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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. 

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.

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. 

This study highlights that small errors, some of which are well within machine tolerance specifications, can have significant dosimetric impact on spine SABR plans. Furthermore, it was shown that PSQA can miss some of these errors. The robustness of such plans to subtle errors and PSQA tolerances should be carefully considered.