Photographic representation of isodose patterns by isodensity tracing. A preliminary report.

Abstract

A definitive method for representing isodose patterns around radioactive sources has been sought for many years. Tsien and Robbins (1) used the Sabattier effect (photographic image reversal phenomenon) with some success. Bogardus, White, and Powers (2) and Hertsch and Maris (3) described electromechanical devices to scan films and print isodensity contours. Tochilin (4) used densitometry for measuring radiation fields surrounding multiple radium sources. Miller, Parsons, and Kofsky (5) used microdensitometer scanning technics to examine density patterns on industrial and medical radiographs, photographic negatives, and even biological specimens. They describe density contour mapping, using an automatic “isodensitracer”2 (IDT). The isodensity tracer is designed to scan a transparency and record the photodensity distribution. The recording pen changes its symbol whenever the density (or object brightness) changes by a small preselected density increment. As the density along a scan decreases, the pen prints a solid line until the limit of this discrete density increment is reached. For the next density increment, a series of equally spaced dots are printed. A blank space is used for the third increment, and the pattern, line-dots-blank, is repeated as long as the density is decreasing. For increasing density, the pattern is reversed (i.e., blank-dots-line). From a series of successive parallel scans, one can quickly visualize an isodensity contour map. Hilger (6) suggested this method as ideal for studying the isodose distribution patterns around radiotherapy applicators and sources. The first example shows an IDT contour map of one side of an Ernst applicator containing a total of 35 mg of radium (Fig. 1). A 5 × 7-in. sheet of Kodak industrial Type M film was placed along the mid-plane and exposed for sixty minutes. A “cut-out” was made in the film for the applicator arm. Both the film and “cut-out” were processed, using standard technics. The “cut-out” was replaced in the film to eliminate edge effects for scans over this area. Figure 2 shows two isodensity tracings from an autoradiograph made with a 3 mg radium needle. Even though the source is unevenly loaded (Fig. 2, B), the IDT contours show that the irregularities along the length of the source cannot be detected beyond 4 to 5 mm. More interesting, however, is the amount of detail obtained by this method. One needs only to vary the IDT settings to view details adjacent to or at a particular distance from the source. Since this procedure gives a more detailed presentation than any other currently known, the IDT can be employed in conjunction with dosimetry methods for routine quality control of new sources. Other possible uses include the plotting of multi-source implant patterns, making comparisons with computer data, correlating experimental data with existing tables, and comparing the isodose patterns of various radionuclides.