S261
Clinical - Breast
ESTRO 2026
constraint protocols. WBI—with or without simultaneous integrated boost (SIB)—followed the FAST-Forward trial constraints, while APBI planning adhered to ASTRO guideline recommendations. All treatments were delivered under free-breathing with daily MVCT guidance. Clinical data, PTV coverage, OAR dosimetry, acute dermatitis grade, and early local control were analyzed. Results: Fifty-six patients were included: 40 received WBI (26 Gy/5 fx), including 10 with SIB (30 Gy in 8 patients; 29 Gy in 2), and 16 received APBI (30 Gy/5 fx) on non- consecutive days. All completed treatment without interruption. PTV coverage was excellent (mean V95% 97.6% for WBI; 98.0% for APBI). In the WBI group, mean heart dose was 1.56 ± 0.48 Gy and ipsilateral lung mean dose 3.56 ± 0.72 Gy, comparable with published 26 Gy/5 fx series using VMAT or IMRT (heart 0.9–1.2 Gy, lung 3.1–3.2 Gy). APBI, though prescribed to a higher dose, involved smaller volumes, resulting in lower mean heart dose 0.82 ± 0.59 Gy and whole- lung mean dose 1.62 ± 0.38 Gy—consistent with the Florence APBI-IMRT trial. Acute dermatitis was mild: Grade 0–1 in 77% of WBI and 100% of APBI patients; no Grade ≥ 2 toxicity occurred. No local recurrence was observed, and all patients were alive at last follow-up. Conclusion: Both 5-fraction WBI and APBI delivered with HT demonstrated excellent feasibility, target coverage, and minimal heart and lung exposure, with uniformly mild acute toxicity. The established constraint sets for ultra-hypofractionated WBI and APBI were successfully achieved within HT planning parameters, supporting its clinical applicability for modern short-course breast radiotherapy. Continued follow-up will clarify late effects and cosmetic outcomes. References: 1. Sigaudi, V., Zannetti, M., Ferrara, E., Manfredda, I., Mones, E., Loi, G., Krengli, M., & Franco, P. (2022). Ultra-Hypofractionation for Whole-Breast Irradiation in Early Breast Cancer: Interim Analysis of a Prospective Study. Biomedicines, 10(10), 2568.2. Choi SH, Kim JS, Kim HJ, Park RH, Lee IJ, Kim YB, Chang JS. Volumetric Modulated Arc Therapy for 26 Gy in 5 Fractions Whole Breast Irradiation for Breast Cancer. Adv Radiat Oncol. 2025 Feb 1;10(4):101733.3. Marrazzo L, Meattini I, Simontacchi G, Livi L, Pallotta S. Updates on the APBI- IMRT-Florence Trial (NCT02104895) Technique: From the Intensity Modulated Radiation Therapy Trial to the Volumetric Modulated Arc Therapy Clinical Practice. Pract Radiat Oncol. 2023;13(1):e28-e34. Keywords: Breast cancer, UHFRT, Tomotherapy
patients had a single metastatic site. Following re- irradiation, 38% of patients achieved CR and 30% partial pain response, while 32% experienced stable or no improvement. Overall, 35 out of 50 patients (70%) received a cumulative biologically effective dose BED ₃ > 90 Gy, and 15 patients (30%) received < 90 Gy.All patients treated with a cumulative dose BED3 > 90 Gy achieved either complete or partial pain response (100% response rate), compared with an 80% response rate among those treated with BED ₃ < 90 Gy.This difference was statistically significant (p = 0.04), indicating that higher cumulative BED ₃ was associated with superior pain control.Within the higher-dose group, 37.2% achieved CR, and the remaining 62.8% achieved PR. The median survival after reirradiation was 20 months, with better outcomes observed in patients with limited metastatic disease and longer retreatment intervals (>12 months). Conclusion: Re-irradiation is a safe and effective palliative modality for bone metastases from breast cancer, achieving meaningful symptom control and maintaining quality of life with minimal toxicity. Higher radiotherapy doses are required for a better pain control. These findings support its use as a valuable option in selected patients requiring re-treatment. Keywords: Reirradiation, breast cancer, bone metastases Digital Poster 1865 Five-Fraction Whole- and Partial-Breast Irradiation Using Helical Tomotherapy: Comparative Dosimetry and Early Clinical Outcomes Pei-Yu Hou, Yu-Hsuan Chuang Radiation Oncology, Far Eastern Memorial Hospital, New Taipei City, Taiwan Purpose/Objective: Ultra-hypofractionated whole-breast irradiation (WBI, 26 Gy in 5 fractions) and accelerated partial-breast irradiation (APBI, 30 Gy in 5 fractions) have become efficient adjuvant options supported by contemporary clinical trials. Despite their increasing adoption, data comparing these two regimens on helical tomotherapy (HT) remain limited. This study aimed to evaluate and compare the dosimetric performance, early toxicity, and short-term oncologic outcomes of WBI and APBI delivered in 5 fractions using HT in daily practice. Material/Methods: We retrospectively reviewed all early-stage breast cancer patients treated with HT between January 2024 and July 2025. Patients receiving adjuvant RT in 5 fractions were included and stratified into WBI and APBI cohorts according to target volume and
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