Evaluation of the Effect of Antibiotic Prophylaxis During Lumpectomy for Breast Cancer: Prospective Randomised Double-blind Noninferiority Study of Placebo vs Cefazolin

The goal of this clinical trial is to determine whether preoperative antibiotic prophylaxis with cefazolin is necessary to prevent surgical site infections in adult women undergoing breast-conserving surgery (lumpectomy or partial mastectomy) for breast cancer. Specifically, the trial aims to establish whether surgery without antibiotic prophylaxis is non-inferior to surgery with cefazolin prophylaxis in terms of surgical site infections within 30 days after surgery. The main questions it aims to answer are: * Is the rate of surgical site infections within 30 days after surgery without antibiotic prophylaxis non-inferior to that observed with cefazolin prophylaxis? * How does surgery without antibiotic prophylaxis affect postoperative complications, including antibiotic-related adverse events, and the subsequent management of surgical site infections? Researchers will compare preoperative cefazolin prophylaxis (2 g administered intravenously) with placebo (0.9% saline solution) to determine whether surgery without antibiotic prophylaxis is non-inferior to surgery with cefazolin prophylaxis in preventing surgical site infections. A total of 1,600 participants will be randomly assigned to one of the two groups. Neither the participants nor the healthcare professionals involved in their care will know which treatment has been administered. Participants will: * Undergo breast-conserving surgery for breast cancer, with or without a sentinel lymph node procedure, as part of their usual care. * Receive a single intravenous administration of either cefazolin or placebo before the surgical incision. * Receive information about the signs and symptoms of surgical site infections and how to contact the study team if an infection is suspected. * Be contacted by telephone approximately 15 and 30 days after surgery to assess the occurrence of surgical site infections and other postoperative complications. No additional hospital visits are required specifically for the study.

Inclusion Criteria: * Adult patients (≥18 years). * Undergoing breast lumpectomy/partial mastectomy for breast cancer, with or without sentinel lymph node procedure. * Patients benefiting from a Social Security system. * Patients able to fully understand the study (involving notably a full command of the French language), and having given their informed consent. Exclusion Criteria: * Patients undergoing breast lumpectomy/partial mastectomy with concomitant complete axillary lymph node dissection. * Breast lumpectomy/partial mastectomy performed when there is local breast inflammation or infection (breast abscess, mastitis, etc.) * Known hypersensitivity to cefazolin or other beta-lactams. * Patients with extremely severe obesity (BMI \>50 kg/m2). * Active bacterial infection treated with antibiotics at the time of surgery or recent antimicrobial therapy (in the 15 days prior to surgery). * Patients with severe immunosuppression or a combination of other risk factors for SSI, which would lead the attending anaesthesiologist and surgeon to decide to administer surgical antimicrobial prophylaxis on an individual basis, outside the national guidelines. * Participation to another clinical trial aimed at reducing SSI or another trial regarding the perioperative anaesthetic strategy. * Patients previously enrolled in this study. * Pregnant or breastfeeding women (in patients of childbearing age, a pregnancy test is performed before surgery, the results of which will be verified before inclusion in the study). * Patients benefiting from enhanced protection (minors, adults under legal protection, patients deprived of liberty, etc.). * Patient being a relative of the investigator or any member of the team directly involved in the study, including assistant physicians, pharmacists, nurses, and coordinators.