SU-E-T-131: Influence of Scanning Speed on Measurements of Field Flatness and Symmetry of Photon Beams.

PURPOSE: The purpose of this study was to investigate influence of different scanning speeds on measurements of photon beam flatness and symmetry. METHODS: Commissioning and quality assurance of linear accelerators require extensive beam measurements. To increase efficacy, we evaluated flatness, symmetry and penumbra of 6MV photon beam using the Varian-TrueBeamTM system. Scanning speeds were 0.3, 0.5, 0.75, 1, 1.5, and 2.5cm/s. Measurements were performed in water phantom (BluePhantom2 , IBA-Dosimetry) at depths of maximum dose, 5,10, and 20cm, for 10×10 cm field size. For each scanning speed and depth, measurements were repeated five times to give results sufficient statistical significance, in both crossline and inline directions. Beam flatness was calculated using variation over mean (80%), whereas symmetry was calculated using point difference quotient (IEC) algorithm. After filed scanning chamber (Wellhofer) was fully stopped, system was paused for stabilization time of 15s to avoid buildup of ripples. RESULTS: It was noticed for all measurements that minimum and maximum flatness and symmetry were recorded when scanning speeds were 0.3cm and 2.5cm, respectively. For depth of maximum dose, maximum flatness and symmetry were 0.82% and 100.58% (crossplane), and 0.94% and 100.96% (inplane). The average was 0.76% and 100.38% (SD 0.04 and 0.12) for crossplane; 0.89% and 100.87% (SD 0.04 and 0.06) for inplane measurements. As the scanning depth increased, flatness and symmetry increased, but SD for all measurements was within the same range (0.04-0.07 and 0.04-0.12). The maximum absolute difference for flatness and symmetry for maximum and minimum speed were 0.16% and 0.34%.However, for scanning speeds from 0.5-1cm/s, results were almost identical with maximum SD 0.03 for both flatness and symmetry. Use of different scanning speeds did not influence penumbra; SD was 0 for all measurements. CONCLUSIONS: This study reveals small influence of scanning speed within predefined range. Consequently, difference in measurements does not have clinical significance.