Simple SummaryA retrospective observational study was proposed to evaluate the clinical response rate and cosmetic outcome after full-dose intraoperative electron radiotherapy (IOERT) in early low-risk breast cancer treated with conserving surgery. 162 patients were included in this analysis (median follow-up: 54 months, range: 1–98 months). IOERT was delivered with a dose of 21 Gy at 90% isodose. The overall response rate was 97.5% (CI 95%: 0.93–0.99%). Locoragional relapse occurred in 2.5% of patients. No patient showed distant metastases. No patient showed radiation-related acute complications, 3.7% of patients late G2–3 toxicity. Only 3.7% of patients showed poor cosmetic results. The results highlighted the safety and effectiveness of the full-dose IOERT treatment for highly selected patients.AbstractTo evaluate the clinical response rate and cosmetic outcome after full-dose intraoperative electron radiotherapy (IOERT) in early breast cancer (BC) treated with conserving surgery. Inclusion criteria were: >60 years old, clinical tumor size ≤2 cm, luminal A carcinoma, patological negative lymph nodes, excluded lobular carcinoma histology. IOERT was delivered with a dose of 21 Gy at 90% isodose. Clinical, cosmetic and/or instrumental follow-up were performed 45 days after IOERT, 6 months after the first check, and every 12 months thereafter. Acute and late toxicities were assessed with the CTCAE v.4.03 and EORTC-RTOG scales, respectively. Cosmetic outcome was evaluated using the Harvard/NSABO/RTOG Breast Cosmesis Grading Scale. Overall, 162 consecutive patients were included in this analysis (median follow-up: 54 months, range: 1–98 months). The overall response rate was 97.5% (CI 95%: 0.93–0.99%). Locoragional relapse occurred in 2.5% of patients. No patient showed distant metastases. No patient showed radiation-related acute complications, with 3.7% showing late G2–3 toxicity. Only 3.7% of patients showed poor cosmetic results. Our data confirmed that IOERT is a feasible and valid therapeutic option in low-risk BC patients treated with lumpectomy. A low local recurrence rate combined with good cosmetic results validates the settings of our operative method in routinely clinical practice.