Dosimetric pre-treatment verification with an electronic portal imaging device

Abstract: A commercially available amorphous silicon electronic portal imaging device (EPID) was studied with regard to its dosimetric properties and to determine its usefulness as a tool for dosimetric pre-treatment verification of radiotherapy treatment fields. The dosimetric properties that were studied include reproducibility over time, linearity with dose, dose rate dependence and ghosting effects. The pre-treatment verification is performed by acquiring dosimetric images with the EPID and comparing these images with predicted images, calculated by the treatment planning system. This method for verification was evaluated. Also, the calibration and configuration of the treatment planning system and of the EPID for dosimetric verification was performed and is presented here. The dosimetric properties of the EPID were found to be suitable for the measurements for which it is intended. It is linear with dose and does not show significant dose rate dependence or ghosting effects. As a pre-treatment verification system it is accurate within 3% and 3mm for ~99% of a region around the irradiated area of the image.