Abstract
Purpose: Latest generation linear accelerators (linacs), TrueBeam (Varian Medical Systems, Palo Alto, CA) and its stereotactic counterpart, TrueBeam STx, have several unique features, including high-dose-rate flattening-filter-free (FFF) photon modes, reengineered electron modes with new scattering foil geometries, updated imaging hardware/software, and a novel control system. We have performed a comprehensive evaluation of three TrueBeam linacs at three different institutions and report on our commissioning experience. Methods: Acceptance and commissioning data were analyzed for three TrueBeam linacs equipped with 120 leaf (5 mm width) MLCs at three different institutions. Dosimetric data and mechanical parameters were compared. These included measurements of photon beam profiles (6X, 6XFFF, 10X, 10XFFF, 15X), photon and electron percent depth dose (PDD)curves (6MeV, 9MeV, 12MeV), relative photon output factors (Scp),electron cone factors, mechanical isocenter accuracy, MLC transmission, and dosimetric leaf gap (DLG). Results: Gantry/collimator isocentricity measurements were similar (0.27–0.28mm), with overall couch/gantry/collimator values of 0.46–0.68mm across the three institutions. Dosimetric data showed good agreement between machines. The largestdiscrepancy was observed with measured MLC DLG (for 6, 10 and 15 MV photons, average DLGs were 1.95 ± 1.15mm, 2.22 ± 1.30 mm, and 2.20 ± 1.24mm, respectively). Photon and electron PDDs were comparable for all energies. 6, 15 and 10 MV photon beam quality, %dd(10)x varied less than 0.3% for all machines. Electron beam quality specifier (R50) showed less than 1.7% variation for all energies. Output factors (Scp) and electron cone factors agreed within 0.27%, on average; largest variations were observed for small field sizes (0.77% variation, 2×2cm2) and small cone sizes (0.39% variation, 6×6 cm2 cone), respectively. Conclusions: Overall, strong agreement was observed in TrueBeam commissioning data. This comprehensive set of multi-institutional data may serve as a benchmark for other institutions embarking on TrueBeam commissioning, ultimately improving the safety/quality of beam commissioning
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