ESTRO 2021 Abstract Book
offset due to panel recalibration, primary maintenance (e.g. MLC calibration) or system upgrades. Results are reported as one standard deviation (SD) of the measurements. Results A total of 460 EPID QA images recorded at 428 consecutive clinical days were included in the analysis. A total of 14 offsets were applied during that period with a median interval of 28 days (Range 4 – 73 days). Figure 1 shows histograms of variations in output and beam quality. The variation in output was 0.4 % (1xSD), and the variations in beam symmetry and flatness were 0.05 and 0.06 percentage points, respectively. Figure 2 shows the variations in beam geometry. The gantry position variation was extremely small with a SD of 0.002 degrees, while the variations in the jaws and MLC positions were 0.04 mm and 0.09 mm, respectively.
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