Measurement of Effective Half-life of Iodine-131 to Optimize Safety Release Instructions after Treatment for Patients with Differentiated Thyroid Cancer

Iodine-131 ( 131 I) is a common therapy for treatment of differentiated thyroid carcinoma (DTC); however, its radioactivity poses a radiation safety risk to public health. There is inter-facility variation in release instructions to minimize incident exposure to other individuals. Isolation measures are not without harm. Most studies on this topic rely upon cumulative dosimetry to measure exposure, but this does not provide the researcher with critical dose protraction information. Refining estimation of elimination kinetics with more frequent exposure readings would help optimize radiation safety recommendations. Measuring radiation exposure from patients with DTC post-thyroidectomy receiving 131 I would better quantify its elimination kinetics to improve radiation safety recommendations. Patients with DTC post-thyroidectomy undergoing radioiodine remnant ablation with 131 I were instructed to measure exposure at a distance of 1 m, three times a day for 14 d, using an ion chamber at home. These data were used to form an exponential decay model and estimate the time after which cumulative exposure is below a reasonably low threshold. The average effective half-life was 15.8 h when calculated using real-time exposure readings from 32 patients. Among patients administered less than 4.22 GBq, cumulative effective dose is ≤1 mSv after 24 h of isolation. Between 4.22 and 6.03 GBq, cumulative effective dose is ≤1mSv after 48 h of isolation. Cumulative gamma radiation exposure at 1 m remains low enough to consider re-evaluating isolation protocols that encourage long-term distancing past the first 24 h in post-thyroidectomy patients treated with 131I for remnant ablation.