Radiation safety precautions in 131I therapy of Graves' disease based on actual biokinetic measurements

Abstract

Context: Radiation protection is an integral part of targeted radionuclide therapy. How to offer rational radiation precautions to patients with Graves' disease (GD) undergoing 131I therapy is still a matter of ongoing discussions. Objective: The objective of the study was to formulate radiation precautions for GD patients undergoing 131I therapy through actual biokinetic measurements for a particular population of patients. Design: This was a prospective study. Setting: The study was conducted at a university hospital. Patients: From January 2009 through January 2012, consecutive GD patients prepared for 131I therapy were prospectively recruited. Main Outcome Measures: Pretherapy thyroid radioiodine uptake and uptake ratio (4 to 24 h radioiodine uptake) were measured. Serial whole-body dose-rate measurements after therapy were performed to deduce 131I whole-body retention. Calculations based on deduced whole-body retention and measured thyroid radioiodine biokinetics were derived to determine the thyroidal and extrathyroidal compartment uptake fractions and effective half-lives. Precaution times necessary to avoid close contact with family members and the general public were derived from these parameters and regulatory dose limits. Results: A total of 72 patients were eligible for the analysis. A high interpatient variability in 131I biokinetics was observed: the mean peaking 131I uptake (±1 SD) in the thyroid was 68% (±19%), and the range was 18%-89%; the mean effective 131I half-life (±1 SD) in the remainder of the body was 5.1 (±0.9) hours (range 3.5-7.2 h). The mean measured initial dose rate (±1 SD) at 1.0 m after 131I administration was 0.039 (±0.003) μSv·h-1·MBq-1 (range 0.017-0.055 μSv·h-1·MBq-1). The 0.3:1.0 m initial dose rate ranged from 2.9 to 7.1, which was greatly lower than the projected ratio of 11.1 by the inverse square law approximation. On the basis of the measured radioiodine biokinetics and dose rates, detailed instructions were provided to limit nearby individuals' exposure. Conclusion: The use of actual biokinetic measurements may remove the effect of variability errors associated with general default assumptions about the 131I biokinetics in GD patients. The marked variability in 131I biokinetics among GD patients reinforces the need for patient-specific iodine biokinetic measurements for radiation safety precautions.