ESTRO 2025 - Abstract Book

S3224

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

ESTRO 2025

Results: The average prostate motion over all fractions was -0.02 ± 0.21 mm (LR), 0.3 ±0.5 mm (AP) and -0.3 ± 0.6 mm (SI). Maximum prostate motion ranged from -6.0 to 8.6 mm (LR), -12.8 to 5.9 mm (AP) and -15.6 to 33.8 mm (SI), with the largest displacement in the SI direction. In 76% of fractions, 3D motion displacement was < 1mm, resulting in dose difference < 6 cGy for CTV, bladder, and urethra dose metrics. Larger displacements were associated with increased dose differences for bladder and urethra metrics. Among 55 patients, 3 patients experienced Grade > 2 acute toxicities, and 2 patients reported Grade > 2 long-term toxicities. All patients with clinically significant toxicities had at least one fraction with an average 3D displacement greater than 1.4mm.

Conclusion: This study investigated the dosimetric impact of intrafractional motion in prostate cancer patients receiving MR guided adaptive radiotherapy. Intrafractional motion of <1 mm had minimal dosimetric impact on the planned dose distributions. However, larger displacements may result in dose difference and could lead to clinically significant urinary toxicity.

Keywords: Unity, motion, prostate