ESTRO 2021 Abstract Book

S1443

ESTRO 2021

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.