ESTRO 2025 - Abstract Book

S544

Clinical - Breast

ESTRO 2025

Instituto Mexicano del Seguro Social, Monterrey, Nuevo León, Mexico. 3 Radiotherapy Department, Centro Médico ABC, Ciudad de México, Mexico

Purpose: Radiotherapy (RT) is essential in breast cancer treatment, with hypofractionated radiotherapy (HypoF) being a safe, effective, and increasingly common approach. However, data on the relationship between HypoF, molecular profile (MP), and Locoregional Recurrence-Free Survival (LRFS) are limited [1,2]. Personalized RT based on tumor-specific biology could enable individualized fractionation, potentially improving LRFS. Objective: To evaluate the role of the MP as a predictive factor for Locoregional Recurrence-Free Survival (LRFS) in two distinct radiotherapy fractionation regimens in breast cancer. Methods: This retrospective, observational, single-center cohort study included 180 breast cancer patients who received adjuvant radiotherapy in 2016. The study analyzed the relationship between MP (luminal A, luminal B, HER2+, and triple-negative) and RT fractionation (conventional, CF, 50Gy/25fx vs. HypoF 40.5Gy/15fx) concerning LRFS. Descriptive statistics, bivariate and multivariate analyses were conducted. Kaplan-Meier survival curves and log-rank tests were used to estimate LRFS. All analyses were performed using SPSS version 27.

Results:

Results: Among the 180 patients, 64.4% (116) had estrogen receptor-positive tumors, 61.7% (111) had progesterone receptor-positive tumors, and 23.3% (42) showed HER2+. Conventional fractionation (CF) was administered to 52.2% of patients, while hypofractionation (HypoF) was given to 47.2%. For LRFS based on the MP the luminal A group showed the longest LRFS (68.6 months) compared to luminal B (68.4 months), HER2+ (62.9 months), and triple negative (64.3 months), without statistical significance ( p =0.145). Five-year LRFS was highest in luminal A patients regardless of fractionation type: CF at 92.7% and HypoF at 100% ( p =0.133). The triple-negative profile responded better to HypoF than CF, with outcomes of 100% vs. 73.7% ( p =0.030). For luminal B, CF achieved 84.6% and HypoF 88.9% ( p =0.602); in HER2 patients, CF achieved 75% and HypoF 90.9% ( p =0.371). Conclusion: This study suggests that MP is critical in predicting LRFS in breast cancer patients undergoing RT. The luminal A subtype demonstrated the highest LRFS, independent of the radiotherapy fractionation type, consistent with previous literature. Notably, the triple-negative subtype showed a significantly better LRFS outcome with HypoF compared to CF, highlighting HypoF as a potentially more effective option for this subgroup. These findings support the value of MP in guiding personalized RT strategies, potentially improving locoregional control, and optimizing