What is an electron cutout factor?

An electron cutout is placed in the path of an electron beam in order to change the shape of the beam. Because this changes the size and shape of the beam, it will change the dose to the patient due to scatter, field size dependence (especially for small fields), etc. Because MUs are calculated based on the standard 10 cone without any cutout, we must determine how an electron cutout affects measurements regarding this setup. Therefore, one measures dose through an electron cutout (with whatever cone size) and also measures the 10 cone at the same depth and energy. The ratio of these is a cutout factor which can be used to determine MUs (dose required from the machine to produce desired effect in patient).