The effect of gantry and collimator angles on leaf limited velocity and position in dynamic multileaf collimator intensity-modulated radiation therapy

Abstract

The purpose of the study is to evaluate the limiting velocity (LV) of a multileaf collimator and the leaf position in various collimator and gantry angles. Both leading leaves and trailing leaves began to move with a constant acceleration from 0 to 4 cm s−1. When the beam hold occurred, the leaf velocity was defined as the leaf LV. Dynamic irradiation was performed at eight gantry angles of every 45° with three different collimator angles. The analysis of the LV and the leaf position was performed with a log file from a leaf motion controller. The mean LVs for Varian Clinac 21EX (21EX) ranged from 2.51 to 3.10 cm s−1. The mean LVs for Clinac 600C ranged from 2.91 to 3.12 cm s−1. When only central 5 mm leaves of 21EX moved, LVs were significantly higher than those when all 60 pairs of leaf moved, while the leaf position inconsistencies of the two accelerators were within 1 mm at the leaf velocities from 0.5 to 2.0 cm s−1. It was recognized that the LV was affected by gravity. This measurement method can be utilized as routine quality assurance for a dynamic multileaf collimator (DMLC) is and easily reproducible.