ESTRO 2025 - Abstract Book

S3305

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

fractions) using the Varian/Ethos ART system. Eight patients were treated with a 5 mm CTV-PTV margin, while 11 received a 3 mm margin. The ART workflow involved daily CBCT imaging to account for anatomical changes. Intrafraction motion was measured by comparing the spatial alignment of two CBCT scans: the first taken before the ART workflow and the second immediately before treatment. "Door-to-door" time, encompassing imaging and treatment delivery, was recorded. The CTV volume, derived from the gross tumor volume (GTV), remained consistent across patients. CTV coverage was assessed by analyzing D98% (dose covering 98% of the CTV) and comparing results between the 5 mm and 3 mm groups. Results: Reducing the margin to 3 mm significantly decreased the average PTV volume from 903 ± 187 cm³ (5 mm) to 770 ± 97 cm³ (3 mm), indicating reduced irradiation of healthy tissues. Spatial displacement between the first and second CBCT scans was minimal, with an average of 0.16 ± 0.10 cm, confirming precise target localization with the narrower margin. The average door-to-door time was 19:29 ± 2:02 minutes, demonstrating that ART workflows remain efficient, even with narrower margins. CTV D98% values for the 3 mm group fell within the 95% confidence interval of the 5 mm group, indicating no significant compromise in target coverage.