A Modelling Study of Elekta VMAT dMLC Tracking For Realistic Respiratory Motion

Abstract

A VMAT model for calculation of static-tumour and dMLC tracking motion-compensation delivery was applied to five VMAT treatment plans, for three respiratory motion trajectories generated from patient fluoroscopic data acquired during lung radiotherapy treatment. The delivery time was calculated for static-tumour, 1D dMLC tracking and 2D dMLC tracking delivery with the MLCi multileaf collimator and the Elekta Integrity linear accelerator control system. The delivery time was found to increase with increasing amplitude of motion, and the largest increase in delivery time was observed when 2D tracking was employed. The benefit of the dMLC tracking technique was evaluated with fluence reconstructions of the VMAT treatment arc within a digital phantom programmed to move rigidly with the chosen motion trajectories. The accuracy of 1D dMLC tracking compared with 2D dMLC tracking was evaluated and it was found that although 2D dMLC tracking provides the most accurate reconstruction of the desired fluence, this is at the expense of a substantial increase in treatment time. The study has shown that the majority of the errors due to intrafraction respiratory motion can be corrected for in a shorter treatment time with the use of 1D dMLC tracking only, with the MLC direction of motion aligned with the axis of motion of largest amplitude.