Improving agreement between radiation-delineated field edges on simulation and portal films: the edge tolerance test tool.

The anatomy at the edge of a treatment machine portal film may differ from that shown by the delineator lines on a simulator film by up to 10 mm, even if both the simulator and the treatment machine meet accepted criteria for mechanical tolerances. To assure that this possibility is minimized requires some form of overall alignment check between the simulator and the treatment machine. A new test device, the edge tolerance test tool (ET3), has been designed to permit a quick and accurate check on whether portal film disagreement with simulator films is due to an accumulation of tolerances. Its use should eliminate one source of this common problem.