Depth Dose of Electrons from the Betatron

Electrons from radioactive substances or beta rays have been used for many years in radiation therapy. Because of their relatively low energy, the effects are confined to a depth of a few millimeters, and because of the distribution of energy the depth dose characteristics are similar to those of x-rays. With the development of the betatron by Kerst, the acceleration of electrons to very high energies became possible. Experiments to liberate a beam of electrons from the betatron, with the ultimate goal of using them for therapy, were successful at the University of Illinois in 1946. The advantage of using electrons for therapy is expected to lie principally in the depth distribution of dose. Quastler observed no qualitative difference between the biological effects of 20-Mev and 200-Kev x-rays. Quantitatively there is a dosage factor which is probably partially real and partially due to an energy variation in the response of the measuring instruments. The only physical difference between the secondary electrons of 20-Mev x-rays and the primary particles of the electron beam which might influence biological effect lies in the fact that in the former case the ends of the electron paths with their high specific ionization are more or less uniformly distributed throughout the depth, while in the latter case the ends of the electron paths are predominantly in the latter half of the range of penetration of the beam. However, the number of ions produced in the region of high density by very high-energy electrons is such a small fraction of the total ionization that it is to be expected that biological effects due to high ion density will be unobservable. The expected depth dose of an electron beam is quite unlike that of x-rays. The electron beam will penetrate only to a certain depth characteristic of the energy of the beam. Beyond this depth there will be a very weak residual ionization of practically constant intensity due to an x-ray background produced by the electron beam. The maximum depth of penetration or range of electrons in various materials is well understood, and an adequate theory is available. To a first approximation in the region of 10 to 40 Mev, the range in water in centimeters is equal to one-half the energy in Mev minus 0.5 cm. There are a number of factors affecting the shape of the depth distribution of ionization of an electron beam. Whenever a particle loses energy in discrete steps, as is the case when an electron produces ionization, the energy lost by particles which pass through a finite thickness of material will vary about a mean value. This will result in a variation in the position of the end of the range of the particle or a straggling of the range. If, however, the initial energy of the particle is very large compared to the energy lost in an individual encounter, then the straggling will be a very small fraction of the total range. The same conclusion is not true for radiative energy losses by electrons.