S1276
Clinical - Urology
ESTRO 2026
prostate anatomy across fractions. Material/Methods: Patients were enrolled in the international
MOMENTUM study (NCT04075305) and received curative radiotherapy for prostate cancer in five fractions. Treatment followed either adapt-to-shape (ATS) which involves daily recontouring or adapt-to- position (ATP) which involves rigid shifts without daily recontouring. QoL was assessed at baseline, three, six, 12 and 24-months follow-up using the EORTC-QLQ- PR25 questionnaire. Propensity score matching was performed based on age and risk category. Multivariable mixed model analysis was used to assess changes in QoL over time as compared to baseline, adjusting for hormonal therapy and differences in reference plan dose distributions in rectum and bladder. Results are presented as the difference in mean difference (MD) in QoL scores between ATP- and ATS-treated patients.A research tool segmented the prostate on each patient’s daily MR-scans, computed corresponding bounding boxes, and rigidly registered prostate structures from fractions 2-5 onto that of fraction 1. Volume change, overlap metrics (DICE score and Hausdorff distance), and deformations along spatial axes were quantified as indicators of shape changes uncaptured by ATP, potentially explaining QoL score differences. Results: 296 ATP patients were successfully matched to 296 ATS patients. Mixed model analyses showed significantly smaller increases in urinary symptoms for ATS patients at 3 months (ATP MD: 7.67, ATS MD: 2.22, difference: -5.45, 95% CI: -8.69 to -2.21) and 12 months (ATP MD: 8.79, ATS MD: 2.79, difference: -5.99, 95% CI: -9.41 to –2.57). No differences were found in the bowel and sexual domain. Daily MR-scans were available for 200 ATP and 206 ATS patients. Prostate volume increased after the first fraction and peaked at fraction three. Overlap (DICE score and Hausdorff distance) decreased after the first fraction and stabilized after the third (Figure 1). Prostate deformation was asymmetric and largest in the superior-inferior direction (-2 to +6 mm) (Figure 2).
Conclusion: Online daily recontouring was associated with less deterioration in urinary QoL. Daily MR-scans suggest that radiotherapy induces prostate swelling, reflected by loweroverlap metrics after the first fraction. Deformation was largest along the superior-inferior axis, indicating shape changes not captured by rigid shifts (ATP). This supports the rationale for daily recontouring and suggests that fixed margins may fail to accommodate directionally asymmetric deformations. Keywords: Quality of life, prostate, online adaptive MRgRT Digital Poster 3806 MRI-Guided Salvage Stereotactic Body Radiotherapy (SBRT) for Prostate Cancer After Non-Radiotherapy Ablative Techniques DANIELA GONSALVES 1,2 , Castalia Fernandez 1 , Lisselott Torres 1 , Abrahams Ocanto 1 , María Gonzalez 1 , Macarena Teja 1 , Gloria Gaurdia 1 , Fernando Lopez Campos 1 , Miren Gaztañaga 1 , Luis Glaria 1 , Raul Herranz 1 , Maria Mateos 3 , Alvaro Flores 4 , Ana Bezares 4 , Jose Antonio Gonzalez 5 , Julia Rodriguez 6 , Eduardo Meilan 7 , Felipe Couñago 1,2 1 Radiation Oncology ¡, GenesisCare, Madrid, Spain. 2 Department of Medicine, School of Medicine, Health and Sport, European University of Madrid, Madrid, Spain. 3 Radiation Oncology ¡, GenesisCare, Torrejon, Spain. 4 Radiation Oncology ¡, GenesisCare, Algeciras, Spain. 5 Radiation Oncology ¡, GenesisCare, Sevilla, Spain. 6 RTT, GenesisCare, Madrid, Spain. 7 Physicis, GenesisCare, Madrid, Spain Purpose/Objective: Focal therapies such as high-intensity focused ultrasound (HIFU) and cryoablation are increasingly used in localized prostate cancer, yet local recurrence rates up to 20% have been reported. Stereotactic body radiotherapy (SBRT) offers a non-invasive salvage option with promising efficacy and tolerability. MRI- guided radiotherapy (MRgRT) enables daily plan
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