Intraoperative Radiation Therapy Delivered by Brachytherapy in Breast Cancer: An Interim Analysis of a Phase 2 Trial

Intraoperative breast cancer radiotherapy offers an alternative to external beam radiotherapy after breastconserving surgery due to breast cancer. In this trial we present the safety and toxicity of IORT delivered by high dos rate brachytherapy. After a median follow-up of 3.1 years the risk of recurrence was low as well as any toxic side effects. Purpose: Intraoperative breast cancer radiotherapy (IORT) offers an alternative to external beam radiotherapy (EBRT) after breast-conserving surgery (BCS). The Intraoperative brachytherapy (IOBT) trial applies high dose rate (HDR) brachytherapy with a new applicator prototype as IORT after BCS. In this interim analysis of the IOBT trial, we present the oncological safety and toxicity of the method Methods: Eligible patients were women, >= 50 years old with an unifocal nonlobular, estrogen-receptor-positive, HER2-negative breast cancer, cN0, <= 3 cm, treated with BCS and sentinel node biopsy (SNB). Toxicity was registered according to the LENT-SOMA scale. Cumulative incidence of local (LR) and regional recurrence (RR) were calculated through cumulative incidence function whereas overall survival (OS) was illustrated through Kaplan-Meier curve. Results: Until February 2023, 155 women (median age 68 years) were included in the trial. Twenty-nine women (18.7%) received supplemental EBRT, mostly due to positive SNB. Three-year cumulative incidence of LR and RR were 1.0% (CI 95 % 0.1%-2.3%) and 2.1% (CI 95% 0.8%-4.2%) respectively. Five- year cumulative incidence of LR and RR were 3.9% (CI 95% 1.8%-6.4%) and 2.1% (CI 95% 0.8%-4.2%) respectively. Five-year OS was 96.3% (CI 95% 93.6%-98.4%). Side effects were limited, low grade, and transient. Conclusion: Acknowledging the short median follow-up time at interim analysis, our initial results indicate that delivering IORT through HDR brachytherapy in carefully selected breast cancer patients is feasible and oncological safe so far. A long-term follow-up is essential to confirm the initial results.