ESTRO 2025 - Abstract Book

S3280

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

ESTRO 2025

To gauge potential impact and motivate the development of software tools to enable plan recycling, we retrospectively assessed a strategy whereby all pre-existing plans were considered treatment candidates for subsequent fractions in a cohort of 9 patients who had ultrahypofractionated RT to their prostate on the MR- linac. Material/Methods: Each structure set from the daily planning MRI was propagated backwards onto the reference and daily MRIs from previous fractions to calculate the dose to each structure that would have been received had the corresponding plan been delivered. The resulting dose statistics were assessed against: (A) standard target objectives and organ at-risk constraints, (B) population-based estimates of current practice, and (C) exceptional circumstances where failures/violations in the online plan would have been improved or matched by a pre-existing plan. We counted plan acceptability assuming a workflow whereby online plans generated at fractions where a pre-existing plan was acceptable were not considered candidates for future fractions. Results: The median ± interquartile range percentage of fractions with at least one acceptable pre-existing plan was 25%±20%, 60%±35%, and 60%±20% for criteria A, B, and C respectively. The rate of pre-existing plan acceptability varied from patient to patient: there were no acceptable plans for all 5 fractions for P3, while acceptable doses would have been delivered in 60% - 80% of fractions for other patients (e.g. P7)(Figure 1).

The CT reference plan alone was only acceptable in 0%, 0%, and 20% of fractions according to criteria A, B, and C, respectively. The likelihood of pre-existing plan acceptability increased over the course of treatment (Figure 2).