In the last few years we have all witnessed with varying degrees of interest a tremendous increase in the upper limits of supravoltage energies for possible therapeutic application. A variety of radiations have become available, some in adequate quantity for the first time. We have been using a 24,000,000-volt betatron which yields a very powerful beam of x-rays and a less well developed external beam of electrons (3, 6). Most of our work in the last year and a half has been with the x-ray beam, although at present we are diverting some of our attention to engineering problems and animal tissue effects of the electron beam. The betatron x-ray beam offers some very appealing distribution advantages in tissue (4). In Figure 1 is shown the distribution of density in a film phantom with a 200-kv. beam path and with the 24,000,000-volt betatron x-ray beam path. In the latter the sparing effect on the superficial tissues at the site of entrance of the beam (on the left in the illustration), the concentration of...