Physical and biologic aspects on the optimum choice of radiation modality

The most generally used radiation modalities, low energy photon and electron beams, have been gradually optimized with regard to their physical properties during the last decade. The advantages of increasing the energy range of these beams as high as 50 MeV are discussed based on the assumption that high quality beam flattening systems are used. Possible further developments of the conventional low LET electron and photon beams using grid irradiation and ultra short pulses are also indicated. The dose distributional properties of high quality electron and photon beams are compared with those of heavy particle beams indicating that in several cases only marginal advantages should be expected for the heavy charged particle beams.