There is a concern that image lag may reduce accuracy of real-time target tracking in radiotherapy. This study wasperformed to investigate influence of image lag on the accuracy of target tracking in radiotherapy. Fluoroscopic imageswere obtained using a direct type of dynamic flat-panel detector (FPD) system under conditions of target tracking duringradiotherapy. The images continued to be read out after X-irradiations and cutoff, and image lag properties in the systemwere then determined. Subsequently, a tungsten materials plate with a precision edge was mounted on to a motor controldevice, which provided a constant velocity. The plate was moved into the center of the detector at movement rate of 10and 20 mm/s, covering lung tumor movement of normal breathing, and MTF and profile curves were measured on theedges covering and uncovering the detector. A lung tumor with blurred edge due to image lag was simulated using theresults and then superimposed on breathing chest radiographs of a patient. The moving target with and without image lagwas traced using a template-matching technique. In the results, the target could be traced within a margin for error inexternal radiotherapy. The results indicated that there was no effect of image lag on target tracking in usual breathingspeed in a radiotherapy situation. Further studies are required to investigate influence by the other factors, such asexposure dose, target size and shape, imaging rate, and thickness of a patients body
