S255
Clinical - Breast
ESTRO 2026
retrospectively. Dosimetric parameters including mean, maximum and minimum dose, were extracted. Univariable analysis and stepwise logistic regression were used to identify predictors of 5-year patient- and clinician-reported arm swelling. Final multivariable models were adjusted for dose/fractionation received. Results: 456/469 had CT-planning DICOM-RT data available. Visible axillary seroma was present in 27/203 (13%) patients who underwent sentinel node biopsy (SLNB) and 196/253 (77%) patients who had axillary lymph node dissection (ALND). 5-year patient-reported arm swelling data were available for 299 patients (64%), with 13% (39/299) reporting moderate/marked arm/hand swelling. Multivariable analysis identified ALND (OR 3.70, 95% CI [1.61-8.48], p=0.002) and BMI ≥ 35kg/m2 (OR 3.31 [1.08-10.12], p=0.036) as significant predictors of 5-year patient-reported moderate/marked arm/hand swelling (Table 1). 27Gy/5Fr showed borderline significance for the primary endpoint (OR 2.36 [0.99-5.65], p=0.053). 5- year clinician-reported arm lymphoedema data were available for 346 patients, of whom 45 (13%) developed lymphoedema. Multivariable analysis identified ALND (OR 3.67 [1.55-8.69], p=0.003) and number of positive lymph nodes (OR 1.08 [1.00-1.18], p=0.049) as significant predictors of 5-year clinician- reported arm lymphoedema. No significant associations were found between individual dosimetric parameters and 5-year arm swelling on multivariable analysis.
Conclusion: Axillary surgery remains the main predictor of long- term arm lymphoedema following locoregional radiotherapy for node-positive breast cancer. These findings support de-escalation of axillary surgery as the main means of reducing risk of long-term arm swelling. Future research investigating dosimetric factors in the subgroup undergoing SLNB could help optimise radiotherapy dose constraints in the evolving axillary management landscape. References: 1. Brunt AM, Cafferty FH, Wheatley D, Sydenham MA, Kirby AM, Coles CE, et al. Patient- and clinician- assessed five-year normal tissue effects following one- week versus three-week axillary radiotherapy for breast cancer: Results from the phase III FAST-Forward trial randomised nodal sub-study. Radiother Oncol. 2025;207:110915. ISRCTN19906132. Keywords: arm lymphoedema, nodal, predictors ESTRO’s FALCON breast cancer target volume contouring workshops: Where are we in 2025? SENEM ALANYALI 1 , Sandra Hol 2 , Miika Palmu 3 , Andrea Collavini 3 , Maja V Maraldo 4 , Orit Kaidar-Person 5 , Philip Poortmans 6 1 Radiation Oncology, Ege University, Faculty of Medicine, Izmir, Turkey. 2 Radiation Oncology, Institute Verbeeten,, Tilburg, Netherlands. 3 European Society of Radiotherapy and Oncology, ESTRO, Brussels, Belgium. 4 Oncology, Copenhagen University Hospital- Digital Poster Highlight 1735 Rigshospitalet, Copenhagen, Denmark. 5 Radiation Oncology, Gray School of Medicine, Tel-Aviv University, Tel-Aviv, Israel. 6 Radiation Oncology, Faculty of Medicine and Health Sciences, University of Antwerp, Antwerp, Belgium Purpose/Objective: ESTRO launched the educational project FALCON (Fellowship in Anatomic deLineation and CONtouring) within the activities of the ESTRO School in 2009. The FALCON EduCase tool has been used in ESTRO congresses, workshops, and teaching courses. Over the last 15 years, 50 FALCON breast cancer (BC) contouring online and onsite workshops have been held, totalling >1200 participants throughout the years. The 2-step contouring program provides participants with the opportunity to contour on the EduCase® platform before and after educational presentations. Material/Methods: On 2 and 16 September 2025, an online FALCON BC contouring workshop was held. Sixty participants from around the world joined the course. Fifty-five
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