Skin Uptake of 131I in a Veterinary Practice

Accidental trace 131I skin contamination resulted in an intake to a veterinary nuclear medicine technician at Colorado State University (CSU). The resulting dose to the technician was determined through an in-vivo measurement of radioactivity in the thyroid. Inhalation and ingestion of the isotope were ruled out due to the chemical nature of the 131I, as determined by a previous study. The CSU Radiation Control Office performed measurements of the technician's thyroid daily and then weekly to quantify the uptake [2.2 kBq (60.1 nCi)] as well as the effective half-life of 131I (7.61 days) for the technician. Establishing a technician-specific effective half-life by graphing in-vivo measurements also showed near agreement with established effective half-life determinations specified in ICRP 30 for inhalation. The university assessed the committed dose equivalent (CDE) to the thyroid using three different methods and assigned a the highest CDE calculated (4.1 mSv), and an SDE of 0.22 mSv from external contamination. The average of the CDE calculated via each method was 3.68 ± 0.38 mSv. A comparison of the 3 thyroid dose calculation methods resulted in a 10% coefficient of variation. Close agreement between the various calculations demonstrates that any of these methods would be sufficient in determining a committed effective dose to the thyroid, while providing a level of confidence to the technician that the determination is accurate and appropriate. A key aspect of this report is how even trace amounts of radioiodine on intact skin can result in measurable doses.