Physical aspects of radiation treatment. Some past and present developments with implications for the future

Some aspects of the development and initial use of high energy photons and electrons are discussed, including the initial roles of Kerst and Quastler. Analytical developments in brachytherapy are also described. Relative to orthovoltages, the availability of high energy photons and electrons have provided the opportunity for concentration of ionizing energy anywhere in the body. Computer methods became necessary to handle the number of computations required in treatment planning, and the development of computerized treatment planning over the past 3 decades is sketched, with emphasis on current multidimensional planning. Both anatomical detail and dose distributions are now displayed in any arbitrary plane with radiation beams incident at any angle. The physical advantages of high energy photons are assessed, as well as increasing limitations with increasing energy, leading to determination of optimum photon energy ranges relative to lesion sites. A recent advance in the analysis of interstitial implants is discussed, which provides a basis for quantitative evaluation of implant systems and their application to specific cases. Further anticipated developments of these current advances are summarized.