An oscillating sweeping gap test for VMAT quality assurance

Abstract

The objective of this study was to develop an oscillating sweeping gap test for volumetric modulated arc therapy (VMAT) quality assurance (QA). A novel test was designed and used to simultaneously determine uncertainties associated with linac performance, dose calculation and dosimetric MLC parameters during VMAT delivery. Delivered doses were measured with Matrixx, ionization chamber A12 and EDR2 films, and compared to calculations from the treatment planning system (TPS) Eclipse. A new gantry and MLC motion pattern, called here 'oscillating sweeping gap', is developed as an extension of the standard sweeping gap MLC pattern developed for IMRT QA. Specifically, in the oscillating sweeping gap test, a uniform MLC gap is moving repeatedly back and forth across the field at a constant speed during a full rotation of the gantry. The dose distribution generated by the combined gantry and MLC motion pattern is designed to be quasi-uniform within a cylindrical target volume with a sharp penumbra. The test design allows for an easy detection of dose errors as deviations from the uniform background. MLC gap sizes, gantry and MLC speeds and monitor units (MU) are selected according to a formula determining the magnitude of dose delivered to the target. Both measured and calculated dose distributions were analyzed as a function of the number of control points in the TPS, MLC gap size and magnitude of the gantry angle error. Dose calculation errors due to the insufficient number of control points in the gantry and MLC motion pattern appear as streak artifacts. The magnitude of these artifacts is increasing with the decreased number of control points, and with the decreased MLC gap size. The spatial distribution of dose errors due to the gantry angle errors (unsteady rocking motion) appears as high-frequency noise for higher wobble frequencies and as large hot/cold spots for lower wobble frequencies. The actual MLC leaf position as a function of time (or the gantry angle), determined from the Matrixx snaps (dose images measured per time interval) of the moving gap and compared to the ideal leaf positions, reveal discrepancies in agreement with theoretical calculations. The MLC parameters determined for VMAT with the oscillating sweeping gap test, their uncertainties and the associated dose errors are similar to those determined for IMRT with the standard sweeping gap test. The oscillating sweeping gap test has been developed for the gantry and MLC QA. Applications include commissioning of the planning system for VMAT and performing routine linac QA. The test is sensitive to several errors in dose calculation and delivery.