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

Purpose: To study the impact of dose rates on the position accuracy of a Varian multi-leaf collimator (MLC) in dynamic IMRT. Methods: Four nasopharyngeal carcinoma (NPC) patients who received dynamic IMRT were chosen. The dosimetric verification at different dose rates (400–600 MU/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. Results: The passing ratios of dosimetric verification both of the criteria of 3%/3 mm and 2%/2 mm did not show statistical differences between two dose rates (P>0.05). But using the criteria of 2%/1 mm, 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 600 MU/min were statistically significant (P<0.05). Conclusions: The higher dose rate will affect the accuracy of MLC leaf position in dynamic IMRT. - Highlights: ► Higher dose rate affected the accuracy of MLC leaf position in dynamic IMRT. ► Leaf position deviations caused the differences in the dose distributions. ► Technicians should be cautious in choosing higher dose rate. ► Dynalog file information can be used for QA to assess the function of the MLC.