ESTRO 2025 - Abstract Book

S3038

Physics - Image acquisition and processing

ESTRO 2025

gamma analysis passing rate using 3%-3 mm criteria was 99.7 (99.0-100.0%). Soft tissue matching using MR-CBCT was equivalent to CT-CBCT ( p > 0.05 ), with median alignment differences of 0.3 ± 0.3 cm in the x, y, z directions.

Conclusion: Implementing an MRI-only workflow is feasible in a large center, providing accuracy for dose calculation and online matching.

Keywords: MRI-Only,Pelvic Radiotherapy

3319

Digital Poster Automated MR-only radiotherapy outperforms conventional MR-CT based radiotherapy in accuracy for head-and-neck cancer (MEC-2019-0805) Iris Lauwers 1 , Marta Capala 1 , Sandeep Kaushik 2,3 , László Ruskó 4 , Cristina Cozzini 2 , Eszter Szabo 4 , Adam Kekesi 4 , Borbala Deak-Karancsi 4 , Jonathan Wyatt 5,6 , Rachel Pearson 5,6 , Gerda Verduijn 1 , Florian Wiesinger 2 , Juan Hernandez Tamames 7,8 , Steven Petit 1 1 Department of Radiotherapy, Erasmus MC Cancer Institute, Rotterdam, Netherlands. 2 GE Healthcare, GE HealthCare, Munich, Germany. 3 Department of Quantitative Biomedicine, University of Zurich, Zurich, Switzerland. 4 GE Healthcare Magyarország Kft.,, GE HealthCare, Budapest, Hungary. 5 Translational and Clinical Research Institute, Newcastle University, Newcastle, United Kingdom. 6 Northern Centre for Cancer Care, Newcastle upon Tyne Hospitals NHS Foundation Trust, Newcastle, United Kingdom. 7 Department of Radiology and Nuclear Medicine, Erasmus MC, Rotterdam, Netherlands. 8 Department of Imaging Physics, TU Delft, Delft, Netherlands Purpose/Objective: MR-only radiotherapy is of high interest for head-and-neck cancer patients as it eliminates the need for a CT scan and the corresponding registration inaccuracies. Moreover, the superior soft tissue contrast achievable with MR compared to CT facilitates accurate autocontouring of organs at risk (OAR). Here for the first time we dosimetrically evaluated automated MR-only RT for head-and-neck cancer patients, where after MR acquisition, the MR scans are automatically converted into synthetic CT (synCT) scans including autocontouring of all relevant OARs. Automated MR-only RT was compared to current clinical workflow where the tumor is delineated based on MR, and OAR delineations and planning performed on CT scans. Material/Methods: Fifteen head-and-neck patients were prospectively included between January 2022 and July 2024 (MEC-2019-0805). Patients were treated using VMAT according to the current clinical standard using, which consisted of MR-based tumor delineation, and CT-based OAR delineation and treatment planning. For automated MR-only RT, the ZeroTE MR scan was converted into a synCT scan 1 , and the OAR contours were automatically delineated on the T2w 2 . To