ESTRO 2025 - Abstract Book

S1999

Clinical - Urology

ESTRO 2025

2378

Digital Poster Clinical Application of CBCT-based Online Adaptive Radiotherapy (oART) for Bladder Cancer Kynann P. Aninditha 1 , Tarek Ellethy 1 , Saib Rinnawi 1 , Danilo Trijic 1 , Nora Tremmel 2 , Jannis Herkommer 2 , Nils Wegner 2 , Marc Münter 1 1 Department of Radiation Oncology, Klinikum Stuttgart, Stuttgart, Germany. 2 Department of Medical Physics and Radiation Protection, Klinikum Stuttgart, Stuttgart, Germany Purpose/Objective: Online adaptive radiotherapy (oART) allows daily adjustments to treatment plans based on the patient’s current anatomy, improving precision and reducing toxicity through advanced workflows [1,2]. In bladder cancer, daily oART leverages imaging modalities like CBCT to address interfractional bladder volume changes, enabling structure and plan adaptation with the potential of reducing treatment margins [3]. This retrospective analysis investigates the feasibility and efficacy of daily oART in bladder cancer patients using AI guided, CBCT-based radiotherapy with the Varian Ethos® system. Material/Methods: Nine patients with muscle-invasive bladder cancer were treated with definitive radiotherapy using CBCT-based oART with the Varian Ethos® system between 02/2022 and 07/2023 at Klinikum Stuttgart, Germany. For each session, two plans were generated after recontouring the target volume and anatomy of the day. The "scheduled plan" was the reference plan rigidly recalculated for the daily anatomy, while the "adapted plan" was a reoptimized reference plan tailored to the patient’s anatomy of the day. Key metrics, including PTV coverage (D98%) and D2cc doses to OARs, were evaluated for both scheduled and adaptive plans. Results: From a total of 200 treatment sessions, the adapted plan was selected as the plan of the day in 98% of cases. The median duration for adaptive sessions was 24 minutes and 50 seconds. Bladder volume showed a change of 28.1% relative to the reference plan in average, resulting in a 22.9% change in PTV volume after adaptation. The trend indicated a reduction in bladder volume, with a mean decrease of 36.2 cc in bladder filling between fractions compared to the reference plan, leading to a mean reduction of 74 cc in PTV volume. The mean PTV coverage (D98%) was 89.3% for scheduled plans and 97.7% for adaptive plans, demonstrating an improvement of over 8% with adaptive planning (p < 0.05). Significant reduction of D2cc were observed in the OARs, with a 7.3% reduction for the rectum (p < 0.05), 6% for the sigmoid colon (p < 0.05), and 4.5% for the intestine (p = 0.003). Conclusion: Our results highlight the potential of oART to address daily volumetric changes in the bladder, maintaining dose coverage in the PTV while minimizing radiation exposure to OARs through plan adaptation. The ability to dynamically adjust target volumes based on these changes could enhance treatment precision and improve clinical outcomes. Further studies are necessary to validate these findings and optimize the implementation of this innovative approach in clinical practice.

Keywords: online adaptive radiotherapy in bladder cancer

References: 1. Glide-Hurst, C. K., Hackett, C. ... & Michalski, J. M. (2021). Adaptive radiation therapy (ART) strategies and technical considerations: A state of the ART review from NRG Oncology. International Journal of Radiation Oncology, Biology, Physics, 109(4), 1054 – 1075.