Evaluation on the stability of positioning precision of the leaves of accelerator MLC under different gantry angles / 中国医学装备

ABSTRACT

Objective:To evaluate the stability of the positioning precisions of the leaves of accelerator multi-leaf collimator(MLC)at different gantry angles,so as to determine whether the positioning precision of leaf is affected by the gantry angle.Methods:The HD 120 MLC of American Varian Edge medical linear accelerator at our hospital was selected,and the Varian Eclipse radiotherapy planning system was used to design test cases.One test plan included 5 dynamic subfields(named PMLCE)was designed,which ray amount was 100MU,and small machine head was kept at 0°position,and the numbers of the positions of corresponding leaves of each control point were respectively-20,-15,-10,-5,0,5,10 and 12 that were 8 in total,and the dose rate was set at 500MU/min.In test,all leaves slid as uniform velocity from one extreme position to the opposite extreme position,and the movement distance of leaves between the adjacent control points was 5 cm.The PMLCE test plan was executed on the EDGE linear accelerator by using the IEC1217 coordinate system,and each test was conducted at eight different gantry angles(0°,45°,90°,135°,180°,225°,270° and 315°),and a total of 8 treatment log files of accelerator were obtained.A program was written by using Matlab2016a(MathWorks,USA)software to read the log files of accelerator,and extract out leaf position data,and calculate the deviation between the actual position and planned position of all leaves,and record a total of 960 data of positioning deviations of 120 leaves at 8 leaves positions which included in each file.The above acquisition scheme of data was performed per three days for 10 times,and then the average value was obtained.There were the data of positioning deviations of a total of 8 groups of HD 120 MLC at different gantry angles.The above 8 groups of data were analyzed as statistical method.Results:Under various angles,the range of the percentage of"leaf positioning deviations≤10 μm"was between 45.7%and 58.3%,and that of"leaf positioning deviations≤30 μm"was between 95.6%and 99.2%.When the gantry angle was at 0° and 180°,the leaf positioning accuracy was relatively higher,and the percentages of"leaf positioning deviations≤10 μm"were respectively 58.3%and 57.9%.The maximum positioning error of the HD 120 MLC did not exceed 43.3 μm.The results of the one-way ANOVA showed that there were significant differences in the average positioning precisions between 0° and each one of 45°,90°,135°,225°,270° and 315°,and between 90° and 180°,and between 135° and 180°,and between 180° and each one of 225°,270° and 315° under the gantry angles were respectively 0°,45°,90°,135°,180°,225°,270° and 315°,and under the observation values of the positioning precision of 8 leaf positions(F=4.32,F=5.67,F=3.98,F=3.85,F=6.14,F=2.87,F=4.89,F=4.56,F=4.12,F=5.01,F=6.77,P<0.05).There were no significant differences in the positioning precision among other gantry angles.Conclusion:The positioning precisions of HD 120 MLC leaf under different gantry angles can be kept at higher level,but the influences of gantry angles on the positioning precision of leaf indeed existed.Therefore,doctors and physicists should fully consider these factors under the design stage of treatment plan,so as to optimize the dose distribution and enhance the treatment outcomes.