Room: Exhibit Hall | Forum 3
Purpose: To evaluate the performance of the EPID and customize the tolerance levels for the image-based automatic QA on Halcyon using a control chart.
Methods: Machine Performance Check (MPC) is an image-based automatic QA that verifies the geometric and beam performances of the system. Before any patient treatments each day, the automated checks based on the EPID imaging system have to pass a vendor-specified tolerance. For the stability check of the EPID imager, raw Flood Field data was analyzed over a period of 5 months (n=123) across the entire imager area. Dosimetric accuracy of the MPC-reported beam output was evaluated against Farmer chamber dose measurements at 10 cm depth (n=57). Using Shawhart and EWMA charts, we designed customized tolerances for the geometric checks (isocenter size, coincidence, positioning accuracy, collimator, gantry, MLC leaves and couch position) and dosimetric checks (beam output, uniformity). These customized tolerances for the daily MPC were then compared to the vendor-specified tolerances.
Results: Results were analyzed as a set of time-series data. Pixel response across the panel was stable but the standard deviation of raw response increased over time. Over the observational period, MPC-reported output agreed with the Farmer chamber measurements to within 0.45%. For on-going process control, we determined the customized tolerances for the MPC and compared these to the vendor-specified tolerances. Some of the key customized tolerances include: isocenter size = 0.70 Â±0.01mm (vendor-specified tolerance = Â± 0.9mm), beam output = 2.02 Â±0.33% (vendor-specified tolerance = Â± 4.0%), distal MLC position deviation = 0.02 Â±0.03mm (vendor-specified tolerance= Â± 0.6mm), and gantry absolute position deviation = -0.09 Â±0.1mm (vendor-specified tolerance = Â± 0.5mm).
Conclusion: The image-based automatic QA is a suitable replacement of the routine daily QA. It can be a reliable and efficient tool for checking the dosimetric and geometric performances of Halcyon.