ESTRO 2022 - Abstract Book

S787

Abstract book

ESTRO 2022

Figure 1: Distribution of dose-volume metrics for MR- and CT-based treatment plans. Dotted lines indicate institutional evaluation criteria. Asterisks indicate statistically significant changes in dose.

Figure 2: The relationship between dosimetric and PTV overlap differences for a) MR- and CT- based ML plans, and b) each treatment plan and its respective selected atlases. Conclusion The dosimetric differences observed between MR- and CT-based treatment plans are attributable to anatomical differences between the images, rather than altered ML behaviour resulting from changing the input imaging modality to MR. Therefore, we have demonstrated that an atlas-based ML model, previously trained on CT imaging only, can be safely repurposed for use in the MR guided setting.

PD-0895 Evaluating a method of treatment plan selection from daily plans on the MR-Linac

J. Stickley 1,2 , R. Chuter 1,3

1 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering, Manchester, United Kingdom; 2 Nottingham University Hospitals Trust, Radiotherapy Physics, Nottingham, United Kingdom; 3 University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom Purpose or Objective Adaptive radiotherapy using the MR-Linac has the potential to greatly improve the quality of patient treatments. Plans can be adapted based on a daily scan that account for changes in position using an adapt-to-position (ATP) workflow, or adapted to both anatomy and positioning using an adapt-to-shape (ATS) workflow. A full optimisation is currently used to give the best plan for the anatomy that day. However, the mean time taken for optimisation alone has been approximately 14 minutes. As a library of adapted plans is built up for each day that a patient is treated, it is possible that on a given day a suitable plan already exists. This work investigates a method for automatically selecting a plan from this dynamic library to improve the workflow efficiency. This could improve the patient experience, and reduce uncertainties due to anatomical change between scanning and delivering treatment. Materials and Methods Evaluation of using library plans based on image similarity has been carried out for 4 prostate and 2 cervix patients at the 10 th fraction. Image similarity of daily scans with the previous 9 treatment images was assessed using ADMIRE (Elekta). This performed a rigid registration and extracted similarity measures in a region of interest including the planning target volume (PTV) with an additional 3 cm margin. ATS plans from previous fractions (library plans) were copied on to the 10th fraction using an adapt segments method which shifts segments based on rigid registration between the two scans.