Replacement corrections of a Farmer-type ionization chamber for the calibration of Cs-137 and Ir-192 sources in a solid phantom.

Calibrating Cs-137 and Ir-192 brachytherapy sources in a solid phantom has the advantage over calibration in air that the positioning of the sources is very accurate and straightforward. In order to determine the air kerma rate at the point of measurement it is, however, necessary to take the replacement of the phantom material by the ionization chamber into account. The replacement correction factor pr of a Farmer-type ionization chamber has been determined for a few types of 137Cs and 192Ir sources at a source to chamber distance of 5 cm. For spherical 137Cs sources the replacement correction was determined by means of measurements with chambers with decreasing diameter and length. Additional measurements were performed for some other source configurations in order to determine pr for 137Cs micro-seed trains. For an 192Ir-HDR source pr was determined for a source chamber configuration equal to that for spherical 137Cs sources by comparing measurements in-phantom with measurements free in-air. Finally, measurements were performed with source configurations that yielded pr values for 192Ir seed trains and 192Ir wires. The resulting replacement correction factors are all within +/- 2% of unity. It can be concluded that, although the dose nonuniformity over the chamber volume caused by each source is rather substantial, the replacement correction that has to be applied is rather small.