RADIATION-DOSE TO PATIENTS AND RELATIVES INCIDENT TO I-131 THERAPY

Radioiodine long has proven to be a safe and effective treatment for t hyroid disease. Nonetheless, persisting concerns regarding radiogenic stochastic risks (e.g., carcinogenesis) to patients, their families, a nd the general public have led regulators to establish criteria for re lease of I-131-containing patients from medical confinement, with limi ts ranging from as low as 2 mCi in some parts of Europe to as high as 30 mCi in the United States. To optimize clinical efficacy and cost-ef fectiveness of I-131 therapy, such regulations should be based on logi cal dosimetric considerations. The thyroidal absorbed dose, proportion al to maximum uptake and effective half-life and inversely proportiona l to mass, is typically similar to 1500 rad/mCi of I-131 administered to a euthyroid adult (based on a thyroid maximum uptake of 25%, effect ive half-life equivalent to the physical half-life of I-131 (8.04 days ), and mass of 20 g). As thyroid uptake increases from 0% to 100%, ext rathyroidal absorbed doses range from a minimum of 0.15 to 0.5 rad/mCi for breast and gonads to a maximum of 1.5 to 2 rad/mCi for stomach an d salivary glands; the absorbed doses to the urinary bladder wall, in contrast, decrease with increasing thyroid uptake, from 2 to 0.6 rad/m Ci. In hyperthyroid patients (similar to 15%) with a small iodine pool (so-called small pool patients), the short effective half-life of rad ioiodine in the thyroid and high serum concentrations of long-lived pr otein-bound I-131 re sult in a standard 7000-rad absorbed dose for tre atment of Graves' disease requiring an administered activity of 28 mCi of I-131 and yielding a prohibitively high blood absorbed dose of 150 rad. Importantly, once the fetal thyroid begins to function and accum ulate radioiodine at a gestational age of 10-12 weeks, fetal thyroid a bsorbed doses as large as 5000 rad/mCi of I-131 administered to the mo ther can result. Thus, pregnancy is an absolute contraindication to ad ministration of I-131 because of the risk of radiogenic cretinisim. Ba sed on actual measurements of thyroidal activity and of external absor bed dose, the total thyroid and mean extrathyroidal absorbed doses to adult family members from immediately released I-131-treated patients are similar to 0.01 and similar to 0.02 rad/mCi administered, respecti vely, yielding an effective dose of similar to 0.02 rem/mCi. A maximum permissible effective dose of 0.5 rem for adults therefore is consist ent with a release criterion of 30 mCi of retained I-131. Lower-activi ty release criteria therefore may be unnecessarily restrictive.