Half-Value Depth and Fall-Off Ratio as Functions of Portal Area, Target-Skin Distance, and Half-Value Layer

Abstract

The purpose of this communication is to present, in convenient form, values for the half-value depths and fall-off ratios corresponding to the entire range of radiation qualities, portal areas, and target-skin distances likely to be encountered in clinical roentgen therapy. Compilation of these data is believed to be useful for several reasons. First, these parameters have proved extremely useful in specifying superficial radiation beams, and it is thought that a graphic comparison of the data will facilitate treatment planning for superficial lesions. Second, though these values have seldom been considered in the planning of intermediate or deep therapy, they do provide, in some measure at least, an index of the distribution of radiation of higher energy in tissue. It is believed, therefore, that the data will enable the student to visualize readily the effect of changing kilovoltage, external filter, portal size, and target-skin distance on the distribution of radiation within a patient's tissues. Third, interest in the calculation of the integral dose (7) delivered in roentgen diagnosis and therapy is increasing, because of its apparent correlation with the systemic effects of irradiation. Among the factors required for the calculation of the integral dose is the half-value depth of the incident radiation. The data presented here should therefore be of aid in this computation. Definitions Half-value depth, or D1∕2, is the depth in tissue at which the radiation dose equals 50 per cent of the surface dose. It is thus a clinical expression of the penetrating power of a specified radiation beam. Similarly, D9∕10 is the depth in tissue at which the radiation dose equals 10 per cent of the surface dose. Fall-off ratio, or F.O.R., is defined as the D9∕10 of a specified radiation beam divided by its D1∕2. The fall-off ratio is, therefore, a clinical expression of the absorption rate of the specified radiation in tissue. The Data 1. Range of Data: Data are presented for half-value layers of 1 to 8 mm. Al and 0.4 to 15 mm. Cu, for portal areas varying from 5 to 400 sq. cm., and for target-skin distances commonly employed in this clinic. In addition, data applying specifically to the Philips and Chaoul contact-therapy units, and to a beryllium-window x-ray tube (Machlett O.E.G.-60) superficial-therapy unit are presented for half-value layers, portal areas, and target-skin distances appropriate to these generators. 2. Sources of Data: The data presented here for half-value layers of 1 mm. Al through 4 mm. Cu are all based on depth-dose tables published in Physical Foundations of Radiology (13). These tables were chosen as a basis because of their almost universal acceptance in clinical roentgen therapy in America.