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.