Age-dependent thyroid absorbed doses for radiobiologically significant radioisotopes of iodine

In light of the post-Chernobyl increase in pediatric thyroid cancer incidence, among other recent events, there is renewed interest in radioiodine thyroid dosimetry and effects. Among the radioiodines produced in fission of 235U, only 131I [(T1/2)p = 8.04 d], 132I (2.3 h), 133I (20.3 h), and 135I (6.7 h) may undergo significant environmental dispersion. Age-dependent thyroid absorbed dose estimates for these radiobiologically significant radioiodines and for the "medical" radioisotopes 123I (13.2 h) and 125I (60 d) have been derived, incorporating the effect of absorption following inhalation or ingestion. This effect has generally been ignored in previously derived estimates of radioiodine absorbed doses to the thyroid. Based on the latest ICRP lung and gut models, inhaled radioiodine is absorbed at a rate of 0.175 h(-1) and exhaled at 0.101 to 0.118 h(-1) (depending on age) and ingested radioiodine is completely absorbed in the stomach at a rate of 1 h(-1). Whole-body compartmental models (SAAM II) were fit to previously published 24-h thyroid uptakes, thyroid half-times, and 48-h plasma concentration of protein-bound iodine. The resulting fitted models were used to calculate thyroid residence times of radioiodine. The mean thyroid absorbed doses [cGy/37 kBq (rad/microCi) injected intravenously] were then calculated using the age-dependent S(thyroid<--thyroid) factors (MIRDOSE III), with the highest doses (from 0.49 for 123I to 36 for 131I) in newborns and the lowest doses (from 0.014 for 123I to 1.4 for 131I) in adults in inverse relation to the thyroid mass. Although the thyroid absorbed dose for inhalation is substantially (30 to 70%) less than that for injection for all radioiodines and at all ages, it is markedly (25%) less for ingestion only for short-lived 132I.