Use of an amorphous silicon electronic portal imaging device for multileaf collimator quality control and calibration

Abstract

Multileaf collimator (MLC) calibration and quality control is a time-consuming procedure typically involving the processing, scanning and analysis of films to measure leaf and collimator positions. Faster and more reliable calibration procedures are required for these tasks, especially with the introduction of intensity modulated radiotherapy which requires more frequent checking and finer positional leaf tolerances than previously. A routine quality control (QC) technique to measure MLC leaf bank gain and offset, as well as minor offsets (individual leaf position relative to a reference leaf), using an amorphous silicon electronic portal imaging device (EPID) has been developed. The technique also tests the calibration of the primary and back-up collimators. A detailed comparison between film and EPID measurements has been performed for six linear accelerators (linacs) equipped with MLC and amorphous silicon EPIDs. Measurements of field size from 4 to 24 cm with the EPID were systematically smaller than film measurements over all field sizes by 0.4 mm for leaves/back-up collimators and by 0.2 mm for conventional collimators. This effect is due to the gain calibration correction applied by the EPID, resulting in a 'flattening' of primary beam profiles. Linac dependent systematic differences of up to 0.5 mm in individual leaf/collimator positions were also found between EPID and film measurements due to the difference between the mechanical and radiation axes of rotation. When corrections for these systematic differences were applied, the residual random differences between EPID and film were 0.23 mm and 0.26 mm (1 standard deviation) for field size and individual leaf/back-up collimator position, respectively. Measured gains (over a distance of 220 mm) always agreed within 0.4 mm with a standard deviation of 0.17 mm. Minor offset measurements gave a mean agreement between EPID and film of 0.01+/-0.10 mm (1 standard deviation) after correction for the tilt of the EPID and small rotational misalignments between leaf banks and the back-up collimators used as a reference straight edge. Reproducibility of EPID measurements was found to be very high, with a standard deviation of <0.05 mm for field size and <0.1 mm for individual leaf/collimator positions for a 10x10 cm2 field. A standard set of QC images (three field sizes defined both by leaves only and collimators only) can be acquired in less than 20 min and analysed in 5 min.

Task Group 142 report: Quality assurance of medical acceleratorsa)

“…Although the task group report was published during initial IMRT implementations using multileaf collimation, it did not make recommendations specific for MLCs as used for IMRT. Subsequent publications, 9,30,[56][57][58][59][60][61] particularly those by Cosgrove et al 62 and Chang et al, 63 pointed to tests for MLC QA along with tools for such tests. We have subsequently recommended testing ͑Table V͒ that depends on whether or not the MLC system is used for IMRT.…”

EPID‐based dosimetry to verify IMRT planar dose distribution for the aS1200 EPID and FFF beams

“…Linear accelerators (linacs) are equipped with EPIDs originally designed for patient positioning, (4) but because EPIDs have high sensitivity, spatial resolution, and immediate digital format, they have also been utilized to determine dose for routine QA of linacs or dose verification of treatments 5 , 6 , 7 , 8 . The Varian aS1200 EPID detector (Varian Medical Systems, Palo Alto, CA) was released recently and has a large area ($40\times 40\hspace{0.17em}{\text{cm}}^2$), small pixel size (0.0336 cm), and advanced acquisition electronics, and is potentially an improved design for dosimetry (9) .…”

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

“…The accuracy of the EPID at determining MLC positions was found to be within 0.5 mm compared to film measurements, and reproducibility of measurements $\text{was}\hspace{0.17em}<\hspace{0.17em}0.01$ mm (19) . Prior to use with the VMAT plans, the system was tested under dynamic conditions using both a conformal ($10\times 10\hspace{0.17em}\text{cm}$) and dynamic arc (a 2 cm sliding window defined by MLCs, similar to that described in Bedford and Warrington (14) ).…”

“…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.…”