S5
Track-Topic
ESTRO 2026
95], respectively. Although DFS (92% vs 88%, p=0.03) and OS (91% vs 94%, p=0.02) slightly favoured the ≥ 2- day regimen, outcomes were comparable between single-day and multi-day schedules regarding 5-y LRCI and CSS (2% and 98% in both groups). Similar results were observed when comparing number of fractions. Late toxicity was observed in 189 patients (33%), with a total of 236 events. Grade 2 and 3 late toxicities represented 15.7% and 0.4% of cases, respectively, with no grade 4 events. The most frequent toxicities were fibrosis (61%), dyschromia (20%), and pain (15%). Rates of G ≥ 2 late events were not influenced by the treatment schedule (1 day: 8.6% vs ≥ 2 days: 9.2%, p=0.80), but were significantly higher for CTV >50 cc (p=0.001). Conclusion: VAPBI delivered with MIB in one to four fractions provides excellent 5-year local oncological outcome with minimal long-term toxicity. The treatment is feasible both in single-day and multi-day regimens, offering a safe, effective, and time-sparing de- escalation strategy for carefully selected patients with low-risk breast cancer. These mature results consolidate the role of vAPBI as a promising alternative to conventional whole-breast irradiation in modern breast-conserving therapy. References: VAPBI delivered with MIB in one to four fractions provides excellent 5-year local oncological outcome with minimal long-term toxicity. The treatment is feasible both in single-day and multi-day regimens, offering a safe, effective, and time-sparing de- escalation strategy for carefully selected patients with low-risk breast cancer. These mature results consolidate the role of vAPBI as a promising alternative to conventional whole-breast irradiation in modern breast-conserving therapy. Keywords: Breast cancer, brachytherapy, fractionation Single fraction based-very accelerated partial breast irradiation: 10-year results of the SiFEBI Phase 2 prospective trial Laura Haas 1,2 , Jocelyn Gal 3 , Mathieu Gauthier 1 , Renaud Schiappa 3 , Jean-Michel Hannoun-Levi 1 1 Radiation Oncology, Antoine Lacassagne Cancer Center, University Côte d’Azur, Nice, France. 2 Radiation Oncology, Henri Becquerel Cancer Center, Rouen, France. 3 Statistics, Antoine Lacassagne Cancer Center, University Côte d’Azur, Nice, France Purpose/Objective: This analysis updates the SiFEBI phase II trial (NCT01727011) evaluating very-accelerated partial Proffered Paper 557
Proffered Paper 218
Very accelerated partial breast irradiation in 1 or 2 days: Five-year oncological outcome of the GEC- ESTRO VAPBI cohort Jean-Michel Hannoun-Levi 1 , Cristina Gutierrez 2 , Marta Gimeno Morales 3 , Jocelyn Gal 4 , Javier Anchuelo 5 , Jose- Luis Guinot 6 , Miren Gaztañaga 7 , Norbert Meszaros 8 , Renaud Schiappa 4 , Vratislav Strnad 9 , Csaba Polgar 10 1 Radiation Oncology, Antoine Lacassagne Cancer Center, University Côte d’Azur, Nice, France. 2 Radiation Oncology, Institut Català d’Oncologia, Barcelona, Barcelona, Spain. 3 Radiation Oncology, Clínica Universidad de Navarra, Pamplona, Spain. 4 Statistics, Antoine Lacassagne Cancer Center – University of Côte d’Azur, Nice, France. 5 Radiation Oncology, Hospital Universitario Miguel Servet, Santander, Spain. 6 Radiation Oncology, Instituto Valenciano de Oncologia, Valencia, Spain. 7 Radiation Oncology, Hospital Clínico San Carlos, Madrid, Spain. 8 Radiation Oncology, National Institute of Oncology Budapest, Budapest, Hungary. 9 Radiation Oncology, Erlangen University Hospital, Erlangen, Germany. 10 Radiation Oncology, Semmelweis University, Budapest, Hungary Purpose/Objective: The aim of this updated analysis was to evaluate the long-term oncological outcomes of very accelerated partial breast irradiation (vAPBI) delivered with multicatheter interstitial brachytherapy (MIB) in patients with low-risk breast cancer. Material/Methods: This retrospective GEC-ESTRO observational international study (HDH F20220713143949) enrolled patients with early-stage low-risk breast cancer treated with lumpectomy followed by MIB-based vAPBI. Treatment regimens included one fraction (16–18Gy in 1 day), three fractions (3 × 7.45Gy over 2 days), or four fractions (4 × 6.2Gy over 2 days). Primary end point was oncological outcome including cumulative incidence of breast cancer local relapse (LRCI), regional relapse (RRCI) and metastatic relapse (MRCI) and disease-free (DFS), cause-specific (CSS) and overall (OS) survivals Secondary endpoints included late toxicity assessment. Results: Between 2012 and 2024, 579 patients were analysed. Median age was 72 years [40–101], median tumour size 12 mm [0.5–35]. Postoperative versus perioperative implantation was balanced (47.5% vs 52.5%). Treatment was delivered in 1 day in 37% (215 pts) and in ≥ 2 days in 63%. (3f: 220 pts; 4f: 144 pts). At a median follow-up of 64 months [62–67], 5-year oncological outcomes were excellent: LRCI 2% [1–3], RRCI 1% [0–1], and MRCI 1% [0–2]. Five-year DFS, SS, and OS were 91% [88–93], 98% [97–100], and 93% [90–
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