Potential Errors Caused by Variable Radionuclidic Purity of Iodine-123

The commercial accelerator production of iodine-123 in the United States has changed from the Te-122 (d,n) 1-123 reaction to the Te-124 (p,2n) 1-123 reaction-and a greater amount of 1-124 contamination results. This impurity causes only slight elevation in thyroid uptake value when counted 24 hr after calibration time of the capsule, but a significant elevation in uptake value 48 hr after calibration time (when 1-123 is administered 24 hr after calibration time). There is no image degradation on 24and 48hr scintigraphs obtained with a pinhole collimator. A significant time-dependent change in potentiometer settings on dose calibrators is required to assay the capsules.