Computer Evaluation of Dose Rates Around Multiple Radium Sources

Abstract

Two programs have been developed to determine the dose-rate distribution resulting from an arbitrary array of radium sources. Both programs have been written for an IBM 1620 Data Processing System with 20 K memory and with the automatic division and indirect addressing special features. The constants incorporated in the programs pertain to radium sources of 1 cm. active length with 1 mm. Pt filter, and to any number of such sources up to 6. The constants and the number limitation may be changed rather easily. The first program is written in FORTRAN, the second in SPS. Either program may be used to calculate the dose rate at selected points (entered on punched cards) or at an automatically indexed grid of points (horizontal or vertical plane). For these alternatives, the computer yields a tabular print-out of the coordinates and of the corresponding total dose rates (r/hr.). An additional alternative is available with the SPS program, that of a graphic print-out (again, horizontal or vertical plane). Here the typed output page is an array of points characterized by specific letters which have been assigned to predetermined dose-rate levels. Sketching connections through like letters gives all isodose chart for the distribution. For each patient, as many input cards are required as there are sources. The following information is required on each card: measured angles and relative spatial position of the source (taken directly from anteroposterior and lateral roentgenograms), the estimated magnification of each roentgenogram, the activity of the source, and indexing numbers to distinguish the sources. Information such as patient name and number, date, and number of the application, if included on the first card, will be repeated in the output as the first line typed. The strength of the several sources will also be repeated in the output headings. The method of calculation is the same in both programs. Greenfield et al. (1) have recalculated dose-rate tables for linear radium sources. These data have been revised to the present value for the emission constant of radium and then reduced to analytical form. Six zones of polar angle (measured from the axis of a source) suffice to describe the tabular data in the analytical form chosen. The basic form is common throughout all zones; the constants selected are specific to each zone, and in three a calculated correction is also included. With such selection, the dose rate at a point is evaluated for each source in turn and accumulated for output presentation. The time required depends on the number of sources and the number of points required. A typical gynecological application (5 sources) requires about fifteen minutes for tabulation or plotting of the results for one plane covering an area 16 × 16 cm. Upon request, arrangements can be made to duplicate the programs.