Clinical Safety in Low-Megavolt Electron Therapy

Abstract

Low-megavolt electrons produced by accelerators offer clinical advantages over low-energy x rays in that they can be used to treat large-area superficial tumors and they yield more favorable dose distributions in depth. In electron therapy it is also important that special precautions be taken to eliminate the chance of an overdose, since the electron dose rates can be extraordinarily high. The accompanying x-ray background, which results in whole-body irradiation, should be kept below an average dose of 20 rads for a treatment series to avoid the production of irreversible blood changes. Electron therapy planning must take into account the gram rad dose due to both electrons and x rays (1). These considerations tend to limit the maximum electron energy used for total skin surface area treatment. In the low-megavolt energy range, the x-ray production efficiency increases in the forward direction approximately as the cube of the electron energy and directly with the atomic number of the material. The accompanying x-ray background can be minimized by utilizing the electron-beam current efficiently, by using the lowest electron energy consistent with the tumor thickness, and by selecting field-defining material of low atomic number. Low-megavolt electrons have been employed for treatment of extensive superficial disease. In many cases, particularly mycosis fungoides, the complete skin surface area has been treated. Technics developed (2–4) at the Massachusetts Institute of Technology, National Institutes of Health, and Stanford University School of Medicine are essentially the ones currently being used for treatment of large areas. In these treatments, typical daily doses involve an incident electronic charge of about 10−9 coulombs per square centimeter. To deliver such a dose in one minute would require a current of 1.7 × 10−11 amps/cm2; in 10 nano-seconds a current of 0.1 amps/cm2 is required. For treatment of less extensive superficial tumors a method has been developed to obtain uniformity over an area of 700 cm2. A low atomic number absorber shaped like the hub and spokes of a wagon wheel and placed midway between the exit window of the accelerator tube and the patient has been used to reduce electron flux in the central part of the beam so that the dose distribution with the aid of air scattering is quite uniform over a circle 30 cm in diameter. The energy of the electrons has not been degraded by use of scattering foils, and the x-ray background has been kept to a low level. Various monitoring devices have been used and considered which could be automated to switch off the electron beam at preselected dose levels. This method, for electron therapy with a linear accelerator, has been used by Dr. Martin Levene at the Peter Bent Brigham Hospital.