SU-FF-T-136: Design of a Jig for Thermoluminescence Dosimetry of Brachytherapy Sources in Liquid Water and the Determination of a Correction Factor for Water-Equivalent Plastics

Abstract

Purpose: To develop a method for measuring the characteristics of brachytherapy sources in water, rather than in water-equivalent plastics, and to use this method to determine the correction factor for water-equivalent plastic. Method and Materials: Small thermoluminescence dosimeter (TLD) capsules were constructed from capillary tubes to hold 14 mg of lithium fluoride powder. Plastic jigs were designed to hold the capsules in circular pattern around a brachytherapy source, or in a spiral pattern radiating away from the source. The radioactive source was mounted on the tip of a thin graphite rod with its long axis either parallel or perpendicular to the TLD pattern. The jigs were placed in a water phantom to enable measurement of all TG-43 parameters. A Solid Water™ phantom was constructed to hold the TLD capsules in exactly the same circular pattern around the source. TLD measurements were made in water and Solid Water™ at 1.00 cm distance from a model 6711 source to determine a correction factor for the Solid Water™. Similar measurements were also made with a model CS-1 source. Results: The measured correction factor for Solid Water™ was 1.05 +− 0.02 at a distance of 1.00 cm from the model 6711 source. This value is in good agreement with a Monte Carlo-based value published previously. Similar measurements with the model CS-1 source produced the same result within experimental uncertainties. Discussion: The preferred medium for therapy dose measurements is liquid water. However, dosimetry measurements reported in the literature are limited to water-equivalent plastics. The correction factor for plastic phantoms is based on Monte Carlo calculations, and must be validated by actual measurements.