Correction for dose-response variations in a scanning liquid ion chamber EPID as a function of linac gantry angle

The response of electronic portal imaging devices (EPIDs) of the scanning liquid ionization chamber (SLIC) type is known to vary with linear accelerator gantry angle. This work considered several contributing factors, quantified the artefacts, monitored their reproducibility and investigated the effects of repeated gantry rotations. Unflatness of up to 5% was found. A correction technique was devised using nonlinear regression of a three-variable sinusoidal modulation. Comparison with two existing techniques found our method to be the most effective, providing a flatness well within 2%. This improved accuracy is expected to benefit more accurate dosimetric studies in particular. The post-acquisition correction process required no change in imaging protocols. Applicability of the new technique was demonstrated on images acquired on different days and with different beam sizes. Since the artefacts compromise both accurate dosimetry and image quality, their successful removal should benefit a broad range of SLIC EPID applications.