S2180
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
Material/Methods: Population mean electron density (ED) values for bone, prostate and external contour were derived from 30 previously treated patients. The T2-weighted reference MRI scan from a patient with representative anatomy was selected as an anonymized phantom reference dataset. While preserving bony anatomy and external contour with population-based mean ED assignment, a central cubic region surrounding the prostate was voided to clearly denote the dataset as artificial (see Figure 1). For new patients treated in the simulation-free workflow, this phantom dataset was re-identified and a reference plan generated using the institutional prostate template for MRI-guided treatment. At the first treatment fraction, AI-based auto-segmentation [1] delineated target volumes and organs at risk on the daily MRI. External and bony structures from the phantom dataset were deformably propagated and used for online adaptive planning. Prior to clinical implementation, the workflow was validated by deformably propagating the bone and external contour from the phantom dataset to the remaining 29 previously treated patients using visual assessment and dose recalculation. For the first five prospectively treated patients using the simulation-free workflow, fraction time and plan quality were compared with the conventional simulation-based pathway.
QA ( γ -index > 99 %). Adaptive re-optimization significantly improved PTV V95 % coverage (+1.8 %, p < 0.001) while reducing organ-of-interest doses: Rectum V41 ( − 6.3 %, p < 0.001), Rectum V100 ( − 1.2 %, p < 0.001), Bladder V68 ( − 4.5 %, p < 0.001), Bladder V100 ( − 0.8 %, p = 0.004), and Bowel V68 ( − 2.6 cc, p = 0.021). Bowel D195 showed a non-significant trend toward reduction ( − 4.3 cc, p = 0.065). Conclusion: Daily ART using a ring-gantry SGRT-integrated platform was clinically feasible with efficient workflow (< 25 min/fraction). Adapted plans achieved superior target coverage and significant rectal and bladder dose reductions compared with scheduled plans, without compromising QA or setup accuracy. This study represents an initial Indian institutional experience demonstrating that daily online ART can be safely implemented with consistent dosimetric benefit and streamlined clinical throughput. Keywords: Precision Radiation, SGRT, motion management Simulation-free phantom-based reference planning for MRI-guided online adaptive radiotherapy in intermediate-risk prostate cancer Søren Nielsen Agergaard 1,2 , Rasmus Lübeck Christiansen 1,2 , Christina Junker Nyborg 2 , Uffe Bernchou 1,3 , Anders Smedegaard Bertelsen 1,2 1 Laboratory of Radiation Physics, Odense University Hospital, Odense, Denmark. 2 Department of Oncology, Odense University Hospital, Odense, Denmark. 3 Department of Clinical Research, University of Southern Denmark, Odense, Denmark Purpose/Objective: MRI-linacs facilitate online adaptive radiotherapy (oART) but conventionally require generation of a patient-specific reference plan from dedicated simulation imaging, a plan which is never delivered. This prerequisite entails additional imaging, Digital Poster 4279 contouring, and planning procedures that consume substantial physician, physicist, and radiographer time, exacerbating staff constraints and potentially limiting access to MRI-guided oART. We report the development and initial clinical implementation of a simulation-free, phantom-based reference planning workflow for MRI-guided oART in intermediate-risk prostate cancer designed to eliminate non-value- adding steps while preserving dosimetric accuracy and operational feasibility.
Results: Propagation of population-based ED structures was robust; residual geometric discrepancies did not compromise target coverage or organ-at-risk constraints. Recalculated plans showed a mean CTV
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