Impact of dose rates on the position accuracy of multi-leaf collimator

Abstract

PurposeTo study the impact of dose rates on the position accuracy of a Varian multi-leaf collimator (MLC) in dynamic IMRT. MethodsFour nasopharyngeal carcinoma (NPC) patients who received dynamic IMRT were chosen. The dosimetric verification at different dose rates (400–600MU/min) was performed using a PTW 2D ion chamber Array in a Varian Clinac iX linear accelerator. Meanwhile, the actual MLC leaf position data was taken by the MLC controller. The passing ratios of dosimetric verification were compared at different dose rates, and the MLC leaf position deviations were evaluated through the parameters of Error Histogram and Error RMS. ResultsThe passing ratios of dosimetric verification both of the criteria of 3%/3mm and 2%/2mm did not show statistical differences between two dose rates (P>0.05). But using the criteria of 2%/1mm, significant differences were observed between two dose rates (P<0.05). Moreover, according to the analysis of the Error Histogram and Error RMS, the MLC leaf position deviations were higher with the increase of dose rate, and the differences between 400 and 600MU/min were statistically significant (P<0.05). ConclusionsThe higher dose rate will affect the accuracy of MLC leaf position in dynamic IMRT.