Recommended restrictions after 131I therapy: measured doses in family members.

Absorbed doses to family members of patients treated with (131)I were measured using thermoluminescent dosimeters worn on the chest. Twenty-two patients with thyroid cancer were hospitalized for 2 d for treatment with 3,700-7,400 MBq, and 18 hyperthyroid patients were treated on an outpatient basis with 200-600 MBq. Doses were measured over periods of 15-21 d following the administration of radioiodine in 35 partners and 38 children, aged 4 mo to 25 y. These results were correlated with dose rate measurements performed with an ionization chamber, and residual thyroid uptake was assessed by scintigraphy over the same period. In the cancer group, the residual activity in thyroid remnants was less than 50 MBq in all cases at day 4 following treatment and decayed with a mean half-life of 2.2 (SD: 0.8) d. The dose measured with thermoluminescent dosimeters was lower than 0.5 mSv in all partners and children. In the hyperthyroid group, the effective half-life averaged 6.2 (SD: 1.2) d. The median of the doses measured in partners and children were 1.04 mSv (range: 0.05-5.2) and 0.13 mSv (range: 0.04-3.1), respectively. Fifteen children (88%) received less than the dose constraint of 0.5 mSv. The ICRP recommend an annual limit of 1 mSv for the members of the public. In addition, dose constraints (for example: 0.5 mSv) should be complied with whenever possible. The recommended dose limits are generally well met among family members of patients treated with 1311 for cancer. The higher doses measured in hyperthyroid patients, compared to thyroid cancer patients, relate to a higher (131)I retention by the gland and justify more extended and stringent restriction periods, based on residual thyroid activity.