ESTRO 2025 - Abstract Book

S3303

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

ESTRO 2025

3293

Poster Discussion Reduced CTV-to-PTV margins for online adaptive radiotherapy for prostate cancer Judith H Sluijter, Igor G Lenting, Kim C De Vries, Andras G Zolnay, Maarten LP Dirkx Department of Radiotherapy, Erasmus MC Cancer Institute, Rotterdam, Netherlands

Purpose/Objective: This study aimed to optimize CTV-to-PTV margins in online adaptive radiotherapy (OART) for prostate cancer patients. Material/Methods: We included 30 prostate cancer patients treated with OART on the Ethos therapy system (Varian Medical Systems). Based on our IGRT experience, we apply 5 mm (7 mm to caudal) CTV-to-PTV margins for the prostate and, if applicable, 8 mm for seminal vesicles. Each OART session included a pre-treatment CBCT (CBCT1) for plan reoptimization, a verification CBCT (CBCT2) to adjust for patient motion, and a post-treatment CBCT (CBCT3). To determine the required CTV-to-PTV margins for OART, we evaluated the outward motion of the target structures on CBCT3 compared to CBCT1. Prostate and seminal vesicles contours were segmented on CBCT1 and CBCT3 using AI-based software (MIM Contour ProtégéAI, MIM Software Inc.), reviewed and manually adjusted if needed. Then, for each of these organs, a union structure was created by combining the outer contours from both scans. This structure was registered to the organ delineation on CBCT1 using an in-house, symmetric, feature-based non-rigid registration method with 2000 points across each surface, thereby evaluating the outward displacement in six directions [1,2]. For each fraction and organ, the 1st percentile displacement was recorded for negative directions (right, caudal, ventral), and the 99th percentile for positive directions (left, cranial, dorsal). Systematic (Σ) and random (σ) errors were calculated for each direction using the error parameters from Brand et al. [3]. Effective Σ and σ were then calculated to account for a 7-fraction ultrahypofractionation scheme [4], after which the Van Herk margin formula was applied to calculate the CTV-to-PTV margin [5]. Results: After excluding 31 fractions due to missing CBCT3 scans, 29 patients and 179 fractions were analyzed. The average time between CBCT1 and CBCT3 acquisition was 24.1 ± 4.2 minutes, with typically 8 minutes between the verification CBCT2 and CBCT3. The mean displacement of the prostate ranged from 1.8 to 2.4 mm, and for the seminal vesicles from 2.1 to 4.3 mm (Figure 1, Table 1). The derived CTV-to-PTV margins for ultrahypofractionation (7 fractions) ranged from 2.1-4.2 mm for the prostate and 3.2-7.0 mm for seminal vesicles (Table 1).