A review of new technological developments in the radiotherapeutic treatment of cancer Colin G. Orton Radiation therapy is one of the major tools employed for the treatment of cancer but it is successful in producing cures only about 50% of the time, even when the cancer has not spread to other parts of the body at the time of treatment. Reasons for failure can be either physical or biological. Physically, because radiotherapy is a "targeted" modality, failure can result if the exact position and/or extent of the cancer are not known accurately. Biologically, it is well known that many, maybe most, cancers contain cells that are relatively resistant to conventional radiations used in radiotherapy, such as x rays and electrons. To be successful, either very high doses of these radiations must be delivered, or else different types of radiation must be used for which these cells are not resistant. In this presentation we will review new ways being developed to accurately localize and delineate cancers and any regions within these cancers that are likely to be resistant to radiation. Exciting new developments in computerized tomography (CT), magnetic resonance (MR), and positron emission tomography (PET) will be presented. We will then examine new ways being studied to overcome cellular resistance, such as methods that allow for the delivery of very high doses of conventional radiations without exceeding the tolerance of surrounding normal tissues, or by the use of different types of radiation, such as neutrons, protons, or heavy ions. By combining these new physical and biological technologies, it is likely that the failure rate of radiotherapy will be reduced significantly in the near future.