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Purpose: Comparative outcome data after intraoperative radiation therapy and whole breast irradiation (WBI) for breast cancer at >10 years median follow-up are rare. We present a mature, single-institution, matched-pair comparison reporting survival and relapse rates in patients treated with either modality., Methods and Materials: Complete data sets for 258 intraoperative electron radiation therapy (IOERT) patients treated between 2000 and 2010 were matched with 258 patients postoperatively treated with WBI by age/histology/tumor size, grading/lymph-node-status/hormone receptors/type of adjuvant therapy/surgical margins, and treatment date. Relapse at surgical intervention site was classified as true local recurrence (LR). All recurrences in the treated breast (any quadrant) were classified as ipsilateral recurrence (IR)., Results: Median follow-up was 157 months (12-251) for the IOERT group and 154 months (31-246) for the WBI group. Cumulative incidence of IR at 5, 10, and 15 years was 2.4%, 7.9%, and 12.7% for IOERT and 1.2%, 4.1%, and 5.0% for WBI (P = .02). Cumulative incidence of LR at 5, 10, and 15 years was 1.6%, 5.1%, and 8.3% for IOERT and 0.4%, 2.1%, and 2.5% for WBI (P = .02). No differences in overall survival, disease-free survival, second cancer incidence, or cardiac events were recorded in either treatment group. Outcome was better in the accelerated partial breast irradiation (APBI)-suitable group than in the APBI-unsuitable group (2009 criteria) (cumulative incidence of IR at 5, 10, and 15 years was 0% vs 7.3%, 6.1% vs 13.3%, and 7.3% vs 19.9% for IOERT and 0% vs 1.8%, 2.0% vs 3.9%, and 3.1% vs 3.9% for WBI) and in the revised APBI-suitable group than in the APBI-cautionary group (2017 criteria) (cumulative incidence of IR at 5, 10, and 15 years was 1.1% vs 6.4%, 6.2% vs 13.3%, and 7.8% vs 27.5% for IOERT and 1.7% vs 0%, 4.1% vs 4.4%, and 5.4% vs 4.4% for WBI)., Conclusions: The IR and LR rate were higher after IOERT than after WBI for the American Society for Radiation Oncology suitable patient group, although without reaching statistical significance. Thus, IOERT could be an alternative to WBI upon stringent patient selection, but patients should be counseled carefully about the potential for increased IR rate with IOERT. Second cancer incidence and cardiac events did not differ between IOERT and WBI., (Copyright © 2023 Elsevier Inc. All rights reserved.)