Abstract
For accurate delivery of volumetric‐modulated arc therapy (VMAT), the gantry position should be synchronized with the multileaf collimator (MLC) leaf positions and the dose rate. This study, therefore, aims to implement quality control (QC) of VMAT synchronization, with as few arcs as possible and with minimal data handling time, using portal imaging. A steel bar of diameter 12 mm is accurately positioned in the G–T direction, 80 mm laterally from the isocenter. An arc prescription irradiates the bar with a 16mm×220mm field during a complete 360° arc, so as to cast a shadow of the bar onto the portal imager. This results in a sinusoidal sweep of the field and shadow across the portal imager and back. The method is evaluated by simulating gantry position errors of 1°–9° at one control point, dose errors of 2 monitor units to 20 monitor units (MU) at one control point (0.3%–3% overall), and MLC leaf position errors of 1 mm ‐ 6 mm at one control point. Inhomogeneity metrics are defined to characterize the synchronization of all leaves and of individual leaves with respect to the complete set. Typical behavior is also investigated for three models of accelerator. In the absence of simulated errors, the integrated images show uniformity, and with simulated delivery errors, irregular patterns appear. The inhomogeneity metrics increase by 67% due to a 4° gantry position error, 33% due to an 8 MU (1.25%) dose error, and 70% due to a 2 mm MLC leaf position error. The method is more sensitive to errors at gantry angle 90°/270° than at 0°/180° due to the geometry of the test. This method provides fast and effective VMAT QC suitable for inclusion in a monthly accelerator QC program. The test is able to detect errors in the delivery of individual control points, with the possibility of using movie images to further investigate suspicious image features.PACS numbers: 87.55.Qr, 87.56.bd, 87.56.Fc
Highlights
Clinical use of volumetric-modulated arc therapy (VMAT)(1-4) brings with it the requirement for regular quality control (QC) of arc delivery on the linear accelerator
The specific tests required for QC of VMAT are well-established,(6-8) and a proposal has been made for the frequency of these tests.[7]. Broadly, there are three types of test needed: 1) measurement of beam flatness and symmetry at the range of dose rates used for VMAT; 2) demonstration of accurate multileaf collimator (MLC) calibration;(9-16) and 3) ensuring accurate synchronization of gantry, MLC, and dose rate during VMAT delivery.[17]. Carrying out these tests explicitly represents a significant workload during monthly accelerator QC
With a 4° gantry position error at one control point and an 8 monitor units (MU) dose error at one control point, P9′5 increases to 0.25 (67% increase) and 0.20 (33% increase), respectively
Summary
Clinical use of volumetric-modulated arc therapy (VMAT)(1-4) brings with it the requirement for regular quality control (QC) of arc delivery on the linear accelerator. Commercial array phantoms are designed primarily for patient-specific quality assurance rather than accelerator quality assurance, but may be used successfully to test synchronization.[17] The general approach for machine-specific quality assurance of synchronization is to use a prescription which produces a dose distribution sensitive to the delivery parameters.[20,21] A method previously proposed for testing delivery synchronization[7] irradiates a transaxial film in a cylindrical phantom with a narrow aperture, the aperture being designed to move in a sinusoidal pattern around an offset isocenter This suffers from two drawbacks: firstly, the use of a transaxial film in a cylindrical phantom with the collimator at 0o only tests the synchronization of one MLC leaf pair; and secondly, it requires the use of radiochromic film, which is increasingly unpopular for radiation therapy dosimetry. This paper investigates the sensitivity of the method to potential errors and gives some representative results following its implementation into clinical use
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