Thyroid remnant ablation using 1,110 MBq of I-131 after total thyroidectomy: regulatory considerations on release of patients after unsealed radioiodine therapy

Abstract

This study was undertaken to measure the radiation exposure level of caregivers following outpatient NaI (I-131) 1,110MBq therapy for remnant thyroid ablation after total thyroidectomy in patients with differentiated thyroid cancer, and to evaluate the influence of activities of daily living on radiation exposure level, with the goal of proposing an optimum method of I-131 therapy. The study included 37 patients with differentiated thyroid cancer, who had undergone total thyroidectomy and received outpatient based remnant thyroid ablation using NaI (I-131) 1,110MBq, who were satisfying the following requirements: (1) patients who have no evidence of distant metastases, (2) whose living environments were appropriate for outpatient I-131 (1,110MBq) therapy, and (3) patients who gave written informed consent. The dose rate at a distance of 1m from the body surface of the patient at the moment of release was measured using survey meters of the GM type or ionization chamber type. The dose level for the caregiver was measured with a personal dosimeter in all cases. The dose rate at a distance of 1m from the patient's body surface 1h after I-131 administration was in the range of 29-115μSv/h (mean 63.8μSv/h). The 7-day cumulative effective dose of caregivers was 0.11±0.08mSv, on an average, in 34 dosimeters. In 31 of 34 dosimeters, cumulative effective dose of caregivers was below 0.2mSv. Dose levels exceeding 0.2mSv were recorded in 3 cases (0.21, 0.35 and 0.43mSv in one case each). These results suggest that the exposure level of family members (caregiver and others) was minimal and is lower than the radiation levels affecting human environments. Outpatient-based remnant thyroid ablation with I-131 (1,110MBq) performed after total thyroidectomy in patients with differentiated thyroid cancer is safe if applied in accordance with the appropriate supervision and guidance by experts with certain qualifications.