Whole-body effective half-life of radioiodine in children and young adults with papillary thyroid cancer.

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The lack of radioiodine-131 (RAI) kinetic study is a serious challenge for rational dosing for children and young adults (CYAs) with papillary thyroid cancer (PTC). The present study was conducted to investigate the whole-body effective half-life (EHL) and absorbed dose in RAI ablative therapy of CYAs with PTC. In the period 2017-2022, all consecutive PTC patients 20 years or younger prepared for ablative RAI therapy after thyroid hormone withdrawal were prospectively recruited. Serial whole-body dose-rate measurements after administration were performed to deduce whole-body RAI retention. Calculations based on the deduced whole-body retention and the schema of Medical Internal Radiation Dosimetry were derived to determine whole-body EHL and absorbed doses. A multivariate linear regression analysis was employed to assess the association between whole-body EHL and potential predictors. A total of 52 patients (median age 17 years [range, 6-20 years]) were recruited. The mean whole-body EHL (±SD) was 10.3 (3.3) hours (median, 9.4 h [range, 6.3-21.7 h]). In univariable linear regression analysis, whole-body EHL was significantly associated with gender, body surface area (BSA) and body mass index (p < 0.05). Creatinine, Cystatin C, glomerular filtration rate (GFR) and positive post-ablation scintigraphy approached significance with respect to EHL (p ≤ 0.2 and ≥0.05). At multivariable analysis, BSA, GFR and positive post-ablation scintigraphy was associated with EHL. A median activity of 3.7 GBq of RAI (range, 1.85-7.40 GBq) was administered and a median whole-body absorbed dose of 0.22 Gy was delivered (range, 0.11-0.79 Gy). A wide variation of whole-body EHL was observed in CYAs with PTC treated with RAI. The whole-body EHL is significantly longer in CYAs with larger BSA, decreased GFR and presence of extra-thyroidal disease. Understanding these predictors may improve our ability to dosing strategies in RAI therapy of CYAs with PTC.