Detection of multi-leaf collimator leaf position errors in implementing static intensity-modulated plans and its effects on dose distribution / 中华放射医学与防护杂志

ABSTRACT

Objective To design a method for detecting multileaf collimator (MLC) leaf position accuracy in implementing a static intensity-modulated plan and to analyze the impacts of leaf errors on dose of targets and normal organs.Methods Static intensity-modulated planning for twenty lung cancer cases through dose verification was sorted in an ascending order according to the number of segment,and then the first and the last 10 plans were sorted as the simple plan group and the complex plan group,respectively.These plans were transmitted to a Varian 600CD accelerator and implemented by it.Photos were taken with PV aS500 electronic portal imaging device (EPID) and actual position of leafs was determined by gradient algorithm to calculate the pass rate for leaf verification.MLC files were modified according to examination results and the plans were re-calculated while keeping other parameters unchanged.Thus,difference of targets and normal organs dose distribution before and after the appearance of leaf errors were obtained.Results The dose distribution of most organs after leaf errors were increased or decreased,and the maximum dose of spinal cord in the sixth and thirteen cases exceeded the limit of 45 Gy.In the group of simple plan only the changes of maximum dose to the spinal cord were statistically significant(t =-3.08,P ＜ 0.05),while in the group of the complex plan all changes of D95％ of PGTV and PTV,maximum dose of the spinal cord,V20 of lung and V40 of heart were statistically significant(t =-1.89,-1.99,-2.36,-2.55,-1.85,P ＜ 0.05).Conclusions To ensure the safety and effects,it was necessary to detect leaf position,particularly the complex intensity-modulated planning.Electronic portal imaging devices and treatment planning system could detect leaf positions during the implementation of a plan and obtain the actual dose of targets and normal organs.