Establishing a threshold for 131I bioassay in nuclear medicine personnel

Since the late 1970's, manufacturers in nuclear medicine have reformulated the I solution to reduce the volatility of the iodine. There has also been an increase in use of the iodide in encapsulated form. Per the requirement of the current U.S. Nuclear Regulatory Commission (U.S. NRC) regulation, with the available results on the volatility of the reformulated radioiodine, we review the I bioassay program for nuclear medicine workers. Our analysis shows the threshold quantity for bioassay monitoring for the routine use of I in nuclear medicine is much higher than the criteria set in U.S. NRC Regulatory Guide 8.20. The latter is a broad bioassay guideline for the general usage of radioactive iodine. For treatment of thyroid carcinoma and hyperthyroidism, a single therapeutic I dose large enough to yield a detectable thyroid burden is very unlikely to occur in a nuclear medicine clinic. Accidental ingestion or inhalation would be an exception to our conclusion. Based on this analysis, we propose a new bioassay policy for the routine use of I in nuclear medicine clinics.