Development of the Betatron for Electron Therapy

Abstract

Introduction Electrons, also called cathode rays, or beta rays, have been suggested for therapeutic use at various times. Successful attempts were made in 1928 to liberate cathode rays from a modified x-ray tube, and these rays were used in the treatment of superficial skin lesions. The therapeutic effects were similar to those of x-rays, but the method did not become popular because the limited penetration power of electrons produced with 200,000 to 250,000 volts made them unsuitable for the treatment of most malignant lesions. Lange and Brasch, by using an impulse generator, were able to produce electron beams with 1.7 and 2.4 million electron volts. Even with these considerably higher tensions, the penetration was limited to several millimeters of tissue. Trump, Van der Graaff, and Cloud produced electrons with a high-voltage generator and an acceleration tube of up to 3 million volts, and it was hoped that a further increase in voltage on the same basic principle might provide for the possibility of treatment with electrons to a greater depth. The development of the betatron by Kerst, however, opens an entirely new approach to this problem and makes it possible to consider actual use of fast electrons in the treatment of deep-seated cancer. Although there is no difference to be expected in the effect of fast electrons on the tissue itself, there is a very marked difference in the range of the beam and its distribution within the tissue, as compared with x-rays or gamma rays of radium. The limitations in the use of radiant energy from traditional sources are not due to the fact that x-rays or gamma rays do not sufficiently destroy cancer cells, but rather that, in doing so, they also damage the normal tissue. This effect, which is always greater in the overlying tissue, spreads to the tissues underlying and surrounding the actual cancer and limits the amount of energy which can safely be applied to the malignant tumor. In contrast to x-rays or to the gamma rays of radium, the concentration of energy in a beam of fast electrons is not at the source but at the end of the radiation. Furthermore, the beam itself is limited in its range by the amount of voltage used in its production. Therefore, electrons will reach only a certain predetermined depth and will not go beyond this calculated range. Since the concentration of energy is at the end of the beam, the overlying tissue will receive less intensive radiation than the actual tumor, in this way reversing the conditions under which radiation therapy is at present administered. For the first time, it seems possible to concentrate almost the entire radiant energy in the tumor itself, without simultaneously irradiating underlying or surrounding tissue, and with relatively small amounts of energy affecting the overlying tissue. For these reasons, it seems logical to use the betatron for the liberation of an electron beam with sufficient range to reach any malignant growth.