New gantry angle-dependent beam control optimization with Elekta linear accelerator for VMAT delivery.

Introduction: This paper describes a method to improve gantry-dependent beam steering for Elekta traveling wave linear accelerators by applying the measured and filtered beam servo corrections to the existing lookup table (LUT). Beam steering has a direct influence on the treatment accuracy by affecting the beam symmetry and position. The presented method provides an improved LUT with respect to the default Elekta method to reduce treatment delivery interruptions. These interruptions are known to contribute to unwanted intrafraction motion and longer treatment times.

Methods: Compared to the default method of the manufacturer, this new method takes both clockwise and counterclockwise rotation to compensate for magnetic hysteresis as well as previous configuration and noise filtering into account. The improved method to determine the lookup table uses service graphing information from the linac without the need for additional symmetry information. The clinical configuration of the flattened beam energies remains untouched during the data record.

Results: This method results in a configuration where the gantry-dependent steering is optimized over the full arc with optimal balance in the hysteresis and minimizing the effect of errors in the steering values. This method is a less error-prone process compared to the methodology described in previous research, still achieving a reduction of interruption of about 60 percent compared to the Elekta method.

Conclusion: This study shows a simplified way to optimize linac stability with improved LUT. The optimized LUT results in a lower number of interruptions, preventing downtime, and a lower risk of intrafraction motion due to longer treatment time.