Determination of half-value layer.

Abstract

In the determination of half-value layer, the geometry involved is usually described as “good” geometry or “poor” geometry. In general, these terms have reference to measurements made under narrow-beam and broad-beam conditions. Such conditions have not been too well explored. Narrow-beam determinations can always be made under laboratory conditions, but such conditions may not lend themselves to measurement technics in a working therapy installation, where restrictions may be imposed by equipment limitations on filters and diaphragms. The object of our investigation is the derivation of information that might be used to correct a half-value layer determination made under generally available conditions to some common denominator that could be described as narrow-beam conditions. We are presenting here the first phase of a study, the full report of which will cover the variables involved and their influence on the resulting determination. It has been found, as had been expected, that the irradiated area of the filter which is added to the treatment filter has a major influence on the determination. An effort to plot half-value layer against filter area does not yield a straight line and extrapolation to zero area therefore becomes difficult. It has been found that half-value layer is a linear function of the diameter of the measuring filter. Data so plotted are presented in Figure 1. To provide readily usable data, it has been found advisable to plot relative half-value layer against filter diameter The relative half-value layer is obtained by dividing the half-value layer for a given filter diameter by the half-value layer obtained by extrapolation to zero diameter. The half-value layer for zero diameter is taken from a plot similar to Figure 1. A plot of values obtained for a series of voltages and treatment filters is shown in Figure 2. By means of such a plot it becomes possible to take data under usual working conditions and from them arrive at the half-value layer that would have been obtained for any other set of conditions. A similar system for square fields is in cource of development. In the interests of providing widespread agreement as to what constitutes an acceptable half-value layer determination, it appears desirable that some group, preferably the International Commission on Radiological Units and Measurements, should establish some common denominator to which all half-value layer measurements might be reduced. In a subsequent report, reference will be made to an exploration of all the variables influencing half-value layer determinations over a voltage range from 100 kvp to 2,000 kvp and the usual range of treatment filters used at these voltages.