Dynamic MLC quality assurance program using EPID

Abstract

The introduction of intensity modulated radiotherapy (IMRT) allowed delivering high uniform dose distributions in the target, while sparing the surrounding healthy tissue. This technique is performed thanks to multileaf collimators (MLCs) in continuous motion. Therefore, it is crucial for patient safety to carry out a routinely quality assurance (QA) program of the MLC to ensure that leaf motion is precise, smooth and reproducible on every fraction. In this work a set of tests was gathered in order to quantitatively examine the stability of the electronic portal imaging device (EPID) signal, the accuracy of leaf positioning, the steadiness of leaf velocity and gap sizes and the influence of gravity for different gantry angles. These tests were accomplished using EPID (amorphous silicon aS1000, Varian) calibrated for dosimetric purposes for a 120-leaf Millennium MLC. The detector consists of matrix of 1024 ×784 pixels for 40 ×30 cm2 area at the isocenter, providing a resolution of 0.39 mm/pixel. Hence, the EPID not only presents an adequate resolution to detect potential faults in the MLC performance but it is also agile and accessible. Additionally, it allows exporting .dxf files for each of the tests to be quantitatively assessed with help of a series of routines developed in Matlab. Corroboration of the implemented routines was performed using a detector array. Both methods showed an exceptional agreement. In conclusion, a MLCQA program using the EPID was presented to provide a reduction in time of quantitatively analysis of the tests and an early detection of errors.