MLC positioning checks and calibration with a portal imaging system

For checking the leaf positions of a MLC (Multi Leaf Collimator) images are acquired with an EPID (Electronic Portal Imaging Device) and then evaluated with a programme (MLC check) developed in-house. During image acquisition a Perspex tray with two metal markers of known position (in the radiation field) is inserted in the satellite tray holder. After determination of the marker positions within the image coordinate system, the image can be transformed to the radiation field coordinate system. This allows the exact determination of the leaf tip positions relative to the radiation field. This evaluation can be applied to images of arbitrary field shapes, provided they were acquired in the same geometry (EPID position, gantry and collimator angles). The entire measurement procedure is based on images in bmp (Windows Bitmap) format, with a 1024 x 1024 matrix and a pixel depth of 24 bit (8 bit per color channel). A suitable marker plate can be manufactured without sophisticated workload; thus the method may be easily and cost effectively adapted at other locations.