STUDY OF AMORPHOUS SILICON ELECTRONIC PORTAL IMAGING DEVICE AS A DOSIMETRY FOR DYNAMIC INTENSITY MODULATED RADIATION THERAPY VERIFICATION

Abstract

The purpose of this study is to investigate the basic dosimetric characteristics of Varian flat panel amorphous silicon electronic portal imaging detectors (EPID) for the possibility of using a silicon portal imager for absolute dosimetric verification of the delivery of dynamic intensity modulated radiation treatment (IMRT) fields. The measurements were performed with 6 MV X-ray beams from Varian Clinac 23EX. The studies included field size dependence, dose rate, dose response, effect of dead time, relative and absolute dosimetry. The portal dose image was tested by comparing the EPID profiles with the ion chamber both for open and wedge fields. The portal dose image calculated by EPID dosimetry was compared with the EPID measurement for clinical IMRT fields. The results showed field size dependence of EPID, which was more sensitive than ion-chamber for larger field size but less than ion chamber in smaller field size, the maximum deviation of 4.9% was observed. The EPID was linear with dose rate and integral dose. The effect of dead time in frame acquisition due to transfer to the CPU was found to start at 40 MU of field size studied. The dead time resulted in dynamic field caused error that increased with leaf speed, the error was 17.62% fora 1 cm leaf gap moving at 1cm/s. The comparison of profiles from EPID and ion chamber measurement for 10x10 cm? normal field showed the good agreement. For wedge field, both of EPID and ion-chamber profiles showed the agreement in the center part but slightly shift in the penumbra region. The pre-treatment verification for IMRT fields of 15 plans showed the agreement stribution between EPID calculation and EPID measurement within 3% difference in dose and 3 mm. difference in distance. The profile in the direction of MLC movement also showed good correlation between calculation and measurement.