ESTRO 2024 - Abstract Book

S4367

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

ESTRO 2024

Ten primary prostate cancer patients treated previously with a regime of seven fractions (7 x 6.1 Gy) on the 1.5 T MR Linac (Elekta AB, Stockholm, Sweden) were retrospectively included. Several T2-weighted MRIs (voxel size: 0.83 x 0.83 x 2 mm3) were acquired for each fraction: pre-treatment for daily plan adaptation (pre-MRI), position verification scan (PV-MRI) shortly before treatment application and post-treatment (post-MRI). OAR and prostate CTV were automatically delineated on pre-MRI/PV-MRI and post-MRI using deep-learning auto-segmentation (ADMIRE Research, v.3.48, Elekta AB, Stockholm, Sweden). Since the pre- and post-treatment scans were allocated in the same coordinate space, no registration was needed. A voxel-based analysis was performed, calculating the voxel-wise displacements of the prostate contour on the post-MRI scan with respect to the pre-MRI/PV-MRI in superior-inferior (SI), anterior-posterior (AP) and left-right (LR) directions (Python v.3.8.5). Population-based motion histograms were created for SI, AP and LR directions and histogram percentiles (5%, 10%, 15% and 20%) were used as cut-offs to iteratively derive motion values for each direction. Anisotropic PTVs were then created by expanding the pre treatment CTV, using the determined motion cut-off values according to the respective histogram percentiles. Percentage CTV coverage was analyzed for the different derived PTVs with respect to post-treatment prostate CTV. Finally, a non-isotropic margin was determined, assuming a minimum CTV coverage of 95% in 90% of all treatment fractions.

Results:

The determined motion histograms for SI, AP and LR direction are displayed in Figure 1. Depending on the histogram cut-off used for anisotropic PTV generation, target coverage of the post-treatment prostate CTV by the PTV was found to be ≤ 95% in zero (PTV5%), four (PTV10%) and five (PTV15%) fractions of overall 70 fractions (Figure 2). PTV20% resulted in a target coverage ≤ 95% for eight fractions (11.42% of total fractions). Consequently, these results suggest that a margin of 2.5 mm, 1.7 mm and 4 mm in AP, LR and SI directions is required for online adaptive MRgRT in prostate cancer to account for residual IFM during the online procedure.