A TECHNIC OF EXPERIMENTAL DOSIMETRY FOR INTRACAVITARY RADIUM APPLICATORS.

Abstract

The attainment of experimental isodose curves around an intracavitary radium applicator is frequently time-consuming. To determine the complete dosage distribution, many individual measurements are usually made at discrete points around the radiation source. Their accuracy is dependent upon the stability, range of sensitivity, and actual physical size of the measuring device as well as upon the ability of the investigator to position the instrument accurately at the designated points. The radiation field around such a radium applicator might be more easily mapped by a technic utilizing the Potsaid-Irie radiation detector calibrated by lithium fluoride dosimeters. Potsaid-Irie Radiation Detector The Potsaid-Irie radiation detector consists of a paraffin-wax base containing chloroform and an acid-base indicator, methyl yellow (p-dimethyl aminoazobenzene). In 1928 Günther, von der Horst, and Cronheim (1) showed that ionizing radiation broke down chloroform-releasing free chlorine radicals. These in turn produced an acid in amounts directly proportional to the radiant energy absorbed. Methyl yellow can serve as an indicator for this reaction. In the presence of acid a quinoid structure is formed, changing the normal yellow color to red (2). Potsaid (3, 4) utilized this reaction in conjunction with a paraffin-wax matrix to produce a solid colorimetric radiation dosimeter. He reported that this dosimeter has a specific gravity of approximately 1.0, it has little or no energy dependence in the range from 50 to 2,000 kev, and it is not dependent on dose range. The red color changes produced by radiation are directly proportional to the absorbed dose. Method With this wax as a relative dosimeter, the isodose pattern around an Ernst intracavitary radium applicator was determined. Initially the radiation distribution in the median plane was of interest. To map this, a uniform layer of wax 3.0 mm. on either side of this plane was required. It was also necessary to enclose the Ernst applicator and the wax dosimeter in tissue-density material to duplicate the scattering encountered clinically. Both of these requirements were satisfied by the use of a special Lucite phantom (Fig. 1). This consisted of a 6.0-mm. deep machined Lucite tray accurately grooved to hold the Ernst applicator with its intrauterine tandem flush with the tray surface. Wax was poured into the tray to surround the applicator in the desired median plane. The tray was then covered with a second grooved Lucite block to complete the scattering effect. The fully loaded applicator was left in the phantom long enough to produce the desired color changes in the wax dosimeter. A total dose of 2,000 gamma r at Point B was usually satisfactory for this purpose. The wax layer was then removed from the phantom and photographed with transmitted light using Kodak commercial ortho film.