ESTRO 2025 - Abstract Book

S3129

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

References: [1] Zaki P et al. Proton Beam Therapy and Photon-Based Magnetic Resonance Image-Guided Radiation Therapy: The Next Frontiers of Radiation Therapy for Hepatocellular Carcinoma. Technol Cancer Res Treat. 2023 doi: 10.1177/15330338231206335. [2] Winkel D et al. Adaptive radiotherapy: The Elekta Unity MR-linac concept. Clinical and Translational Radiation Oncology, 2019, 18, 54–59. https://doi.org/10.1016/j.ctro.2019.04.001 [3] Kierkels RGJ et al. Multileaf collimator characterization and modeling for a 1.5 T MR-linac using static synchronous and asynchronous sweeping gaps. Phys Med Biol. 2024 Mar 14;69(7). doi: 10.1088/1361-6560/ad2d7d.

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Mini-Oral Enabling 68Ga-PSMA-11 (Illuccix)-response Adapted SAbR for High-Risk Prostate Cancer Using MR- and CBCT Based Online Adaptive Systems (NCT06044857) Ruiqi Li 1 , Nghi Nguyen 2 , Erica Salcedo 1 , Elizeva Phillips 3 , Fan-Chi Su 1 , Sean Domal 1 , Aurelie Garant 1 , Raquibul Hannan 1 , Daniel Yang 1 , Robert Timmerman 1 , Minjae Lee 4 , David Parsons 1 , Mu-Han Lin 1 , Orhan Oz 3 , Neil Desai 1 1 Department of Radiation Oncology, UT Southwestern Medical Center, Dallas, USA. 2 Department of Nuclear Medicine, MD Anderson Cancer Center, Houston, USA. 3 Department of Radiology, Division of Nuclear Medicine, UT Southwestern Medical Center, Dallas, USA. 4 Department of Public Health, UT Southwestern Medical Center, Dallas, USA Purpose/Objective: This study evaluates the feasibility of incorporating PSMA-PET-guided adaptive microboosting into stereotactic ablative radiotherapy (SABR) workflows for high-risk prostate cancer. By refining microboost volumes during treatment course, this approach aims to minimize organ-at-risk (OAR) toxicity while maintaining robust target coverage, without requiring specialized biology-guided radiotherapy (BGRT) systems. Material/Methods: This investigator-initiated trial enrolls men with HRPCa planned for SAbR with androgen deprivation therapy (ADT) and staging PET PSMA/CT (68Ga-PSMA-11; Illuccix, Telix). Per protocol prescriptions included 36.25Gy to the prostate/seminal vesicles, 25Gy to pelvic lymph nodes, and 50Gy micro-boost to the PSMA-PET-defined DIL over 5 fractions (Schema Figure 1 ). As opposed to micro-boost of pre-therapy defined DIL (GTVinitial), this study refined DIL-microboost based on treatment response after neoadjuvant ADT (GTVmb1) and after fraction-3 SAbR (GTVmb2) using pre-defined %SUVmax thresholds. Daily online adaptation additionally addressed anatomical variations. For this preliminary analysis, men completing full treatment on either MR- or CBCT-guided-adaptive therapy (ART) systems were identified. Retrospective comparison plans assessed OAR and target dosimetric differences between delivering 50 Gy to GTVinitial, acquired before treatment, versus the protocol-specified as treated GTVmb1/GTVmb2.