Journal of Medical Physics and Applied Sciences

Quality Assurance of Dynamic Multi-Leaf Collimators (MLC)

The purpose of this study was to provide a list of critical performance tests used in establishing and maintaining a safe and effective QAP of Dynamic Multi Leaf Collimators (DMLC). Positional accuracy of MLC was assessed by using EPID based Picket Fence test. Pylinac v3.0 and Dynalog File Viewer (DFV) were used for evaluation of leaf position error. To assess accurate control of leaf speed ImageJ software was used. PTW30013 Farmer type chamber with sensitive volume of 0.6 cm3 was used in water phantom fixed at a depth of 10 cm for 6 MV and 15 MV photon energies to measure leaf transmission. The DLG values were measured using DICOM QA plan files applying the sweeping gap technique. The average and maximum MLC positional error was below AAPM TG-142 tolerance limit (1 mm). The range of values of ROI deviation (0.13-1.4%) and the average of the absolute value of all the ROI deviations (0.7%) for the four dose strips were well within the tolerance values indicated by references and varian provided recommendations for the rapid arc QA. The result of leaf transmission factor from the current study for 6 MV and 15 MV are within the limit of AAPM TG-50 report. The DLGs obtained were 1.42 mm and 1.59 mm at 6 MV and 15 MV respectively, which are comparable with similar studies. EPPID based PF analysis using Pylinac and Dynalog File Viewer are fine and comparable to analyze the MLC positional errors as required by the AAPM TG-142 report for MLC QA and a tighter action criterion of ± 0.5 mm for MLC position error for RapidArc is proposed. MLC speed accuracy could be evaluated using ImageJ and manufacturer provided DICOM QA plan. These concepts and findings have broad implications for VMAT treatments with DMLC and the procedures are applicable to other MLC designs as well.

Author(s): Alemayehu Debbo Telila*

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