131I diagnosis and therapy capsules and low dose rate 125I and 137Cs brachytherapy sources. Calibration of the NPL secondary standard radionuclide calibrator

Abstract

The radionuclide 131I has important thyroid-related diagnostic and therapeutic applications. The response of the NPL secondary standard radionuclide calibrator has been measured for commercially available 131I capsules used in these applications. The capsules were assayed in the secondary standard ionization chamber using an appropriate jig. They were then dissolved to enable the activity to be accurately measured. A calibration figure relating ionization chamber current to capsule activity was thus derived. As a result of this work, the user has a simple yet accurate method of measuring source activity immediately prior to administration. The saturation characteristics of the ionization chamber have been considered. Treatment of disease by the use of brachytherapy sources is a well established technique. Recent incidents together with an increased awareness of the importance of quality assurance has led to the requirement for a simple but accurate procedure for the measurement of dose rates from such sources. Using a simple holder, the responses of the NPL secondary standard radionuclide calibrator to a range of commercially available brachytherapy sources of 125I and 137Cs have been determined, providing the user with a convenient, traceable in-house calibration facility that meets the necessary accuracy requirements.