ESTRO 2025 - Abstract Book

S3275

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

ESTRO 2025

margins to 2 mm. The margin calculations took into account the residue errors due to intrafraction rotations and MRI geometric fidelity. Using the derived action levels we applied CMM with reduced planning margins and evaluated the performance in the first 25 patients treated. Results: The prostate, rectum and bladder were clearly visible on the cine-MR and could be tracked with high accuracy 1 . Simulations predicted that an isotropic action level of 2.5 mm for the drift correction (based on the running average over the last minute of data as provided by CMM) and a 3 mm isotropic gating level achieved 2 mm margins with an average number of corrections per fraction of 0.8 (range 0-6). For an example with 2 corrections, see Fig. 1. This result was insensitive to the gating level. Because the drift correction takes 1 minute, the overall average treatment time was hardly affected. When applying these settings clinically, the results closely followed simulations: the number of corrections per fraction ranged from 0 to 5, with an average of 0.9. The high image quality allowed for easy visual inspection of the tracking process.

Conclusion: We optimized and clinically introduced a CMM procedure to achieve margins of 2 mm in prostate SBRT on a 1.5T MR linac. The procedure has become our clinical standard, with 54 patients treated to date.

Keywords: Intrafraction, prostate, MR-linac

References: 1 L.T. Meijers et al., Validation of T2-TSE-cine images for an intrafraction drift correction workflow for pelvis treatments at the Unity MR-Linac, submitted Rad Onc