ESTRO 2025 - Abstract Book

S3282

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

ESTRO 2025

intrafraction bowel motion. We hereby present the maximum positional difference and drift within the bowel bag, which together with the associated displacement vector fields may find applications in margin calculation and dose accumulation. Material/Methods: MRI scans (bTFE, 1.8 s/dynamic, FOV 250x328x74 mm, voxel 0.98x1.13x0.98 mm) were acquired for patients (n = 20, 7.5 minutes scan time, 245 frames) and volunteers (n = 10, 15 minutes scan time, 495 frames). Motion estimated was performed via an optical flow algorithm 1 and subsequently quantified by calculating two metrics: 1. The largest positional difference per voxel over the full scan duration, achieved by finding for the three orthogonal direction separately the voxel wise maximum peak-to-peak (2.5%-97.5%) difference. A Motion Volume Histogram (MVH) was created from this metric, from which the M90%, M50% and M10% were determined 2 . 2. The drift of the mean motion within the bowel bag, determined by smoothing the mean motion signal plotted against the time. Results: The MVHs of the 95% motion range for each subject can be observed in Figure 1. The main motion direction is in craniocaudal. The lateral and anteroposterior have similar values.

The drift in mean positions within the bowel bag, see Figure 2, mainly takes place in craniocaudal direction, where most long-term (15 minutes) drift end more cranially. However, in some cases mean drifts in the other directions were observed.