Some Physical Aspects of Electron Beam Therapy

Abstract

The physical development of the high-energy electron beam has progressed sufficiently to permit investigation of its action on animals and more recently the initiation of human therapy. The present report is concerned with aspects of this physical development of the electron beam. The long-used therapeutic action of x-rays on tissue is essentially effected through the interaction with the tissue molecules of the secondary electrons produced in the absorption of the x-rays. Accordingly, the direct action of electrons from beta-ray sources has long been exploited in therapy. Examples are the use of Ra D applicators, and of such isotopes as 53I131, 20Ca45, and 79Au198, whose therapeutic action is primarily due to beta emission. The primary differences to be expected in the action of x-rays and electrons are only quantitative, involving such factors as dose distribution, dosimetry, and the dependence of the biological action on specific ionization. At the time of his development of the betatron in 1940, Prof. D. W. Kerst expected that the therapeutic possibilities of both the high-energy electron and x-ray beams made possible by this circular electron accelerator would be utilized. The two beams produce entirely different dose distributions in tissue and require different handling technics because of their different physical properties. These aspects of the high-energy x-ray beam have already been discussed in the literature (31, 17, 23). Pioneer work in the therapeutic application of accelerated electrons has been done in Germany, primarily with the 6-mev Siemens (11) betatron. Problems encountered in this work concerning beam extraction, electron range determination, and dosimetry were considered and reported by several investigators, including Gund (11), Paul (29), Glocker (8), and Wachsmann (37). Comprehensive clinical and biological investigations with the electron beam from the Siemens betatron have also been reported. Source of Electrons The electrons used in our experiments are accelerated in a 22-mev betatron and extracted as a beam at any desired energy between 5 and 22 mev. The betatron is a circular electron accelerator whose operation has been described in the literature (19–22). A 22-mev electron makes approximately 200,000 complete circular revolutions in a little over a thousandth of one second. The electrons are held in this circular orbit by the action of the magnetic field. After the electrons have been accelerated to the desired energy, it is necessary to extract them from the magnetically defined circular orbit for external use. Various methods of extraction have proved successful. Dr. K. Gund (11) successfully removed the electron beam from the Siemens 6-mev betatron by means of a scattering foil and a deflecting electrode located just outside the stable orbit.