ESTRO 2022 - Abstract Book

S1307

Abstract book

ESTRO 2022

indicated that the differences in the central region are negligible, while small discrepancies are in the penumbra region (30%-70%). Changes in field sizes of up to 0.2 mm were detectable by RLI. Table 1: FWHM (mm) obtained from the RLI line profiles and compared with the GC films. Field Size (mm) FWHM (mm) Discrepancy RLI GC Film mm % X-direction Circle 10 10.039 10.108 -0.069 -0.68 Circle 40 41.206 41.269 -0.063 -0.15 Circle 60 61.88 61.82 0.06 0.097

Y-direction Circle 10

10.714 41.148 61.984

10.753 -0.039 41.259 -0.111 61.853 0.131

-0.36 -0.26 0.211

Circle 40 Circle 60

Two facing sides Iris 10

9.78

9.80

-0.02

-0.2

Iris 40 Iris 60

37.052 56.817

37.095 -0.043 56.866 -0.049

-0.11

-0.086

Conclusion The first application of a novel RLI approach for CyberKnife® dosimetry was presented and tested. Results are in agreement with GC film measurements. The sensitivity, sub-mm spatial resolution, simple setup, immediate availability of the data and full automation of the readout and processing, make this optical system as an useful and effective tool for robotic radiosurgery quality assurance. Key words: Radioluminescence imaging, CyberKnife, Quality assurance.

PO-1526 A LINACWatch sensitivity study compared with Octavius4D

V. Silvestri 1 , M. Guerrisi 1 , D. Genovesi 2 , D. Raspanti 3 , M.D. Falco 2

1 University of Rome Tor Vergata, Medical Physics, Rome, Italy; 2 University of Chieti, SS. Annunziata Hospital, Radiation Oncology, Chieti, Italy; 3 Tema Sinergie, Medical Physics, Faenza, Italy Purpose or Objective The recent introduction of the log files during patient specific QA requires a deep analysis process in order to integrate them into clinical QA workflow. We investigated the error sensitivity of log files analysis using LinacWatch ® (LW) and compared it with the one determined using a pre-treatment QA phantom (Octavius4D, Oc4D). Materials and Methods A total of 12 clinical plans have been considered: 3 prostate, 3 rectum, 3 head and neck (H&N) and 3 breast. In order to analyze error detection sensitivity, for each of them, new plans were created introducing a set of errors regarding MLC bank shift, jaws deviation, collimator and gantry rotation and monitor unit (MU) over dosage for a total of 180 plans. They were delivered on an Elekta Synergy linac. Fluence maps both for Oc4D and LW were simultaneously acquired. The comparison between error-free plan and plan with intended errors (both using Oc4D and LW) was performed with local γ - analysis. Personalized protocols were chosen: for Oc4D, 3%/3 mm for H&N and 2.5%/2.5 mm for the others; for LW, 2.5%/2.5 mm for H&N and 2%/2 mm for the others. Deviations of the collimator and gantry errors in LW were performed using the RMS evaluation. Results The total sensitivity was 32.1% for Oc4D and 36.9% for LW. For MLC errors, small deviation of ±0.25 mm produced a sensitivity of 8.3% for Oc4D vs 0% for LW. Both the systems were able to detect an MLC shift of ±1 mm with the same sensitivity (58.3%). Jaw deviations of ±0.5 mm had no effect on sensitivity for both systems. LW detected 70.9% and 87.5% collimator and gantry errors compared to 41.7% and 66.7% of Oc4D. The sensitivity of MU errors was 12.5% for Oc4D vs 37.5% for LW. If we lower the γ -criteria in LW for prostate and breast treatments to 1.5%/1.5 mm while maintaining the specificity always at 100%, the sensitivity in detecting small MLC deviations increased from 0% to 8.3% and from 8.3% to 25% for ±0.25 mm and ±0.5 mm errors, respectively. Conclusion Our result in the context of the patient specific quality assurance of all VMAT and IMRT plans, indicate that the log file analysis with LinacWatch can be considered as a complementary system to Octavius4D phantom, in particular to control each treatment session. Anyway, to increase LW sensitivity, we suggest using personalized protocols in the γ -analysis, depending on different treatment sites.