The history of the development of cancer therapy has been marked by a recurring pattern, one of initially exciting and encouraging results as new methods were introduced, followed by dismaying failures. The extremity of the disease and its high mortality have dictated that each means of damaging tumor cells would be rapidly explored and exploited as a mode of therapy, long before the correspond­ ing theory and technique were completely understood and perfected. Thus radiation was used as an antitumor agent almost immediately following recognition of its cytodestructive capability. Equally constant, following the rapid utilization of new therapeutic methods, has been a period of significant technical improvements. This second aspect of the pattern is also illustrated by the field of radiotherapy. New radiation sources, new methods of dosimetry, use of high-energy radiation, and other new techniques allowed the therapist to better focus upon the tumor and to improve the geometry of exposure. Thus, with each technical advance, the "reach" of radiotherapy was increased and damage to normal tissues was decreased. Inevitably, however, a limit was reached, a point at which clinicians and researchers realized they could go no further without returning to a more fundamental search, one based on the biology of the tumor cell itself.