First-day iodine kinetics is useful for individualizing radiation safety precautions for thyroid carcinoma patients

OBJECTIVE There is considerable variation in the national regulations of different countries for the release of patients from hospitals after radioiodine therapy. Individual variations make these practices, when based on the worst case scenarios, too restrictive for the majority of patients. However, there are cases in which strict rules are needed to comply with the dose limits to other individuals, especially children. We have developed a method to individualize radiation safety precautions. MATERIALS AND METHODS Twenty-three patients with differentiated thyroid carcinoma were included in the study. Four weeks after thyroidectomy, 1.1-3.7 GBq of radioiodine was administered and iodine kinetics were followed with external measurements until hospital discharge. The absorbed dose at the wrist holder was measured with thermoluminescence dosimetry (TLD) during hospital stay and after hospital discharge for up to 1 week. The TLD results were compared with the iodine kinetics. The dose to other individuals was estimated with extra TLDs located both on the patient's bed and given to family members. The kinetics data were fitted in both monoexponential and biexponential models and both for the full measurement period (down to the residual activity level<400 MBq) and for the first 24 h after radioiodine administration. RESULTS The biexponential model was capable of predicting the cumulated dose up to 1 week for both the longer and the shorter measured data set. The occupancy factors both for a person sleeping on the same bed and for a person living in the same apartment with the patient were in agreement with the recommended occupancy factor values of the American Thyroid Association. From these findings it is possible to individualize radiation safety precautions by taking into account the iodine pharmacokinetics and living conditions of a patient. CONCLUSION By measuring the activity content within the body for the first 24 h after radioiodine administration it is possible to individualize radiation safety precautions for thyroid carcinoma patients.