Fluorod Dosimetry Close to Radium and Cobalt-60 Sources

The measurement of dose rates at points close to radium needles has always been difficult to achieve because of the large size of the measuring instruments. Calculated dose distributions have been pragmatically confirmed over a period of twenty-five years but have not been satisfactorily checked by experiments. The fluorod (1), because of its small size, permits experimental corroboration of calculated dose distributions close to radium needles. By virtue of this, measurements can be made with 5 per cent accuracy as close as 1 mm. from the surface of any source; the radiation field can be mapped in detail; and measured isodose curves can be constructed around discrete radioactive sources. Experimental Procedures Phantom for Exposure of Fluorods To describe adequately the dose distribution around a radium needle, measurements should be made at many discrete points. Figure 1 illustrates the location of 156 points around a radium needle of 2 cm. active length. Inside the first 5 mm. from the axis of the radium needle the rows of points are 1 mm. apart. The nearest row is situated 2 mm. from the central axis of the needle (or approximately 1 mm. from its surface). Nineteen fluorods are positioned in each row at 2.5-mm. intervals. In order to position the fluorods at the designated points, a phantom was constructed of Lucite (Fig. 2). Since the diameter of the 6-mm.-long fluorod is 1 mm., it was necessary to place every other line of fluorods on the opposite side of the radium needle. This provided sufficient space to accommodate the great number of fluorods and furthermore reduced gamma-ray absorption by the glass. The radium needle was immersed in an accurately machined groove whose depth was half the diameter of the needle; consequently the central axis of the radium source coincided with the surface plane of the Lucite phantom. To hold the fluorods, holes 1 mm. in diameter were drilled into the Lucite for a depth of 3 mm. (No. 60 drill). The fluorods were now rigidly and precisely accommodated perpendicular to the surface so that their geometric centers were in the same plane as the axis of the radium source. Figure 3 illustrates the loaded phantom. During exposure this was completely immersed in a large water bath at 37°C. Distilled water was used to minimize surface contamination of the fluorods. The water bath duplicated the scatter effects existing under therapeutic conditions and eliminated any possible temperature dependence of the fluorods. Accuracy of Fluorod Measurements An accuracy of 5 per cent in radiation dosimetry with fluorods can be achieved, though some of the following variables may contribute to the observed spread of fluorod measurements: weight of each fluorod, silver ion concentration in the glass, background fluorescence, surface contamination, fading, fluorod position in the fluorimeter, and sensitivity changes of the reading instrument.