ESTRO 2025 - Abstract Book

S1406

Clinical - Lung

ESTRO 2025

metastases and >PR in LR disease” (the best responding group to induction chemotherapy) (n=27), higher TRT TD (tBED 10Gy >50Gy) significantly increased LRPFS in UVA (p:0.010).

Conclusion: Increasing the TRT TD in patients with ES-SCLC showing systemic treatment responses of CR in DM and at least PR in LR disease may provide a survival advantage through increasing response to TRT.

Keywords: ES-SCLC, Thoracic RT, RT Dose

3706

Digital Poster The added value of Ethos HyperSight CBCT imaging in lung cancer radiotherapy: structure visibility and suitability for online adaptive treatments Agustinus J.A.J. van de Schoot, Joan J. Penninkhof, Britt Kunnen, Kimm P. Fremeijer, Elise M. Nicolai - Koornneef, Kirsten Offereins - van Harten, Judith H. Sluijter, Nienke D. Sijtsema, Marjan van de Pol, Raymond de Boer, Maarten L.P. Dirkx, Steven F. Petit Radiotherapy, Erasmus MC Cancer Institute, Rotterdam, Netherlands Purpose/Objective: The recently introduced HyperSight CBCT imaging system (Varian Medical Systems, Inc.) promises superior CBCT image quality, however incurs substantial additional costs. Our aim was to determine the added value of HyperSight CBCT imaging (CBCT H ) compared to conventional ring gantry CBCT imaging (CBCT C ) in lung cancer radiotherapy by quantifying structure visibility and adaptive radiotherapy (ART) suitability. Material/Methods: Twenty consecutive lung cancer patients treated between March 2023 and October 2023 were included in this prospective clinical study after providing informed consent. All patients received fractionated, non-adaptive, image guided radiotherapy with a total dose of 55.0Gy, 60.0Gy or 66.0Gy delivered in free breathing on interchangeable Ethos therapy systems equipped with either CBCT C or CBCT H . All CBCT imaging was acquired in free breathing using a predefined acquisition mode (Thorax: 125kV). Per patient, three CBCT imaging pairs consisting of CBCT H and CBCT C acquired on consecutive days were included (Figure 1). All CBCTs were pseudo-anonymized and blinded. Four experienced observers (2 radiation oncologists, 2 radiation technologists) individually evaluated all 120 CBCTs in a random fashion by scoring the visibility of patient-specific targets (primary GTV (GTV p ) and/or nodal CTV (CTV n )) and OARs (esophagus, heart, aortic arch, brachial plexus) on a 1-to-5 scale (i.e., hardly visible to clearly visible). Additionally, ART suitability scores, defined as overall CBCT-based structure delineation confidence, were scored on a 1-to-5 scale (i.e., unconfident to completely confident). Difference scores were calculated for each CBCT imaging pair per observer, averaged over the three CBCT imaging pairs and four observers resulting in patient-averaged differences and tested on deviations from zero (Wilcoxon signed-rank test).