ESTRO 2024 - Abstract Book

S4866

Physics - Quality assurance and auditing

ESTRO 2024

the same instructions as mentioned previously. Three sets of gamma analyses were then performed between plan measurement and TPS calculation (3%/2mm, 3%/2mm with a ‘calc shift’ of ≤ 1mm allowed and 3%/2mm with a ‘calc shift’ of ≤ 1mm allowed and the TPS DICOM data scaled for the linac output on the day). If the gamma passing rate, using a gamma criterion of 3%/2mm and allowing for a ‘calc shift’ of ≤ 1mm and output scaling of the TPS DICOM data, fell within the control limits for each of the 6 robust plans then the linac could be onboarded to the distributed SABR cohort.

Results:

Of the process-based control limits calculated for the 12 robust plans (6 VersaHD/Monaco® & 6 TrueBeam®/Eclipse TM ), 5 were tighter than the universal tolerance limits proposed by AAPM TG 218 (gamma passing rate ≥ 95% using a gamma criterion of 3%/2mm and 10% dose threshold), 5 were within 1% of the universal tolerance limit and the other 2 were for ‘error plans’. Figure 1 below shows the range of gamma passing rates per robust plan for the 30 onboarded Elekta linacs. Figure 2 below shows the range of gamma passing rates per robust plan for the 10 onboarded Varian linacs. Across both platforms, only 1 measured gamma passing rate out of 200 fell outside of the universal tolerance limit proposed by AAPM TG 218. This same measurement is the only one that fell outside of the process-based control limits and was found to pass after a repeat measurement with a different detector with higher resolution. This does not include the gamma passing rates of the ‘error plans’.