Abstract: Two analysis methods for a portal image are disclosed, one to check apparatus setup and the other to check patient positioning. The first involves preparing a reference radiation field image and a sample radiation field image from a reference image and a sample image respectively by, in order, (i) defining a threshold value and (ii) setting pixels within the respective images to a dark or a bright state if darker or brighter than the threshold value, (optionally) comparing the reference radiation field image and the sample radiation field image to determine at least one of their relative rotation, translation and scaling, subtracting one of the radiation field images from the other thereby to obtain a difference image, and inspecting the difference image to ascertain changes in the radiation field between preparation of the reference image and the sample image.