Leaf position testing for multileaf collimator in the dynamic mode

Abstract

Objective To establish a new method to analyze the position accuracy of multileaf collimator (MLC) in the dynamic mode. Methods The MLC test sequence was created in a field, where intentional leaf positional errors ranging from 0.1 to 1 mm per centimeter were introduced. In order to establish the relationship between the ion chamber readings and leaf position, whose slope indicated the leaf position error per centimeter, a two-dimensional ion chamber array was used to measure absorbed dose while leaves were moving at dose rates of 100, 300 and 600 MU/min, respectively. For routine test, leaf position error was easily found via dose profile in y direction of the field created by dynamic leaves, where the position error could be quantitatively calculated as the slope of absorbed dose line of x direction of the same field. Results The error of 0.2 mm or more per centimeter was obviously shown through y dose profile. The calibration curve was linear at different dose rates. At 600 MU/min, a 0.1 mm leaf position error corresponded to a slope variation of 0.74%, and the differences between the tested errors and the introduced errors were within 0.1 mm. Conclusions The simple and reliable method is helpful to establish the intensity modulated radiation therapy (IMRT) quality control (QC) system. Key words: Dynamic multileaf collimator; Two-dimensional ion chamber array; Position accuracy