ESTRO 2025 - Abstract Book

S3288

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

ESTRO 2025

minimum, maximum and average doses received by them are studied by means of another MRI acquired immediately after the end of the treatment, which is closer to the final doses received.

Material/Methods: Five prostate patients were studied, prescribed to 36.25 Gy in 5 sessions. Treatment planning was calculated for a 7 MV FFF Elekta Unity MR-linac with Monaco treatment planning system. For each session, once the dose was delivered, a final MRI was adquired. CTV, PTV, bladder and rectum were recountoured by two radiation oncologist, and exactly the original session plan was recalculated in the new MRI. Differences in volumes, minimum, maximum and average doses were examined. Results: The largest mean volume differences were observed for the bladder, with an increase of 104.98% between the initial and final MRI. The volume of the rectum on average barely changed (-0.51%). CTV and PTV volumes however decreased by 12.45 % and 11.81 % respectively.

The largest dose changes were in the mean dose in bladder (-20.91%) and rectum (12.13%).The maximum and mean CTV/PTV doses changed by less than 1.5% with the largest variation observed in the minimum doses, -8.16% and - 15.54% for CTV and PTV respectively. Conclusion: Adaptive treatments with MR-linacs represent a great advance in radiotherapy, however, it is necessary to take into account anatomical variations during their administration and to try to reduce the times as much as possible to avoid inaccurate doses.

Keywords: MR-linac, adaptive, prostate

References: Winkel, Dennis, et al. "Adaptive radiotherapy: the Elekta Unity MR-linac concept." Clinical and translational radiation oncology 18 (2019): 54-59. de Muinck Keizer, D. M., et al. "Prostate intrafraction motion during the preparation and delivery of MR-guided radiotherapy sessions on a 1.5 T MR-Linac." Radiotherapy and Oncology 151 (2020): 88-94.