ESTRO 2025 - Abstract Book

S4343

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

Conclusion: Our results have confirmed clinical translatability of this MidV workflow to two other RO departments within the accepted IOV delineation thresholds. Despite differences in MidV phase selection and IOV in the GTV contouring, generation of the PTV margins mitigated these differences resulting in all PTV MDAs being <3mm and DSCs of >0.7. It is postulated that ease of this workflow in a single TPS will facilitate more widespread clinical uptake of the MidV method.

Keywords: Mid-Ventilation, Lung Cancer, SABR

References: 1. Thomas S, Evans B, Harihar L, et al. An evaluation of the mid-ventilation method for the planning of stereotactic lung plans. Radiother Oncol . 2019;137:110-116. doi:10.1016/j.radonc.2019.04.031 2. Guzene L, Beddok A, Nioche C, et al. Assessing Interobserver Variability in the Delineation of Structures in Radiation Oncology: A Systematic Review. Int J Radiat Oncol Biol Phys . 2023;115(5):1047-1060. doi:10.1016/j.ijrobp.2022.11.021 3. Selek,U, Hu, K, Tatli, H, et al. Global Interobserver Variations of Clinical Target Volume (CTV) Delineation among International Experts in Given Standard Gross Tumor Volume (GTV) and Organs at Risk (OAR) Volumes in Lung Cancer. Int J Radiat Oncol Biol Phys . 2024;120(2):e788. 10.1016/j.ijrobp.2024.07.1730

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Digital Poster Comparison of target volume changes in T2W and DWI with ADC and cumulative delivered dose in muscle invasive bladder cancer James Tallon 1 , Damien Mchugh 2 , Benedict Dobby 1 , Claire Nelder 1 , Michael Dubec 2 , Marcel VanHerk 1 , Cynthia Eccles 1 , Ananya Choudhury 1 1 Radiotherapy Research and Innovation, The Christie, Manchester, United Kingdom. 2 Medical Physics and Engineering, The Christie, Manchester, United Kingdom Purpose/Objective: Understanding how the bladder target changes using imaging during radiotherapy, may facilitate adaptive radiotherapy or further plan optimisations. To quantify target volume (TV) longitudinal change measured on T2 weighted (T2W), and diffusion weighted imaging (DWI), we compared volume changes with apparent diffusion coefficient (ADC) changes and cumulative delivered dose (CDD). Material/Methods: Ten patients with muscle invasive bladder cancer (MIBC) were included in this retrospective analysis. Patients received 55 Gray in 20 fractions over four weeks on a 1.5 Tesla (T) MR Linac (Elekta, Stockholm), between March 2022 and December 2023. Treatment took place at a single institution as part of the MOMENTUM study (NCT04075305). Patients were imaged immediately following treatment using axial T2 3D (TR=1400 ms, TE=183 ms, TA=147s) and DWI (b=0, 150, 500 s/mm 2 , TR=3521 ms, TE=66 ms, TA=222s) scans once weekly during treatment. ADC maps were generated from b150 and b500 s/mm 2 acquisitions. TVs were contoured on T2W and DWI at the sim appointment (baseline) and for each weekly scan, by a single observer initially. Intra-observer variability was evaluated using 5 contour repeats for each patient for each sequence acquired in week 3; these were compared with a simultaneous truth and performance level estimation (STAPLE) contour generated with sitkSTAPLEImageFilter v2 software using dice similarity coefficient, Hausdorff distance and sensitivity. Inter observer variability was determined using two additional observer contours versus a STAPLE using the same three measurements as intra observer studies. Mean ADC was determined within a fixed 10 mm 2 region of interest (ROI) placed in the gross tumour volume (T ADC) on a single image slice. Non-tumour bladder wall (B-ADC) was determined from the same image slice using a