Gantry angle dependent beam control optimization of a traveling wave linear accelerator to improve VMAT delivery

Increased modulation and dynamical delivery of external beam radiotherapy (EBRT), such as volumetric modulated arc therapy (VMAT) with dynamic gantry rotation, continuously variable dose rate (CVDR) and field shapes that change during the beam, place greater demands on the performance of linear accelerators (linac). In this study, the accuracy of the linac beam steering is improved by the application of a new method to determine the gantry-dependent lookup table.

An improved method of lookup table creation based on service graphing information from the linac is investigated. This minimizes the impact of magnetic hysteresis due to the previous current in the steering magnets, which is dependent on the previous gantry angle. A software tool, programmed with MATLAB®, is used to calculate and export the new optimal lookup table (LUT).

This method is efficient requiring little clinical machine time or analysis time, and leads to an improved VMAT delivery with a reduction of about 60 percent in beam steering errors. If the surrounding magnetic field is changed, for example, ramping a nearby magnetic resonance imaging system (MRI), the beam steering LUT optimization can be quickly performed.

This study shows an improved linac stability using improved lookup tables. Resulting in a lower number of interruptions, preventing down-time, and a lower risk of intrafraction motion due to longer treatment times.