the study is to assess the safety and effectiveness of the combination of anti-PD1 immunotherapy (pembrolizumab) and radiotherapy in the preoperative treatment of HER2-negative breast cancer resistant to classical chemotherapy. The subject of the intervention will be: 1. Randomly assigned in a 2:1 ratio and double-blinded addition of pre-operative immunotherapy with pembrolizumab or placebo to standard chemotherapy 2. Addition of preoperative radiotherapy boost delivered with a CyberKnife radiosurgery system concomitantly with the use of paclitaxel (+/- carboplatin) and pembrolizumab / placebo.