SU-E-T-342: Dynamic Accuracy and Stability of Novel Multi-Leaf Collimator (MLC) for Volumetric Modulated Arc Therapy (VMAT)




We have investigated the dynamic accuracy and long-term stability of an Agility (Elekta, Stockholm, Sweden) multi-leaf collimator (MLC) for volumetric modulated arc therapy (VMAT).


The MLC leaf position accuracy during VMAT delivery was evaluated with the use of three different quality assurance tests for several months: (1) a leaf gap-width test between opposing leaves by measurement of the isocenter dose during constant-gap sliding-window delivery with varied dose rates, MLC leaf speeds, and gantry angles; (2) a leaf position test by picket-fence delivery with and without gantry rotation; and (3) the complicated VMAT test plans were evaluated by measurement using gamma analysis with criteria of 3 mm distance-to-agreement (DTA) and 3% percent dose difference (DD).


The leaf gap-width deviation once a week for five months was within 0.15 mm under all the test fractions. There were not clear differences of the accuracy depending on the dose rates, the leaf speeds, and gantry angles. Agreement in the static vs. rotation picket fence test was better than 0.5 mm. This difference is near to the limit of the measurement and is not the difference between static and rotation. The pass rates of each complicated VAMT test plans once a day for a month were more than 90% in the gamma analysis. These pass rates are accuracy enough for VMAT because the test plans included strict test plans.


MLC leaf position accuracy and long-term stability were confirmed by the proposed procedures. The Agility MLC showed enough MLC leaf position accuracy and long-term stability for VMAT.