S2248
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
V36Gy (cc), bladder neck V37Gy (cc) and urethra V38Gy (%) for OAR assessment. These OAR constraints are part of the constraints used in the trial. Results: Comparing the predicted and adapted doses showed that the mean dose to the GTV remained constant at 42.10 Gy ± 0.64 and 42.10 Gy ± 0.46. The PTV coverage (V36Gy) increased from 94.30% ± 2.54 to 95.75% ± 1.24, and this improvement was statistically significant (paired t-test, p < 0.01).For the OAR, the mean rectum V36Gy showed a non-significant decrease from 1.21 cc ± 0.81 to 1.13 cc ± 0.48 (paired t-test, p = 0.18). In contrast, significant reductions were observed for the bladder neck and urethra, decreasing from 2.50 cc ± 1.72 to 2.23 cc ± 1.23 (paired t-test, p = 0.01) and from 21.68% ± 14.77 to 18.29% ± 11.85 (paired t-test, p < 0.01), respectively. (See Figure 1)
is cheaper, simpler and straightforward in clinical radiotherapy settings and also avoids any clinical issues with supplementary oxygen. References: 1 van Kesteren Z, Veldman JK, Parkes MJ, et al. Quantifying the reduction of respiratory motion by mechanical ventilation with MRI for radiotherapy. Radiat Oncol. May 21 2022;17(1):99. doi:10.1186/s13014-022-02068-5.2 Veldman- Landegent JK, van Kesteren Z, Parkes MJ, et al. Noninvasive Mechanical Ventilation Is a Promising Way to Improve Lung Cancer Radiation Therapy. Adv Radiat Oncol. Feb 2025;10(2)doi:ARTN 101679 10.1016/j.adro.2024.101679.3 Parkes MJ, Green S, Stevens AM, Parveen S, Stephens R, Clutton-Brock TH. Safely prolonging single breath-holds to >5 min in patients with cancer; feasibility and applications for radiotherapy. Br J Radiol. Jul 2016;89(1063):20160194. doi:10.1259/bjr.20160194. Keywords: Motion, Breathing Management, NIMV Digital Poster 3195 Dosimetric analysis of an MR-guided online adaptive workflow: the Proseven trial Tim Everaert, Benjamin Vanspeybroeck, Jacques Bezuidenhout, Adrián Gutiérrez, Guy Soete, Mark De Ridder, Thierry Gevaert Radiotherapy, UZ Brussel, Brussels, Belgium Purpose/Objective: The Proseven trial (MR-guided prostate stereotactic body radiotherapy in seven days) is a phase II study evaluating whether shortening the overall treatment time of prostate SBRT can improve patient comfort and compliance. [1] The trial also investigates focal dose escalation to the gross tumor volume (GTV) as a strategy to enhance oncologic outcomes. [1] The present research examined the dosimetric effects of an online adaptive workflow on target coverage and organ-at-risk (OAR) doses in this trial. Material/Methods: Dosimetric data from the first 50 patients enrolled in the Proseven trial were retrospectively analyzed. For every fraction, online recontouring of the targets and OAR was performed. The original treatment plan was then recalculated on the adapted contours. If the predicted dose failed to meet the dose criteria, an online plan adaptation was performed. The predicted and adapted dose distributions were afterwards compared. For the analysis, 190 out of 250 fractions were included. The remaining fractions were either treated with the original plan or excluded due to loss of prediction data caused by machine failure. The analysis focused on the GTV mean dose (Gy) and PTV V36Gy (%) for target evaluation and on the rectum
Figure 1: Distribution of the analyzed dose points for all fractions. Conclusion: Online plan adaptation shows a statistically significant increase in PTV coverage accompanied by a significant reduction in bladder neck and urethral dose. These results are clinically relevant given the considerable effect of urinary toxicity on quality of life. [1] References: [1] Vanspeybroeck B, Bezuidenhout J, Soete G, et al. Proseven trial: MR-guided prostate stereotactic body radiotherapy in seven days - First results. Radiother Oncol. 2025;213:111208. doi:10.1016/j.radonc.2025.111208 Keywords: MR-guided, Dosimetry, Prostate
Poster Discussion 3199
Assessment of Intra-Fraction Motion in Patients Treated using MANIV-DIBH in Radiotherapy: Can we Reduce the Intra-Fraction PTV Margin? Alicia Hidoud 1 , Nicolas Audag 2,3 , Aude Vaandering 4 , Geneviève Van Ooteghem 1,4 1 Institut de Recherche Expérimentale et Clinique, Molecular Imaging-Radiotherapy and Oncology IREC/MIRO, Université Catholique de Louvain,
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