The Effective Attenuation in Water of the Gamma Rays of Gold 198, Iridium 192, Cesium 137, Radium 226, and Cobalt 60

Abstract

The absorption and scatter of radiation in tissue alter the dose distribution around interstitial and intracavitary radioactive sources. Within the treated volume of an interstitial implant, the presence of tissue may cause the dose to vary as much as ±3 per cent from that anticipated with the same geometry in air. At more distant points, such as at the pelvic wall in the treatment for uterine cervix by intracavitary radiation sources, the absorption in tissue may reduce the dose by 10 to 15 per cent. With manual methods of calculation, the absorption and scatter corrections are usually ignored, but computer methods which have become available in the last few years usually include these corrections. There have been a number of determinations of the attenuation in water of the gamma rays of gold 198, iridium 192, cesium 137, radium 226 (in equilibrium with daughter products), and cobalt 60 (1–7). Calculations presented here, based upon the theoretical consideration of the diffusion of gamma rays in water by Goldstein and Wilkins (8) and by Berger (9), have yielded results in fair agreement with experiments and calculations by Loevinger (10). In the experiments and calculations reported here, the effective absorption for the above-mentioned isotopes was determined. These sources are most widely used in interstitial and intracavitary therapy. Experimental Data A few measurements on gold-198 gamma rays were made with a small ionization chamber. These measurements were taken to check the response of the anthracene crystal detector used throughout the experiment. The measurements were made with a cylindrical anthracene crystal 3.0 mm in diameter and 3.0 mm in length. The crystal was coupled to a 1P-28 photomultiplier tube by a long Lucite light pipe. The total photomultiplier tube current was measured without discrimination by the apparatus shown in Figure 1, using the various sources described in Table I. Small corrections for dark current from the phototube, the effect of radiation on the photocathode, and the light generated in the light pipe by Cerenkov radiation were determined for each experimental situation by measurements made with the crystal removed. The sources and detector were positioned in a jig at increments of 1 cm to a maximum of 10 cm source-detector distance. The detector and source were mounted inside a Lucite water tank 25 × 25 × 30 cm from which water could be added or withdrawn into a reservoir without disturbing the source-detector geometry. All measurements in air were made with a Lucite cap on the detector to provide full electronic buildup and to prevent the electrons generated in the source container from reaching the detector. The method of determining the proper thickness of the cap and correcting gamma attenuation in the cap was similar to that used by Wootton et al. (3).