The Stanford medical linear accelerator. II. Installation and physical measurements.

Abstract

The electron linear accelerator is an instrument for the acceleration of electrons to high energies by means of guided electromagnetic waves. It may serve as a source of both energetic electrons and x-rays, the latter being obtained by bremsstrahlung conversion in materials of high atomic number. Both types of radiation are of interest in the field of cancer therapy. Although very high energies have been achieved with linear accelerators (1) to serve the purposes of investigations in nuclear physics, moderate energies are sufficient for medical purposes. Thus, for x-ray work all the advantages of supervoltage therapy may be realized in the energy range of 3-10 MEV, while technics which make use of direct electron irradiation require energies up to about 30 MEV. On the basis of such considerations, the Stanford medical accelerator was designed to operate at 6 MEV. At this energy, it serves primarily as a source of supervoltage x-rays, and secondarily as a source of low-energy electrons which may be employed in the treatment of superficial skin lesions. The electron applications are currently under development and will not be discussed in this report. Various technical considerations which influenced the final design of this accelerator are described by Ginzton, Mallory, and Kaplan (2). The machine was completed in 1955 and installed at Stanford Hospital. The treatment of patients began in early 1956. It has been customary to operate the accelerator at an energy between 4 and 5 MEV. At 4.2 MEV, the forward intensity of the x-ray beam is 0.38 milliwatts/sq. cm. (corresponding to 65 rads/min. at the maximum of the transition curve in water) for an average electron current of 16 microamperes at the target. Under these conditions, the corresponding x-ray dose distributions are similar to those described by Greene and Tranter (3) and by Murison and Hughes (4). Magnetic analysis of the electron beam indicated that satisfactory operation is obtainable in the range of 2 to 5.5 MEV. Linear accelerators must be operated under pulsed conditions. In the present design, the electron beam is incident on the target in bursts of approximately one microsecond duration, repeated 480 times per second. Thus, the x-ray intensity during the pulse is about 2,000 times higher than the average intensity. This report is intended as a description of the physical characteristics of the machine and the relevant instrumentation and technics that have evolved. What is described here is not to be interpreted as optimum in any sense but rather as a step toward the development of a useful tool in radiation therapy. The Accelerator Since descriptions of the physical and electrical characteristics of linear electron accelerators have already been given (1, 2), only those features which are pertinent to radiotherapeutic applications will be described.