ESTRO 2023 - Abstract Book

S213

Saturday 13 May

ESTRO 2023

were 0.97, 0.93 and 0.98. Our goal was the detection of fractions with reduced quality, so specificity was the priority. The three models predicted cases of reduced plan quality, though accurately predicting fractions where the 2% threshold in DVHp was exceeded was difficult (see fig. 1), likely due to the large amount of variability in patient motion. The dual model could predict reduced quality fractions earlier during treatment than the others (see fig. 2). This is desirable as it gives more scope for correction.

Conclusion All models can detect which fractions are likely to have reduced quality due to motion-induced changes in dose. The dose model is preferable to the position model as it is more specific. The model utilising both dose and position data is best as it is most specific and it makes predictions earlier. 1) Goodwin, E., Real time motion-including dose reconstruction on the MR-Linac. ESTRO 2021 OC-0279 Validation of intra-fraction adaptations for the MR-linac: novel drift correction and replanning M. van den Dobbelsteen 1 , S. Hackett 1 , H. de Boer 1 , B. van Asselen 1 , S. Oolbekkink 1 , B. Raaymakers 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective A novel motion management functionality, enabling tumor tracking in real-time, will be available for the Unity MR-linac in 2023. If the tumor location shifts above a limit, the plan can be adapted using a baseline shift correction, called an intra- fraction drift correction. This development is promising for intra-fraction adaptations. The goal of this research is to show in silico dose outcomes of ten prostate patients for the novel intra-fraction drift correction compared to an existing clinically applied intra-fraction adaptation using in-house software, called intra-fraction replanning. Materials and Methods In our department an intra-fraction adaptation is performed for all prostate patients treated on the MR-linac, whereby a replanning adaptation occurs mid-treatment, enabling margin reduction. During delivery, a 3D MR-scan is acquired for replanning, using so-called adapt-to-position to translate all patient structures with adapt segments and weights. This process takes five minutes, and can be executed during delivery of the first half of the treatment. The intra-fraction drift correction developed by Elekta (Stockholm, SE) enables tumor tracking in real-time using two orthogonal 2D cine images and replanning if the tumor location shifts above a pre-defined limit. The plan is calculated without optimization based on