Purpose: To evaluate exact MLC positional error (MLCPE) during volumetric modulated arc therapy (VMAT) delivery by planned with a time-dependent MLC velocity in an exact mathematically derivable trajectory of a straight line and conic sections
Methods: VMAT delivery is planned in such a way that MLC’s are moving in a locus can be defined exactly by a mathematical function like linear, parabolic and circular velocity. The VMAT delivery was interrupted single or multiple times and the projection images were acquired in portal imager (EPID). MLCPE was analysed as a function of dependent and independent variables of Dose rate (DR), and MLC Speed (SP) and interruption.
Results: MLCPE was analysed as a function of dependent and independent variables of Dose rate (DR), and MLC Speed (SP) and interruption. The maximum individual error obtained (DR of 35mu/min, SP of 2.0 cm/sec) was 1.96±0.1 mm. Only 4.4% of MLCs showed = ±1mm positional error. If multiple interruptions are involved in a VMAT delivery, then the interruption overwhelmed the influence of DR and SP in MLCPE. An independent and dependent sub group analysis reveals mean MLCPE were 0.18±0.4 mm 0.19±0.42 mm respectively.
Conclusion: Determination of exact MLC positional error using a mathematical function without approximation indicate MLCPE is not a function of MLC speed. Number of times MLC positional error exceed tolerance (±1mm) limit is less than 5% of the observations. The MLC positional error are significantly less in the modern machines due to advancement in the delivery mechanism
Not Applicable / None Entered.