Improved dose homogeneity throughout the treatment volume defined for electron arc therapy is achieved through superposition of multiple arcs of different electron energy to the same treatment surface. The relative weights for each arc segment and energy are determined by computer optimization which minimizes the variation in radial depth dose across the treatment volume. In addition to the standard electron beam energies of 6 MeV, 9 MeV, 12 MeV, 16 MeV and 20 MeV, a new electron arc field is created by adding bolus to the treatment surface during an additional pass using 6 MeV electrons. This modified field, having maximum dose on the patient surface and a reduced range, supplements the dose delivered by the standard electron arc fields in the buildup region. Through use of this multiple-energy technique, depending on clinical indications, electron arc therapy can be planned and delivered to allow either skin sparing or a uniform dose from the patient's surface to the desired treatment depth.