ESTRO 2022 - Abstract Book

S666

Abstract book

ESTRO 2022

OC-0753 Robustness of spine SABR plans to delivery errors within machine tolerance: multicentre analysis

M. Hussein 1 , J. Lehmann 2 , C. Clark 1,3

1 National Physical Laboratory, Metrology for Medical Physics Centre, Teddington, United Kingdom; 2 Calvary Mater Newcastle, Radiation Oncology, Newcastle, Australia; 3 University College London Hospital, Medical Physics, London, United Kingdom Purpose or Objective Spine SABR is characterised by delivery of a highly localised dose to the PTV with sharp fall-off to spare surrounding tissues, with a narrow trade-off between target coverage and sparing. Many centres use conventional linacs run to machine tolerances specified for standard treatments. The aim of this study was to investigate the robustness of spine SABR treatment plans to simulated delivery errors that are within machine tolerance in a multicentre setting. Materials and Methods Fourteen centres participated in the study. Each created a spine SABR plan following provided guidelines and sent it to the study investigators. Custom Matlab software was used to introduce eight different treatment delivery errors into individual copies of the DICOM plan file, which were within machine tolerance. The same errors were introduced for each centre. Plans were then returned to the centres for dose calculation. Errors included changes in collimator rotation, position of whole leaf banks of the MLC, and combinations of these together with changes in machine dose output. Description of the errors shown in Figure 1. The planning systems were Eclipse, Pinnacle, Raystation, iPlan, Monaco and Oncentra MasterPlan. Linacs were Elekta Synergy, Elekta Versa HD, Varian Clinac and Varian Truebeam. Centre calculated dose distributions were resubmitted from which the near maximum spinal cord dose (D 0.03cc,SC ) and dose to 90% of PTV (D 90,PTV ) were calculated centrally. To assess individual impact of the error plans, the changes in D 0.03cc,SC and D 90,PTV in the plan with the error versus in the original plan were evaluated. Centres performed patient-specific QA (PSQA) of each plan and reported a pass or fail according to their local procedures. Results Figure 1 shows example axial plots of differences in dose between the plans with errors and the original plan. Four of the error plans caused a dose increase relative to the original plan, while the remaining plans caused a dose reduction. Figure 2 shows the increase in D 0.03cc,SC and decrease D 90,PTV for the respective error plans. Plans that passed PSQA are marked by an asterisk over each respective bar. A 1-degree collimator error in either direction did not cause any significant change in any centre’s dose distribution. A 0.5mm retraction of both MLC banks caused a >5% different D 0.03cc,SC in 12 out of 14 centres. In 7 cases where the error caused a >5% difference in D 0.03cc,SC and in 4 cases where the error caused >5% reduction in D 90,PTV , the plans passed PSQA.