Evaluation of MLC QA Software to Determine MLC Accuracy From EPID Images
R Rice1*, G Kim1, M Whitaker3, T Pawlicki1, (1) UCSD Medical Center, La Jolla, CA, (3) Radiological Imaging Technology, Inc., Colorado Springs, ColoradoSU-E-T-146 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To determine the clinical usefulness of commercially available software tools to evaluate MLC accuracy for setting appropriate benchmark levels for mean and maximum leaf deviation errors and FWHM values.
Methods: The RIT Picket Fence and Varian Rapidarc Test 1.1 (from the Rapidarc Acceptance Test) analysis routines were used to analyze EPID images delivered on Trilogy and TrueBeam accelerators over a 2 month period. Profiles are automatically detected across the picket fence pattern at each leaf pair. The leaf pair position is the mid-point between the two half-max positions. The FWHM is the distance between the two half-max positions. Maximum, mean and standard deviation statistics are calculated for the 10 openings for each leaf pair in the picket fence pattern. Process control limits (UCL, LCL) were calculated for the mean leaf position deviation and FWHM for both linacs.
Results: For the RIT Picket Fence test the mean leaf position deviation was 0.017 mm with a standard deviation of 0.005mm with UCL=0.030 mm and LCL=0.003 mm (UCL-LCL=0.027 mm) for the TrueBeam accelerator and 0.013 mm with a standard deviation of 0.021 mm for the Trilogy accelerator with UCL=0.0423 mm and LCL=-0.069 mm (UCL-LCL=0.122 mm). For the Rapidarc Test the mean leaf position deviation was -0.197 mm with a standard deviation of 0.020 mm for the TrueBeam and 0.102 mm with a standard deviation of 0.026 mm for the Trilogy. The mean of the measured maximum leaf position deviations was <0.2mm for both accelerators for the RIT Picket Fence test and <0.5 mm for the Rapidarc test.
Conclusion: This work indicates the potential for quantifying MLC performance with simple test patterns delivered to an EPID and analyzed. Appropriate data analysis and assignment of control limits quantitatively highlights differences between linacs and allows a prospective intervention by physicists or service engineers.