# Long-Term Outcome of Intraoperative Radiotherapy for Early-Stage Breast Cancer

## TL;DR

This study shows that intraoperative radiotherapy for early-stage breast cancer works well when patients are carefully selected using updated guidelines.

## Contribution

The study validates the 2024 ASTRO risk classification for identifying high-risk patients in IORT treatment selection.

## Key findings

- Local recurrence rates were significantly higher in 'conditionally recommended' patients compared to 'suitable' patients.
- Patients with two or more conditional criteria had an 8-fold higher recurrence risk than those with fewer criteria.

## Abstract

Intraoperative radiotherapy (IORT) allows radiation treatment to be delivered during breast-conserving surgery, offering a convenient single-session alternative to conventional postoperative radiotherapy for highly selected patients with early-stage breast cancer. Therefore, appropriate patient selection is critical to achieving optimal outcomes. There is a low level of evidence regarding some features to make this personalized decision. In this multicenter retrospective study, we evaluated long-term clinical outcomes of patients treated with IORT and applied the 2024 American Society for Radiation Oncology (ASTRO) risk classification to assess its ability to stratify recurrence risk. Overall local recurrence rates were significantly low among patients classified as suitable versus patients categorized as conditionally recommended or conditionally not recommended, who experienced higher recurrence rates. These findings support the clinical utility of the updated ASTRO risk classification and highlight its value in guiding risk-adapted patient selection for IORT in routine clinical practice.

Background: Intraoperative radiotherapy (IORT) offers single-session treatment during breast-conserving surgery (BCS). Outcomes depend heavily on patient selection and tumor characteristics. Objectives: To assess local recurrence (LR) and prognosis using the 2024 American Society for Radiation Oncology (ASTRO) risk classification in IORT-treated patients. Methods: This multicenter retrospective study analyzed 358 IORT cases (356 patients) treated between 2014 and 2018 using the Zeiss INTRABEAM system. Cases were classified per the 2024 ASTRO partial-breast irradiation guidelines. The primary endpoint was local recurrence-free survival (LRFS); secondary endpoints included overall survival (OS) and mastectomy-free survival (MFS). Findings: The median age was 66 years (range 48–80); all tumors were invasive with a median tumor size of 10 mm. At a median follow-up of 7.1 years, LR occurred in 14/358 cases (3.9%) at a median of 5.2 years post-diagnosis. Five- and 8-year LRFS were 98.3% and 94.8%, respectively; 5- and 8-year OS were 99.4% and 97.7%; MFS at 8 years was 98.2%. Cases that were classified as “conditionally recommended” or “conditionally not recommended” had significantly higher LR than the “suitable” group (8.5% vs. 2.7%; HR 3.25, 95% CI 1.05–10.08, p = 0.041). Exploratory analysis showed that cases with ≥2 conditional criteria carried a markedly higher risk than those with 0–1 (21.4% vs. 3.2%; Firth-penalized Cox HR 8.26, 95% CI 2.06–26.06, p = 0.005). Conclusions: In appropriately selected patients, IORT achieves local control consistent with contemporary series. The 2024 ASTRO risk classification effectively identifies high-risk cases, supporting its use for risk-adapted candidate selection.