Measurement of leaf position accuracy of dynamic multi-leaf collimator using electronic portal imaging device and EBT3 film dosimeter

Objective: To establish a fast and accurate method for measurement of leaf position accuracy of dynamic multi-leaf collimator (MLC) using electronic portal imaging device (EPID) and EBT3 film dosimeter. Methods: A Varian 6 MV accelerator was used with the gantry angle and the collimator angle fixed at zero degree. A total of 11 sliding window MLC fields were designed. Each field contained a group of strip fields with the same width. The width of a strip field ranged from 1 mm to 10 mm and the distance between two adjacent strip fields was 20 mm. The relationship between the width of the strip field (band width) and the full width at half maximum (FWHM) was calibrated using EPID and EBT3 as measurement tools. A field with a band width of 5 mm was designed in the same way and several MLC leaf deviations were made in different positions. EPID and EBT3 film dosimeter were used to analyze the leaf position accuracy. Results: A good linear relationship between band width and FWHM was achieved when the band width was larger than 4 mm. The accuracy of band width, distance between peaks, and MLC leaf position were determined as ±0.2 mm, ±0.1 mm, and ±0.1 mm by EPID and ±0.3 mm, ±0.2 mm, and ±0.2 mm by EBT3 film dosimeter, respectively. Conclusions: This study provides a fast and accurate method for the measurement of MLC leaf position accuracy using EPID or EBT3 film dosimeter, which is helpful for quality assurance of MLC. (authors)