Identification of a potential source of error for 6FFF beams delivered on an Agility(TM) multileaf collimator

PURPOSE: The performance of the Agility(TM) multileaf collimator was investigated with a focus on dynamic, small fields for flattening filter free (FFF) beams. METHODS: In this study we have developed a simple tool to test the robustness of the control mechanisms during dynamic beam delivery for Elekta’s VersaHD linear accelerator with Integrity 4.0.4 control software. We have programed the planning system to calculate dose for delivery of sweeping gaps. These sweeping gaps have a constant speed, constant size, and are delivered at a constant dose rate. Therefore they specifically identify delivery problems in dynamic mode. RESULTS: The Elekta Agility(TM) control mechanism fails to maintain accurate delivery for small, dynamic sweeping gaps. For small gap sizes, the Agility(TM) control mechanism delivers a field that is more than four times the size of the planned field width without generating an interlock. This has dosimetric implications: The discrepancy between calculated and measured doses increases with decreasing gap size and exceeds 10% and 60% at isocenter for a 3.5 mm and 1 mm gap size, respectively. CONCLUSION: A deficiency of the Agility(TM) control system was identified in this study. This deficiency is a potential source of error for volumetric modulated arc therapy fields and could therefore contribute to relatively high failure rates in quality assurance measurements, especially for FFF beams.