Impact of radiocontaminants in commercially available iodine-123: dosimetric evaluation

Iodine-123 (123I) is considered by some to be the radionuclide of choice for thyroid scintigraphy because of its ideal physical and biological characteristics and low radiation absorbed dose to the thyroid. However, commercially available 123I (p,2n) and (p,5n) have radiocontaminants. The MIRD formalism was used to estimate the absorbed dose to the thyroid for various age groups receiving recommended administered activities at the time of delivery and at two half-lives assuming radiocontamination levels specified by the suppliers. The calculations demonstrate that an 131I uptake with a technetium-99m scan at the time of delivery results in less absorbed dose to the thyroid than an 123I (p,2n) scan and uptake. At two half-lives the absorbed dose triples and becomes equivalent to the dose from an 131I scan. The absorbed dose from an 123I (p,5n) scan at two half-lives is higher than that of an 123I (p,2n) scan at the time of delivery. Iodine-123 capsules should not be decayed down in order to obtain a recommended pediatric administered activity. There appears to be no dosimetric advantage of commercially available 123I for thyroid scintigraphy for adults or most children.