Analysis of the measurement precision of an amorphous silicon EPID used for MLC leaf position quality control and the long-term calibration stability of an optically controlled MLC.

Electronic portal imaging devices (EPIDs) have been shown to be suitable for multileaf collimator (MLC) leaf positioning quality control (QC). In our centre, a continuous dataset is available of 2 years of film measurements followed by 3 years of EPID measurements on five MLC-equipped linear accelerators of identical head design. The aim of this work was to analyse this unique dataset in order to determine the relative precision of film and EPID for MLC leaf positioning measurements and to determine the long-term stability of the MLC calibration. The QC dataset was examined and periods without MLC adjustments that contained at least four successive collimator position measurements (a minimum of 6 months) were identified. By calculating the standard deviations (SD) of these results, the reproducibility of the measurements can be determined. Comparison of the film and EPID results enables their relative measurement precision to be assessed; on average film gave an SD of 0.52 mm compared to 0.13 mm for EPIDs. The MLC and conventional collimator results were compared to assess MLC calibration stability; on average, for EPID measurements, the MLC gave an SD of 0.12 mm compared to 0.14 mm for a conventional collimator. The long-term relative individual leaf positions were compared and found to vary between 0.07 and 0.15 mm implying that they are stable over long time periods. These results suggest that the calibration of an optically controlled MLC is inherently very stable between disturbances to the optical system which normally occur on service days.