Skin dose from a cobalt 60 teletherapy unit.

Abstract

Because of its physical characteristics, high-energy radiation offers several advantages over radiation of lower energy in the treatment of deep-seated tumors. These are: (a) an increase in percentage depth dose, (b) a decrease in side-scatter, resulting in flatter isodose curves with a smaller penumbra, and (c) the establishment of electronic equilibrium at a greater depth, resulting in a smaller skin dose. With the Co60 unit under test at this laboratory, it was found that little could be done to affect (a) or (b), with the exception of a moderate dependence of penumbra on cone design. For short cone-to-skin distances, the third advantage is greatly affected by electrons scattered from the walls of the collimating system. This report is a study of the electron build-up region and the optimum cone design necessary to preserve the third advantage. When gamma rays enter tissue, they lose energy through interactions, and accelerated electrons are produced. These electrons in turn lose their energy as they penetrate tissue by radiation processes or by removing other electrons from their atoms (ionization). If the initial electron acquires a high energy from the gamma ray, thousands of electrons and positive ions are produced by this one electron along its path, since only a small amount of energy (about 32.5 ev) is lost in producing each ion pair. The accelerated electrons penetrate the tissue to an average depth corresponding to the energies given the electrons by the gamma rays. If the beam of gamma rays is completely free of electrons before entering the tissue and if there are no back-scattered gamma rays or electrons, the ionization at the surface will be zero. When the gamma rays enter the tissue, accelerated electrons are produced, and at Co60 energies the acceleration is predominantly in a forward direction. As the tissue is penetrated, additional electrons are accelerated with each increment of depth. Thus the number of electrons, and consequently the ionization, builds up with depth. This build-up progresses until a depth corresponding to the average range of the electrons is reached. At this depth, equilibrium has been established between the primary and secondary radiation, and the maximum number of electrons that can contribute to the dosage has been achieved. Beyond this depth, the number of gamma rays per unit area decreases due to the divergence of the beam and to interactions in the preceding layers of tissue. Hence, with further increase in depth, the ionization decreases relatively slowly. There are two other sources of radiation usually present which add to the ionization in the electron build-up region. The first is back-scattered gamma rays which produce additional ionization at all depths up to and including the surface. The second is electrons (from the collimating system or from air) that contaminate the gamma-ray beam. Several workers have studied the phenomenon of electron build-up.