Dosage tables for linear radium sources filtered by 0.5 and 1.0 mm. of platinum.

Abstract

The dosage tables for linear radium sources filtered by 0.5 mm. platinum computed by Quimby (1) continue to be extremely useful in the planning of radium therapy. No corresponding table exists, however, for linear sources filtered by 1.0 mm. of platinum, which are also commonly used in radium therapy. We have computed such a table, therefore, using Sievert's integrals (2). An extensive check of the table by application of the principles of symmetry and similitude (see below) shows that the computed dose rates are internally consistent to about 1 per cent. A similar check of Quimby's table (1) reveals inconsistencies of the order of 10 per cent. To remove these inconsistencies we have also computed a dosage table for linear radium sources filtered by 0.5 mm. platinum. The tables are presented in the same form as that used by Quimby. Readers are referred to her paper (1) or to the standard text of Glasser et al. (3) for instructions as to their use. Method The tables were computed by linear interpolation from Sievert's integrals. Absorption coefficients used were 0.15 cm.−1 for radium sulfate, 2.00 cm.−1 for 0.5 mm. platinum and 1.94 cm.−1 for 1.0 mm. platinum (2). The radium was assumed to be uniformly distributed on a thin line at the center of the needle. The thickness of the radium salt was taken as the difference between the measured needle thickness and the thickness of the platinum filter. For 1.0 mm. and 0.5 mm. Pt, the radium sulfate thicknesses were, respectively, 1.5 and 0.65 mm. A value of 9.33 r∕mg.-hr. was taken as the dose rate 1.0 cm. from an unfiltered source of radium. This value is consistent with the figure of 8.4 r∕mg.-hr. from a point source of radium filtered by 0.5 mm. of platinum and is in satisfactory agreement with both calculated and experimental values (4). The scattering and absorption of the gamma rays in tissue was ignored. A thorough investigation of the tables applying principles of symmetry and similitude (see below) indicates an internal consistency of 1.0 per cent or better throughout. At least half the entries were checked in this way. In addition, all entries were checked graphically. Assuming the validity of the basic assumptions, the accuracy is about 1 per cent for those regions in which the dose rate is in excess of 0.2 r∕mg.-hr. In the regions where the dose rates are smaller, Sievert's integrals lead to small differences in two large numbers, so that the inaccuracies are somewhat larger. Fortunately, in the low dose rate regions far from the needles, high degrees of accuracy are not necessary. The symmetry principle is readily demonstrated by the following example: the dose rate from the center of a 2.0-cm. needle should be exactly the same as the intensity from the end of a 1.0-cm. needle with the same total radium content.