S1289
Clinical - Urology
ESTRO 2026
followed centre-specific margins reflecting local image- guidance practice. Robustness was evaluated by recalculation of all plans on two repeat CTs per patient (weeks 1 and 3) and by geometric robustness scenarios (14 per nominal proton plan, 6 per nominal photon plan). Dose-volume metrics for CTV coverage (V95%) and organs of interest; bowel bag (V35Gy, mean), anorectum (V75Gy, V30Gy, mean), and bladder (V70Gy, mean), were compared using hierarchical linear mixed-effects models including patient and centre as random effects. Interaction terms tested whether modality differences varied across plan types. Results: Target coverage was robust for both modalities, with only minor and clinically acceptable deviations observed. No significant interactions were found between modality and plan type, indicating consistent proton-to-photon differences across nominal, recalculated, and uncertainty scenario plans. Proton therapy significantly reduced bowel exposure: V35Gy decreased by 11.0 percentage points (95% CI [3.4- 18.6], p = 0.02) and bowel mean dose by 13.8 Gy (95% CI [8.9-18.8], p = 0.001). The bladder mean dose was also lower by 18.4 Gy (95% CI [4.2-32.6], p = 0.02). Differences for high-dose anorectal and bladder metrics were small and non-significant, whereas anorectum V30Gy and mean dose showed larger but non-significant reductions with protons.
comparison between centres with different reporting practices. These findings support HDR brachytherapy boost as a safe and effective strategy for prostate dose intensification. Keywords: Dose, Volume, Toxicity
Digital Poster 4166
Treatment planning outcomes from the first randomised trial comparing proton- versus photon-based whole-pelvic radiotherapy for prostate cancer Sofie Tilbæk 1,2 , Stine Elleberg Petersen 1 , Liliana Stolarczyk 1 , Kasper Lind Laursen 3 , Steffen Bjerre Hokland 4 , Susan Blak Nyrup Biancardo 5 , Kirsten Legaard Jakobsen 6 , Rasmus Havelund 7 , Terje Andersen 7 , Bjarke Mortensen 7 , Christine Vestergård Madsen 7 , Anders Schwartz Vittrup 4 , Henriette Lindberg 5 , Dorthe Yakymenko 6 , Jimmi Søndergaard 3 , Ludvig Paul Muren 1,2 1 Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark. 2 Department of Clinical Medicine, Aarhus University, Aarhus, Denmark. 3 Department of Oncology, Aalborg University Hospital, Aalborg, Denmark. 4 Department of Oncology, Aarhus University Hospital, Aarhus, Denmark. 5 Department of Oncology, Herlev and Gentofte Hospital, Copenhagen University Hospital, Herlev, Denmark. 6 Department of Clinical Oncology, Zealand University Hospital, Næstved, Denmark. 7 Department of Oncology, Vejle Hospital, University Hospital of Southern Denmark, Vejle, Denmark Purpose/Objective: Whole-pelvic radiotherapy (WPRT) improves disease control in high-risk prostate cancer but increases normal tissue exposure. Proton therapy offers superior dose conformity but may be more sensitive to setup and anatomical variations. This study evaluated, within the first ever multicentre randomised clinical trial, whether the dose-volume advantages of proton-based WPRT persisted when also accounting for the inter-fractional variation occurring during the treatment course. Material/Methods: Five patients treated with proton WPRT in the pilot phase of the Danish PROstate PROTON Trial 1 (NCT05350475) were included. For each patient, five Danish photon centres created comparative photon plans according to the trial protocol, resulting in 5 proton and 25 photon nominal plans. All plans prescribed 78 Gy to the prostate and 56 Gy to the pelvic lymph nodes in 39 fractions using a simultaneously integrated boost. Proton plans were generated using robust optimisation (3.5% range and 5 mm setup uncertainties), while photon plans
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