ESTRO 2023 - Abstract Book

S506

Sunday 14 May 2023

ESTRO 2023

Results An overview of the input, output, timing and general objective for a selection of metrics is given in Table 1. As all considered metrics have disadvantages and certain insensitivities, a combination using different inputs and/or assumptions should be used. Evaluations should often include alignment metrics (e.g. image similarity, contour-overlap, target registration error) and deformation vector field (DVF) consistency/plausibility metrics (e.g. inverse consistency, Jacobian determinant). For validation, we recommend the target registration error of a set of manually annotated anatomical landmarks and the distance-to-agreement of manually delineated contours. These should be supplemented by plausibility metric(s) to reduce their disadvantage of being sensitive only locally in high-contrast regions. Digital phantoms are useful for validation (especially of DIR for dose accumulation) but are currently not available for a wide range of anatomies, image modalities and types of deformations. For patient-specific verification of DIR for contour propagation, we recommend at least visual inspection of the registered image and contour. For patient-specific verification of DIR for dose warping and accumulation, we recommend at least visual inspection of the DVF and/or an image similarity map, together with the Jacobian determinant and the distance-to- dose-difference or dose (gradient) maps. These recommendations also hold when warping quantitative information like Hounsfield units or PET data. General objectives for these metrics are also given in Table 1. We acknowledge that some of

these metrics are still missing in some commercial softwares. Table 1. Overview of some metrics and their characteristics

Conclusion We provide reasoning and recommendations for using a set of tools and metrics for specific situations of validation and/or verification of DIR for radiotherapy and acknowledge the need for a broader incorporation of these metrics in commercial software and for more complex and specific (digital) phantoms.

Proffered Papers: Photon treatment planning

OC-0619 Robustness of spine SBRT to sub tolerance MLC errors – a large scale evaluation M. Hussein 1 , N. Hardcastle 2 , J. Lehmann 3 , C. Clark 4,1

1 National Physical Laboratory, Metrology for Medical Physics, Teddington, United Kingdom; 2 Peter MacCallum Cancer Centre, Physical Sciences, Melbourne, Australia; 3 Calvary Mater Newcastle, Radiation Oncology, Newcastle, Australia; 4 University College London Hospitals, Medical Physics, London, United Kingdom Purpose or Objective The narrow trade-off between delivering a highly localised dose while sparing critical structures presents a challenge in the planning and delivery of spine SBRT. Consequently, small, sub-tolerance MLC delivery errors may cause unseen large dose deviations. The aim of this study was to perform a large-scale systematic evaluation of the sensitivity of spine SBRT plans to subtle, within-machine tolerance, MLC errors and to better understand the robustness of these plans to such plans using a multi-centre plan challenge dataset. Materials and Methods Data for 65 Varian Eclipse TPS users from an international spine SBRT plan challenge was used. All plans were created on the same CT and RT structure set, with the same target and OAR constraints, using each centre’s respective planning technique on Varian linacs (Clinac and TrueBeam). 33 had the Millennium MLC (MMLC, minimum 5mm leaf width) and 32 High-Definition (HDMLC, min 2.5mm leaf width). The DICOM plan file for each centre was extracted and MATLAB code was used to introduce errors where both MLC banks were retracted or contracted by 0.1, 0.2, 0.3, 0.4 and 0.5mm, respectively,