Snively, E.J.C. Dolgashev, V.A. Faddegon, B.A. Le Sage, G.P. Li, Z. Mendez, J.R. Nanni, E.A. Palmer, D.T. Pankuch, M. Schulte, R.W. Tantawi, S.G. JACoW Publishing Geneva, Switzerland We report on the development of a 2.856 GHz accelerator system to provide energy modulation and RF-based steering for rapid 3-D beam scanning for proton therapy. Designs for the accelerator and deflector cavities have been modeled in ANSYS-HFSS and used to produce prototype structures. We present high power test results for a single cell energy modulator prototype and a three cell deflector prototype. Using General Particle Tracer, we simulate proton beam transport through the fully rendered accelerator and deflector beamline. System performance is optimized for the case of sub-relativistic protons with 230 MeV kinetic energy and covers an energy modulation range of ±30 MeV. We present simulated beam profile data after energy modulation and lateral steering, achieved using a combination of dynamic RF deflector cavities and static permanent magnet quadrupoles.