ESTRO 2025 - Abstract Book

S3146

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Conclusion: Spine tracking using template matching techniques achieved sub-millimeter accuracy with 7 fps. Using 2 fps fluoroscopy resulted in factor 3.5 dose reduction compared to the current standard of 7 fps, without compromising positional accuracy, at the expense of a lower tracking rate. When TI parameters are adjusted to yield a similar kV dose to 2 fps fluoroscopy, comparable tracking rate was not achieved with the current clinical settings. These findings suggest that additional measurements are required to determine a balance between tracking accuracy and imaging dose.

Keywords: spine positioning, phantom, markerless

References: 1. Haytmyradov et al. Markerless tumor tracking using fast-kV switching dual-energy fluoroscopy on a benchtop system. Med Phys. 2019 Jul;46(7):3235-3244. 2. Hazelaar C et al. Verifying tumor position during stereotactic body radiation therapy delivery using (limited-arc) cone beam computed tomography imaging. Radiother Oncol. 2017 Jun;123(3):355-362. 3. Koo J,et al. Triggered kV Imaging During Spine SBRT for Intrafraction Motion Management. Technol Cancer Res Treat. 2021 Jan-Dec. 2531 Digital Poster Dosimetric parameters in Daily Adaptive RadioTherapy in Postoperative HypofrActionated Salvage radiothERapy for prostate cancer patients (DART-PHASER) Riccardo Filippo Borgese 1 , Margherita Corsi 2 , Luca Nicosia 1 , Nicola Bianchi 1 , Antonio De Simone 1 , Andrea Gaetano Allegra 1 , Chiara De-Colle 1 , Niccolò Giaj-Levra 1 , Davide Gurrera 1 , Stefania Naccarato 1 , Carolina Orsatti 1 , Edoardo Pastorello 1 , Francesco Ricchetti 1 , Michele Rigo 1 , Andrea Romei 1 , Gianluisa Sicignano 1 , Ruggiero Ruggeri 1 , Filippo Alongi 1,3 1 Department of Advanced Radiation Oncology, IRCCS Sacro Cuore Don Calabria, Negrar di Valpolicella, Italy. 2 Medical Physics Unit, IRCCS Ospedale Sacro Cuore Don Calabria, Negrar di Valpolicella, Italy. 3 University of Brescia, University of Brescia, Bresci, Italy Purpose/Objective: Ethos system uses daily imaging and artificial intelligence to adjust treatment plans based on daily anatomical changes, optimizing organ-at-risk (OARs) protection and potentially minimizing side effects. This approach might set new standards in the setting of prostate cancer (PC) postoperative salvage radiotherapy (sRT), balancing efficacy with reduced toxicity for better patient outcomes. We aim to evaluate preliminary dosimetric criteria differences between scheduled and adapted treatment in CT-based adaptive radiotherapy within a prospective clinical trial. Material/Methods: This prospective observational study (NCT05884632) evaluates the safety and efficacy of adaptive hypofractionated sRT in PC. Eligible patients (up to 80 years old, post-prostatectomy, PSMA-PET-CT confirmed M0) receive 20 daily fractions up to 59 Gy for macroscopic relapse and 55 Gy for biochemical-only relapse. The pelvis was included in the case of pelvic metastases with a dose of 45 Gy. The treatment was administered using Ethos, an AI-powered system that adjusts treatment plans based on daily anatomical changes. Ethos system generates two available plans: the scheduled and the adapted. The scheduled plan consists of a dose recalculation on daily synthetic CT (generated from daily CBCT). The adapted plan consists of a reoptimization based on the daily anatomy. The user then selects either the scheduled or adaptive plan for treatment. The primary endpoint was acute gastrointestinal (GI) toxicity reduction compared to the historical series, assessed via CTCAE v5.0. In the present study, we report dosimetric differences for PTV, rectum, bladder, and bowel between scheduled and adapted plans.