ESTRO 2025 - Abstract Book

S530

Clinical - Breast

ESTRO 2025

Figure.1

Results: PRT demonstrated variable pCR rates across regimens (Figure 2a). Extreme Hypofractionation achieved a modest pCR rate of 7% (95% CI: 0.02–0.22), while long-course radiotherapy alone yielded a higher pCR rate of 11% (95% CI: 0.06–0.16). Sequential chemo-radiotherapy showed the best outcomes, achieving a pCR rate of 24% (95% CI: 0.16– 0.37). Surgical complications were notably lower in the extreme hypofractionation group (3%, 95% CI: 0.01-0.08) compared to other long-course RT (+/- sequential chemotherapy)(10%, 95% CI: 0.04-0.22). Despite treating early stage disease, 8% of patients in the hypofractionation group required additional post-surgical radiotherapy, and 1% (95% CI: 0.0-0.07) required mastectomy. Further meta-regression revealed no significant correlation between pCR and biologically effective dose (BED), suggesting limited benefit from dose intensification. Publication bias favoring positive outcomes was also noted. A meta-analysis of exclusive SBRT studies found no substantial improvement in pCR rates, reporting a modest rate of 5% (95% CI: 0.00-0.36) in studies utilizing 1–3 fractions (Figure 2b). Meta-regression identified a significant positive correlation between time-to-surgery and pCR rates (p=0.002), particularly in luminal A subtypes. Performing surgery after 6 months increased pCR rates to 30–40%, compared to 3–5% when performed within a week.