ESTRO 2025 - Abstract Book

S4333

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

Conclusion: The integration of MotionFree PET software with Sentinel Catalyst HD's 4D-CT gating system demonstrated promising results in respiratory motion assessment for radiotherapy planning. Using a phantom simulating 4 second breathing cycles with 2-cm amplitude, the study revealed strong correlation between PET and CT data during mid-respiratory phases. While both modalities showed artifacts during extreme respiratory phases, the consistent patterns maintained overall synchronization. The complementary strengths of both systems - PET's superior temporal resolution during rapid motion and CT's anatomical detail - suggest potential for improved target delineation accuracy in clinical applications. This integrated approach shows promise for enhancing personalized radiotherapy planning and dose painting applications, though further validation in clinical settings with diverse patient respiratory patterns is needed.

Keywords: Lung cancer, Respiratory motion, 4D PET/CT

1317

Digital Poster A comparative analysis of the dosimetric benefits of the library of plans and daily online adaptation on MR Linac for cervical cancer Amerah Alshamrani 1 , Robert Chuter 2,1 , Frank Brewster 2 , Marianne Aznar 1 , Peter Hoskin 2,1 , Claire Nelder 2 , Ananya Choudhury 2,1 , Cynthia L Eccles 2,1 1 Faculty of Biology, Medicine and Health, The university of Manchester, Manchester, United Kingdom. 2 Radiation oncology, Christie NHS Foundation Trust, Manchester, United Kingdom Purpose/Objective: Daily online adaptive MR-guided radiotherapy for cervical cancer has the potential to reduce dose to organs at risk (OARs) compared to a library of plan (LOP) approach. This study provided a dosimetric comparison of daily adaptation with LOP strategy in an MR-guided workflow. Material/Methods: Five women with FIGO stage IIB node-negative cervical cancer received 45Gy in 25 fractions on a 1.5T MR-Linac as part of MOMENTUM (NCT04075305). Treatment followed EMBRACE II guidelines and an adapt-to-shape "MR adapted" workflow (Monaco v5.40.01, Elekta). For each fraction, three images were acquired: (Fx-pre: pre adaptation), (Fx-mid: post-adaptation verification), and (Fx-post: post-treatment evaluation). The LOP was created using the pre-treatment MRI (pMR) and the first three fractions to obtain different bladder fillings. Doses to the target and OARs on the Fx-mid images were determined daily and cumulatively for both the LOP and daily online adaptive workflows for all patients. All Fx-mid contours were peer-reviewed. The method for generating the LOPs is presented in Figure 1. Criteria for LOP selection were prioritised based on i) bladder filling, ii) motion of the CTV-T-LR, iii) optimal matching to OAR and iv)transitional shift within 1 cm. The dose for LOP was recalculated from the best-fit LOP to the daily selected image using the adapt-segment function to simulate the dose in the conventional approach. Doses to target structures (V4275 cGy to CTV_T_LR and ITV ) and OARs (D0.1% to the bladder, rectum, and bowel) were extracted for every fraction. Wilcoxon test assessed significant dose change between the two approaches, per patient and structure.