ESTRO 2025 - Abstract Book

S3196

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

312

Digital Poster Online adaptive MRI-guided prostate radiotherapy without patient-specific images, delineations and plans Georgios Tsekas, Lisa Wiersema, Sanne Conijn, Jega Sundaram, Uulke A. van der Heide, Tomas Janssen Department of Radiotherapy, The Netherlands Cancer Institute, Amsterdam, Netherlands Purpose/Objective: Offline treatment preparation is a time-consuming step in online adaptive MRI-guided radiotherapy (MRIgRT) in which, a patient-specific reference plan is created based on reference images and patient-specific delineations of the tumor and organs at risk (OAR). Additionally, on the day of treatment an adaptation MRI is acquired, followed by re-delineation and plan re-optimization, which potentially reduces the significance of the preceding patient-specific preparation. Generic (compared to patient-specific) offline treatment preparation would omit acquisition of the planning scan, and patient-specific offline delineation and planning, drastically improving overall treatment efficiency. This work aims to explore the feasibility of using such patient-agnostic reference delineations and plans for adaptive re-planning in MRI-guided prostate RT. Material/Methods: Planning MRI, delineations and treatment planning template of a single retrospective patient were selected to serve as a “patient-agnostic” reference (patient agnostic ). Population-based electron density values were used for treatment planning [1]. Five prostate cancer patients, treated with 5x7.25Gy, were retrospectively included in this study. For each patient, daily MRI scans and online adaptive delineations and plans were available. Using the patient agnostic reference, online adaptive MRIgRT was simulated (Figure 1). Initially, in fraction 1, an adapt-to-shape (ATS) step [2] was performed between the patient agnostic reference and the adaptation MRI scan. Organ delineations were adjusted and patient specific adaptations were allowed in the planning template, using maximum 2 iterations. For fractions 2-5, online adaptation using conventional ATS was performed using the patient-specific adaptation of fraction 1 as reference. In accordance with clinical practice, delineations were adjusted, but the planning template was not further modified. The contouring and plan optimization steps were timed and reported. The patient agnostic plans were evaluated in terms of target coverage and OAR dose and were compared to the corresponding clinical plans (patient specific ).