S2276
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
CBCT acquisitions ( Δ T) was recorded as treatment time. Correlations between Δ T and organ motion were assessed using Spearman’s rank correlation and mixed-effects models accounting for repeated measures. Dosimetric parameters (V60, V57, V52.8, V48.6, and V40.8) for the bladder and rectum were compared between baseline and intrafraction CBCTs. Results: A total of 200 paired CBCTs were analyzed, with a mean Δ T of 7.0 ± 1.7 min (range: 4–11 min). Mean displacements were 2.6 ± 2.4 mm for the prostate, 3.1 ± 2.5 mm for the bladder, and 2.8 ± 2.2 mm for the rectum (Figure1). Displacements exceeding 3 mm occurred in 20–25% of fractions, and >5 mm in 5–8% of cases. A positive correlation was observed between treatment time and organ motion, most notably for the bladder ( ρ = 0.25, p = 0.09). Beyond 8 minutes of treatment duration, significantly larger shifts were recorded across all structures (p = 0.025 for bladder and p<0.005 for rectum). (Table 1)
Conclusion: The study quantifies respiratory-induced liver segment motion in MR-Linac cine images, underscoring the importance of motion-adaptive margins and the placement of tracking surrogate structures. As gated workflows with reduced margins and multi-metastases treatments in a single session become more common, understanding and quantifying intersegment displacements will be necessary to maintain adequate dose coverage. Keywords: Surrogat tracking, Liver, Breathing Digital Poster 4903 Real - World CBCT analysis of intrafraction motion and treatment time in hypofractionated prostate radiotherapy in LMICs Raouia Ben Amor 1,2 , Raja Oueslati 1 , Siwar Abdessaied 1 , Zeineb Naimi 1,2 , Roua Toumi 1 , Ghada Bouguerra 1 , Rihab Haddad 1 , Awatef Hamdoun 1 , Lotfi Kochbati 1,2 1 Radiation Oncology, Abderrahmen Mami Hospital, Ariana, Tunisia. 2 Faculty of Medicine of Tunis, Tunis El Manar University, Tunis, Tunisia Purpose/Objective: Intrafraction organ motion represents a key source of dosimetric uncertainty in prostate radiotherapy, especially with hypofractionated schedules delivering high doses per fraction. The aim of this study was to quantify the relationship between treatment duration and geometric displacement of pelvic organs during prostate hypofractionated radiotherapy (60 Gy in 20 fractions). Material/Methods: Ten prostate cancer patients treated with volumetric modulated arc therapy (VMAT) to 60 Gy in 20 fractions were retrospectively analyzed. For each patient, ten treatment fractions were randomly selected. Pre- and post-treatment cone-beam CT (CBCT) scans were acquired to quantify intrafraction displacement of the prostate, bladder, and rectum. Organ centroids were delineated on both image sets, and displacements were calculated in millimeters. The interval between
Dosimetric analysis revealed mean increases in rectal V60–V57 (20–80%) and bladder V48.6–V60 (35–45%) compared with baseline, with a significant correlation observed between Δ T and rectal dose parameters (r = 0.62, p = 0.049).
Conclusion: Intrafraction motion and prolonged treatment time were associated with measurable geometric and dosimetric deviations during prostate hypofractionated VMAT. These findings highlight the importance of minimizing beam-on and imaging duration to preserve target coverage and organ-at-risk sparing. The adoption of adaptive workflows or real- time tracking, when feasible, could substantially improve treatment precision and patient safety despite resource constraints. Keywords: Hypofractionated Prostate Radiotherapy, LMIC
Made with FlippingBook - Share PDF online