ACCURATE PORTAL DOSE MEASUREMENT WITH A FLUOROSCOPIC ELECTRONIC PORTAL IMAGING DEVICE (EPID) FOR OPEN AND WEDGED BEAMS AND DYNAMIC MULTILEAF COLLIMATION

Measuring portal dose with an electronic portal imaging device (EPID) in external beam radiotherapy can be used to perform routine dosimetri c quality control checks on linear accelerators and to verify treatmen ts (in vivo dosimetry). An accurate method to measure portal dose imag es (PDIs) with a commercially available fluoroscopic EPID has been dev eloped. The method accounts for (i) the optical 'cross talk' within th e EBPID structure, (ii) the spatially nonuniform EPID response and (ii i) the nonlinearity of the EPID response. The method is based on a dec onvolution algorithm. Measurement of the required input data is straig htforward. The observed nonlinearity of the EPID response was largely due to the somewhat outdated EPID electronics. Nonlinearity correction s for more modern systems are expected to be smaller. The accuracy of the method was assessed by comparing PDIs measured with the EPID with PDIs measured with a scanning ionization chamber in a miniphantom, loc ated at the same position as the fluorescent screen. For irradiations in open, wedged and intensity modulated 25 MV photon beams (produced w ith dynamic multileaf collimation) EPID and ionization chamber measure ments agreed to within 1% (1 SD).