Preclinical research using well characterized small animal models has provided tremendous benefits to medical research, enabling low cost, large scale trials with high statistical significance of observed effects. The goal of the Small Animal Radiation Research Platform (SARRP) is to make those models available for the development and evaluation of novel radiation therapies. SARRP demonstrates the capabilities of delivering high resolution, sub-millimeter, optimally planned conformal of radiation with on-board cone-beam CT (CBCT) guidance. The system requires accurate calibration of the x-ray beam for both imaging and radiation treatment. In this paper, we present a novel technique using an x-ray camera for calibration of the treatment beam. This technique does not require precise positioning or calibration of the x-ray camera.