Radiation Therapy with High-Energy Electrons

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Abstract
High-energy electron beams offer certain advantages in physical distribution for radiotherapy: (a) There is a relatively homogeneous dose from the surface of the irradiated tissue down to a given depth, and beyond that depth the dose decreases rapidly. (b) The depth to which the dose is effective can be varied by changing the energy of the beam. (c) Per gram of tissue there is no significant differential absorption of electrons in bone as compared to soft tissue (6, 14). Ovadia and Uhlmann have stated that in treating deep lesions the use of parallel opposing electron-beam fields reduces the integral dose to healthy tissues and is an arrangement which can be accurately reproduced for repeated treatments (11). In addition to these physical advantages, Zuppinger and others have claimed that the biological effects of electrons are more favorable for tumor therapy than the effects produced by similar doses of x-rays (16, 18). In May 1958, the Mark IV linear accelerator at the W. W. Hansen Laboratories of Physics, Stanford University, was made available to us for clinical use on a part-time basis. This machine was capable of producing a useful electron beam of energy varying between 10 and 70 Mev. The clinical program terminated in April 1960, when the machine was committed to other purposes. During the two-year period, 42 lesions were treated with electron beams of 10 to 40 Mev. We are reporting our clinical observations and our initial impressions of the usefulness of electron-beam therapy. Extensive studies of dosimetry and collimation are the subject of a companion paper by Loevinger, Karzmark, and Weissbluth (10). Equipment and Methods The electron beam emerged from the linear accelerator in a fixed horizontal direction. After passing through a system of monitors, the electrons were scattered in aluminum to provide a beam of the desired width. Beyond the scatterer the beam was collimated to the desired field size and shape. The means of collimation varied during the period of the study as the physical data were accumulated. In the initial phases, the collimator was a 5.7-cm.-thick aluminum aperture 81 cm. from the scatterer, with a collimator-skin distance of 19 cm. In order to reduce the width of the penumbra, the scatterer-skin distance was subsequently increased to 200 cm. and a plastic collimator which extended to the skin surface was used. Later, in order to reduce the skin dose from electrons scattered by the plastic, the final treatment collimation was changed to a lead aperture 5 cm. from the skin surface. The fixed position of the accelerator required that the position of the patient be adjusted to the beam. A chair that could be elevated hydraulically, with an adjustable back and head rest, was used for this purpose. A light localizer and an optical back pointer aided in accurate positioning.

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