TU‐EE‐A1‐03: Multileaf Collimator Performance and Quality Assurance For Volume Modulated Arc Therapy

Abstract

Purpose: The purpose of this study was to determine the effects of Multi Leaf Collimator (MLC) leaf velocity on leaf positional accuracy during modulated arc delivery. Method and Materials: MLC performance was measured for intensity modulated arcs delivered on a Varian21EX equipped with a 120‐leaf dynamic MLC. The performance of the MLC was tested using arc‐based delivery sequences developed by the investigators for routine MLC leaf velocity testing. MLC performance was also tested using actual VMAT delivery sequences created with a commercially available inverse planning system. Errors in the MLC leaf positions were measured using the MLC controller, which recorded positions of each leaf every 50 milliseconds. To characterize the performance of MLC leaves we introduce the notion of MLC leaf performance curve, which is defined to be a dependency between the MLC leaf velocity and the expected leaf position error. Results: At leaf velocities below 2 cm/sec, the MLC leaves were within 2‐mm of the planned position during 98% of the test sequences. The performance of the MLC leaves did not significantly vary with the MLC moving into or out of the carriage. However, there was a significant difference in the performance between the 5‐mm and 1‐cm MLC leaves. Large leaf velocities translate into large magnitudes of MLC leaf positional errors. The typical shape of the leaf performance curve is linear. Summary: The maximum allowable leaf speed for VMAT treatment delivery will depend on the leaf sequencing. If the intensity is modulated with small MLC leaf separations, then the leaf velocity will need to be limited to 1 cm/sec. Leaf position errors of greater than 1‐mm can have a detrimental impact on the delivered dose distributions for small leaf pair separations. Analyzing the leaf performance curves estimated for each leaf one can easily reveal poorly performing MLC leaves.

Analysis of RapidArc optimization strategies using objective function values and dose‐volume histograms

“…Furthermore, it has been shown that there is a relationship between the leaf speed and the inaccuracy of the leaf positions for arc therapy such that at higher leaf speeds, the magnitude of positional errors increases. ( 14 , 15 )…”

Evaluation of optimization strategies and the effect of initial conditions on IMAT optimization using a leaf position optimization algorithm

“…An additional limitation is that the dose calculation in this study was only performed at fixed gantry samples and not at the intermediate position between the samples, and if the dose calculation were performed for the intermediate positions, then we would observe increased dose to the OAR. This increased dose to the OAR could be avoided by turning the dose rate to zero when the leaves sweep across the OAR; however, this would lead to a plan with large fluctuations in both dose rate and leaf positions which would lead to both delivery inefficiency and possible delivery error (Ramsey et al 2001(Ramsey et al , 2008.…”