Radiation Dose to Patients and Relatives Incident to 131I Therapy

Radioiodine long has proven to be a safe and effective treatment for thyroid disease. Nonetheless, persisting concerns regarding radiogenic stochastic risks (e.g., carcinogenesis) to patients, their families, and the general public have led regulators to establish criteria for release of 131 I-containing patients from medical confinement, with limits 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-effectiveness of 131 I therapy, such regulations should be based on logical dosimetric considerations. The thyroidal absorbed dose, proportional to maximum uptake and effective half-life and inversely proportional to mass, is typically ∼1500 rad/mCi of 131 I administered to a euthyroid adult (based on a thyroid maximum uptake of 25%, effective half-life equivalent to the physical half-life of 131 I (8.04 days), and mass of 20 g). As thyroid uptake increases from 0% to 100%, extrathyroidal 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 and salivary glands; the absorbed doses to the urinary bladder wall, in contrast, decrease with increasing thyroid uptake, from 2 to 0.6 rad/mCi. In hyperthyroid patients (∼15%) with a small iodine pool (so-called small pool patients), the short effective half-life of radioiodine in the thyroid and high serum concentrations of long-lived protein-bound 131 I result in a standard 7000-rad absorbed dose for treatment of Graves' disease requiring an administered activity of 28 mCi of 131 I and yielding a prohibitively high blood absorbed dose of 150 rad. Importantly, once the fetal thyroid begins to function and accumulate radioiodine at a gestational age of 10-12 weeks, fetal thyroid absorbed doses as large as 5000 rad/mCi of 131 I administered to the mother can result. Thus, pregnancy is an absolute contraindication to administration of 131 I because of the risk of radiogenic cretinisim. Based on actual measurements of thyroidal activity and of external absorbed dose, the total thyroid and mean extrathyroidal absorbed doses to adult family members from immediately released 131 I-treated patients are ∼0.01 and ∼0.02 rad/mCi administered, respectively, yielding an effective dose of ∼0.02 rem/mCi. A maximum permissible effective dose of 0.5 rem for adults therefore is consistent with a release criterion of 30 mCi of retained 131 I. Lower-activity release criteria therefore m