S249
Clinical - Breast
ESTRO 2026
Digital Poster 1627
Adaptive radiotherapy in hypersensitive and high locoregional risk breast cancer: preliminary results of the SAHARA04 phase II trial Jessica Prunaretty, Céline Bourgier, Aurélie Morel, Laura Lopez, Pauline Hinault, Morgane Cabaillé, Nicolas Bizot, David Azria, Norbert Ailleres, Pascal Fenoglietto Radiotherapy, Institut du Cancer de Montpellier, Montpellier, France Purpose/Objective: The SAHARA04 phase II trial investigates online adaptive radiotherapy (oART) using the Ethos system in breast cancer patients presenting both a high locoregional recurrence risk and a high risk of radiation-induced fibrosis. This report presents our preliminary clinical experience and feasibility results. Material/Methods: Patients were positioned supine, free-breathing, with both arms above the head on a personalized foam cushion. The prescribed doses were 52.2 Gy to the tumor bed (boost) and 42.3 Gy to the breast, internal mammary chain (IMC), and clavicular lymph nodes (Nodes), delivered in 18 fractions. CTV-to-PTV margins were 2 mm for all targets except the IMC (5 mm). Reference IMRT plans used 13 fields with 6 MV FFF energy. Planning followed ICRU 50/62/83 recommendations, and OAR constraints were based on the HYPOG-01 protocol.Each oART session began with a first CBCT (CBCT1). Artificial intelligence automatically generated influencer structures (both breasts, lungs, and heart). IMC and Nodes volumes were obtained by rigid registration. The CTV_Breast was derived from the breast volume excluding the 5 mm layer beneath the skin, while the CTV_Boost resulted from an automatic expansion around the surgical clips. All contours were reviewed and, if necessary, edited by the physician before plan generation. Adapted (reoptimized) and scheduled (recalculated) plans were then produced, and a second CBCT (CBCT2) verified the final positioning.For each session, the treatment duration (from CBCT1 acquisition to the end of beam delivery) and monitor units (MUs) were recorded. Dose metrics from adapted (ADP), scheduled (SCH), and reference (PLAN) plans were compared to assess CTV coverage and OAR sparing. CBCT1–CBCT2 registrations were also analyzed. Results:
A total of 90 oART sessions from five patients (three right- and two left-sided breast cancers) treated between April and September 2025 were evaluated. The median treatment duration was 34 min [21–61 min]. MU values were comparable between reference and adaptive sessions (2467 vs. 2510). Adapted plans achieved higher CTV D98% than scheduled plans, while OAR doses remained similar across all plan types (Figure 1). CTV coverage constraints (D98% > 95%) were unmet in only 9.7% of ADP sessions versus 93.1% of SCH sessions. In 21.4% of fractions, CBCT1–CBCT2 displacement exceeded 2 mm, indicating positioning variability. Conclusion: Early results confirm the feasibility and dosimetric advantages of oART for high-risk breast cancer, with improved target coverage and maintained OAR protection. Accurate patient setup and intra-session stability remain essential to ensure treatment precision and reproducibility. Keywords: Adaptive, Ethos, phase II
Proffered Paper 1633
Randomized controlled trial comparing 5 and 15 fractions for whole breast and nodal irradiation with boost after breast-conserving surgery NCT03677427 Renée De Noyette 1 , Vincent Huybrechts 1,2 , Vincent Vakaet 1 , Hans Van Hulle 2 , Emma Van Damme 1,2 , Leen Paelinck 2 , Luiza Ana Maria Olteanu 2 , Chris Monten 1,2 , Sarah Deschuymer 2 , Sabine Braems 2 , Pieter Deseyne 3 ,
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