Abstract
PurposeTo evaluate the ability of the machine performance check (MPC) on the Halcyon to detect errors, with comparison with the TrueBeam.Methods MPC is an automated set of quality assurance (QA) tests that use a phantom placed on the couch and the linac's imaging system(s) to verify the beam constancy and mechanical performance of the Halcyon and TrueBeam linacs. In order to evaluate the beam constancy tests, we inserted solid water slabs between the beam source and the megavoltage imager to simulate changes in beam output, flatness, and symmetry. The MPC results were compared with measurements, using two‐dimensional array under the same conditions. We then studied the accuracy of MPC geometric tests. The accuracies of the relative gantry offset and couch shift tests were evaluated by intentionally inserting phantom shifts, using a rotating or linear motion stage. The MLC offset and absolute gantry offset tests were assessed by miscalibrating these motions on a Halcyon linac.ResultsFor the Halcyon system, the average difference in the measured beam output between the IC Profiler and MPC, after intentional changes, was 1.3 ± 0.5% (for changes ≤5%). For Halcyon, the MPC test failed (i.e., prevented treatment) when the beam symmetry change was over 1.9%. The accuracy of the MLC offset test was within 0.05 mm. The absolute gantry offset test was able to detect an offset as small as 0.02°. The accuracy of the absolute couch shift test was 0.03 mm. The accuracy of relative couch shift test of Halcyon was measured as 0.16 mm.ConclusionWe intentionally inserted errors to evaluate the ability of the MPC to identify errors in dosimetric and geometric parameters. These results showed that the MPC is sufficiently accurate to be effectively used for daily QA of the Halcyon and TrueBeam treatment devices.
Highlights
Quality assurance (QA) of medical linear accelerators is standard practice in radiotherapy clinics
We compared the change in beam output reported by the Machine Performance Check (MPC) and the output change measured by the IC Profiler
Changes in the Halcyon symmetry were measured by the IC profiler, and plotted against the uniformity measured by the Halcyon and TrueBeam MPC (Fig. 3)
Summary
Quality assurance (QA) of medical linear accelerators (linacs) is standard practice in radiotherapy clinics These QA tests include checks of x-ray output, energy, and beam profile constancy as well as image-guided radiation therapy (IGRT) functionality as recommended by the American Association of Physicist in Medicine (AAPM) Task Group (TG) 142,1 MPPG5a,2 and TG-179.3 In general, several devices are used each morning to perform this QA. MPC is available for both the new Halcyon (Varian Medical Systems) and the existing TrueBeam platforms (version 2.0 and newer) The tests on these two treatment delivery systems are similar, the vendor-set thresholds vary between the systems.
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