On Depth Doses of X-Rays

Abstract

When a divergent beam of X-rays passes through matter, the intensity of the beam diminishes by reason of its divergence and by its absorption in the medium. The laws governing this loss of energy of the primary beam are well known, but when they are applied to a practical case such as occurs daily in therapeutic practice, the calculated dose at a given depth is, always, less than that measured by an ionisation meter. This is due to scattered radiation produced in the medium which partially compensates for the loss by absorption and divergence. The amount of scattered radiation varies considerably with the conditions of irradiation and the difficulty has been to asses the effect of scattering in the variety of conditions used in clinical practice. It is this problem with which we are concerned. From earlier depth dose measurements in water made by Rees and Clark1, it was found that the depth dose could be expressed in the form: ID=Ip (1+B+AD)........................ (1) where Ip is the ionisation due to the primary rays at a depth D, and A and B are constants depending upon the conditions of irradiation. In these experiments there was always a back-scattering layer of water of constant thickness beneath the measuring instrument. We have since carried out depth dose measurements in wax, with and without back scatter, and have investigated the influence of scattered radiation upon the depth dose in each case.