Page 56 - Fisica In Medicina n° 1/2017

^Äëíê~Åí=cáëáÅá= =where D and σ are respectively the mean dose and the standard deviation of the distribution of the gamma passing rate (35 plans for 6 MV and 15 plans for 10 MV) measured by each dosimeter; the detectability threshold (DT) has been calculated as:DT = 100-CLResultsThe average pass rate with 2%L/2 mm criterion for the 6MV cases was 91.3± 5.2 (no shift) and 93,8±2.2 (merged) for 729, 96.0± 4.1 (no shift) and 97.9±2.5 (merged) for 1500 and 96.3± 2.8 for 1000 SRS. Similar results were achieved for 10 MV plans. The results for the plans with errors, normalized to the DT for a success in error detectability, show that only the SRS system can distinguish the delivery errors, as shown in figure for 6MV. Only a 2%L/1 mm gamma criterion proved to be sensitive enough to detect errors.ConclusionImproving the resolution of the planar detector used for QA increases the points with gamma<1. The SRS matrix can detect delivery errors in almost all cases at 2%L/1mm.1.201.000.800.600.400.200.00729 s2° collimator729 1500 SRS 5° collimator729 1500 SRS 2° gantry729 1500 SRS 5° gantry729 1500 SRS UM1500 SRS sFigure 1. Gamma passing rate for the plans (6 MV) with errors, normalized to the detectability threshold, measured by SRS, 729 (merged) and 1500 (merged) dosimeters.47