Conventional radiotherapy with high energy photons and electrons has been remarkably successful in treating many localized tumors. However, if tumors are large or are located very close to critical normal structures, it becomes very difficult to deliver a tumoricidal dose to the entire tumor volume without compromising the function of these adjacent organs. In such situations the local tumor control probability may be enhanced by improving the therapeutic ratio, defined as the ratio of the probability of tumor control to the probability of normal tissue injury. An increase in the therapeutic ratio may be' obtained by choosing a radiation modality that is more effective at killing tumor cells than normal cells or by employing a type of radiation that provides improved dose distribution relative to that achievable by conventional radiation modalities. Historically, great improvements in local control rates of deep-seated tumors were obtained through an increase in the energy of photon treatment beams. This occurred both at the time of the introduction of the 60 Cobalt teletherapy unit into radiation therapy, replacing X-ray machines with maximum energies of about 300 kVp, and with the introduction of high energy electron accelerators capable of producing bremsstrahlung beams with energies in the supervoltage range, i.e. above 2 MeV, such as Betatrons and linear accelerators (4). Since the introduc­ tion of high energy photon beams, physicists have continued to try to improve dose distributions through the use of sophisticated treatment plans, including multiportal and rotational techniques, beam modifiers such as compensating wedges, etc, and by the development of computer­ directed dynamic therapy techniques (8).