Preparation and use of standards for a comparison exercise among users of (131)I capsules in South Africa.

(131)I is widely used as a diagnostic radiopharmaceutical and for radiotherapy in the treatment of thyroid cancer and hyperthyroidism. Radionuclide/Dose calibrators are used to measure activities for administering to patients. To check the accuracy of calibrators operated by South African nuclear medicine facilities, comparison exercises were conducted among users of (131)I capsules. An (131)I solution was standardized by liquid scintillation counting using the absolute 4pibeta-gamma coincidence extrapolation technique. The standardization was transferred to the nml ionization chamber for the geometry of 3.6 ml of solution in a 5 ml glass ampoule. Using this calibration, an ionization chamber calibration factor was determined for (131)I capsules. Two comparison exercises were conducted, one year apart. For each one, four (131)I capsules were calibrated using the previously determined calibration factor. These capsule standards were sent to different provinces in South Africa where they were measured by all participating facilities. The results indicated that more than 90% of the participants showed deviations from the certified value of less than 10%.