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

Abstract

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.

Multileaf collimator performance monitoring and improvement using semiautomated quality control testing and statistical process control

“…Short and long-term leaf positioning accuracy for the Elekta MLCi model was reported to be within 0.5 mm during "normal" MLC operation 9,10,13-15 and recalibration was recommended every 3 months. 10,15 The MLC monitoring performance in this work supports these results for both MLCi and MLCi2 Elekta models but also demonstrated the difficulty to identify when the MLC is performing normally without regular testing. The observed MLC leaf positioning performances support a monthly QC test frequency to achieve ±1 mm specification as specified in the AAPM task group 142.…”

“…Various methods using films and portal imager have been described to assess short and long-term reproducibility of different commercially available MLCs. [9][10][11][12][13][14][15][16] The MLC performance monitoring system presented in this work includes a QC test that integrates radiation isocenter detection, correction for MLC collimator to panel rotation, detection of all MLC leaves (using a shifted panel and collimator rotation technique), and has built-in validation tests. The MLC QC was successfully performed on 355 different image datasets over 11 months (two treatment units) without requiring manual intervention.…”

Dosimetric impact of systematic MLC positional errors on step and shoot IMRT for prostate cancer: a planning study

“…Our study has demonstrated the need for accuracy in both the modelling of the MLC system in the TPS and in calibration as well as quality assurance (QA) of the MLC in order to minimize systematic errors of IMRT treatment delivery. Various MLC QA for IMRT techniques have been described in the literature [12,16,25,26] and recommendations for modelling of MLC have been proposed by the AAPM Task Group No. 50 [27].…”

The impact of continuously‐variable dose rate VMAT on beam stability, MLC positioning, and overall plan dosimetry

“…A software tool has previously been developed and validated at this center to determine MLC positions using the EPID 19 , 20 . For this study, the software has been expanded to allow for tracking of MLC positioning during VMAT delivery.…”