S2198
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Digital Poster 79 Intra-fractional prostate motion during online adaptive SBRT planning CORAL LAOSA-BELLO 1 , SANDRA MORAGUES- FEMENIA 1 , JUAN-FRANCISCO CALVO-ORTEGA 1,2 1 RADIATION ONCOLOGY, HOSPITAL QUIRÓNSALUD BARCELONA, BARCELONA, Spain. 2 RADIATION ONCOLOGY, HOSPITAL QUIRÓNSALUD MÁLAGA, MÁLAGA, Spain Purpose/Objective: 1) To assess the dosimetric impact of the intra- fractional prostate displacements during the planning stage of an online adaptive SBRT procedure. 2) To evaluate the feasibility of a 2 mm PTV margin proposed by Courtney et al. to compensate for the intra-fraction prostate motion.1 Material/Methods: Prostate SBRT (5 × 7.25 Gy) patients are treated in our department using the online adaptive ‟ plan-of-the- day” (POD) (Figure).2 Each POD is planned for a PTV created by expanding the prostate (CTV) with margins of 2 mm in the left-right direction (X), and 4 mm in anterior-posterior (Y) and superior-inferior directions (Z). During the interval between the CBCT1 and CBCT2 image acquisitions (“planning stage”), the POD is created, independently verified (patient-specific QA) and approved for delivery. This elapsed time takes approximately 20.1 ± 6.1 minutes.3 However, potential prostate shifts can occur during this time lapse. Twenty PODs were randomly selected. For each one, the following workflow was performed: A new PTV using a 2 mm isotropic margin was generated from the CTV on the CBCT1 scan.The retrieved POD was re-optimized for this 2 mm PTV (POD1), by keeping the same clinical goals of the original POD. CBCT1 and CBCT2 images were rigidly registered in 6D, focusing the match on the pelvic bones. It provides the patient setup error before treatment delivery.The CTV and organs-at-risk (OARs) originally contoured on the CBCT1 were propagated and updated on the CBCT2 image. Using the match of 3), the POD1 was mapped to the CBCT2 scan and recalculated while keeping the monitor units (POD2). Comparison of CTV coverage and OAR doses (analyzed metrics in Table) between both plans (POD2 vs. POD1) was performed.
Results: CBCT1-CBCT2 matches detected patient shifts (95% confidence limits) of 2.6 mm (alongside X, Y and Z directions), 1.1˚ (pitch), 0.8˚ (swivel) and 1.1˚ (roll).Clinical goals were met in all POD2 (Table).An average mean difference of 2.5% was found between POD1 and POD2 for the CTV coverage. Negligible differences were observed for the remaining of metrics (Table). Conclusion: Non-negligible patient setup errors occur during the planning stage. However, the 2 mm PTV margin seems adequate to compensate the intra-fractional prostate motion during the online planning stage. Nevertheless, further investigation is required to determine whether this margin ensures the target coverage during the treatment delivery interval (CBCT2-CBCT3 time lapse in Figure). References: 1 Int J Radiat Oncol Biol Phys. 2025 Jun 5:S0360- 3016(25)00553-X2 Pract Radiat Oncol. 2022 Mar- Apr;12(2):e144-e152.3 Pract Radiat Oncol . 2025 Jul- Aug;15(4):e371-e381. Keywords: adaptive, online, prostate
Mini-Oral 275
MRIgRT real-time target tracking in 2D CineMRI from MRI-linacs: TrackRAD2025 challenge report Tom Julius Blöcker 1 , Pia Görts 2 , Yiling Wang 3 , Elia Lombardo 1 , Adrian Thummerer 4 , Coen Hurkmans 2 , Rob Tijssen 2 , Davide Cusumano 5 , Pim Borman 6 , Marco
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