Modern image guided external beam radiation therapy involves the use of a mounted on-board imager (OBI) on external beam units to take images of a patient’s position before each treatment. These images are used to determine the treatment couch shifts required for ideal alignment based on digitally reconstructed radiographs created in treatment planning. The lack of an automatic exposure control (AEC) on Varian OBI systems requires x-ray exposure factors to be selected manually from technique factors tables which are based on general population data. This may result in over or under exposed images for certain patients and compromise the accuracy of the image matching. A software based AEC system has been developed to predict the optimal, patient specific exposure factors based on patient planning data. This system allows us to accurately choose the techniques that will give the highest quality image to use in image matching, while eliminating the need for multiple unnecessary exposures.