ESTRO 2020 Abstract book

S818 ESTRO 2020

For Cyberknife plans higher coverage came at the cost of higher PDS. This was not evident in the Brainlab plans due to the automated nature of the planning system, giving good coverage and conformity from the outset. Brainlab spine SRS performed particularly well at conforming the dose to a target that wrapped all the way around the cord, see figure 2.

Conclusion Our methods of simulating the uncertainty of dose distributions enable a more accurate analysis of the treatment plan robustness than the method currently implemented in Varian TPS. Future works: (1) To increase the automation and efficiency of our procedure, more works on automated scripting of the Velocity and Eclipse is needed. (2) Current version of our procedure is limited to the CTV, which can only be moved. Taking the potential changes of CTV's shape in our procedure needs the forecasting of models (AI, RADIOMICS), which will be the second step of our project. PO-1443 Brainlab spine SRS planning and verification, comparison with CyberKnife C. Jones 1 , E. Wells 1 , C. Meehan 1 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Medical Physics, London, United Kingdom Purpose or Objective Spine SRS provides local disease control and pain relief by delivering high doses with steep dose gradients to spare the spinal cord. Brainlab have developed a new VMAT optimisation algorithm specifically for spine SRS treatments which splits the target into segments and optimises each segment with a duplicate arc. This work compares Brainlab plans to CyberKnife plans, our current standard. Material and Methods Brainlab Spine SRS was commissioned with data collected on a TrueBeam with Millenium 120 MLC. 14 patients were re-planned retrospectively using Brainlab Spine SRS 1.5 and Precision 2.0 following recommendations by the UK SABR Consortium 1 . PTVs were planned to 27Gy with a 2mm CTV to PTV margin and 2mm spinal cord PRV margin. Both planning systems used a 1mm dose grid and a pencil beam dose calculation algorithm; all OAR tolerances were met. Plans were optimised to maximise coverage whilst maintaining OAR constraints. Brainlab plans were verified by measuring a point dose in a solid water phantom. A sagittal or coronal film showing the dose gradient through the cord was obtained for 8 of the plans. Results Mean PTV coverage for Brainlab plans (89.2%±2.2%) was significantly higher (p=0.003) than mean PTV coverage of CK plans (84.4%±4.2%). Lee et al 2 recommend two metrics for evaluating plans where PTV coverage is compromised; prescription isodose spillage (PDS) and modified gradient index (mGI) which are normalised to the volume of the target covered by the prescription dose. The Brainlab plans score significantly better for the mGI, see figure 1.

The Brainlab verification plans were delivered to a solid water phantom with mean deviation from expected dose of -0.7%±1.1. Film verification demonstrated good agreement with calculated dose with all 8 films having >95% gamma pass at 3%/2mm. Conclusion Brainlab Spine SRS creates highly conformal plans with excellent PTV coverage whilst maintaining OAR tolerances and conformity indices. The plans are deliverable with dose to a homogeneous water equivalent phantom agreeing well with the calculated dose. SABR treatments must be image guided with online correction protocols. The 2mm PTV margin used clinically with CyberKnife assumes intrafraction monitoring and correction during treatment (25-50mins beam-on time). Brainlab Spine SRS is used with the ExacTrac delivery system which uses kV- kV imaging for patient setup and allows intrafraction monitoring of the patient during treatment (3-11min beam-on time). 1 UK SABR Consortium Guidelines 2019 2 Lee J et al., “Multi-Center Evaluation of Dose Conformity in Stereotactic Body Radiotherapy,” Physics and Imaging in Radiation Oncology 11 (July 2019): 41–46 PO-1444 Robust optimization with accumulated dose over full treatment course reduces dose to organs at risk E. Engwall 1 , A. Fredriksson 1 , B. Andersson 1 1 RaySearch Laboratories, Research and Development, Stockholm, Sweden Purpose or Objective Current methods for robust optimization available in commercial treatment planning systems are limited to account for errors that are the same through all fractions of the treatment. In this study we propose a new robust optimization approach, where each scenario corresponds to the full treatment course with different errors in each fraction. Material and Methods The novel method is implemented in a research version of the treatment planning system RayStation. It utilizes a large number of CTs created from the planning CT through simulated organ motion. The organ motion is simulated