I-131 Activities as High as Safely Administrable (AHASA) for the Treatment of Children and Adolescents with Advanced Differentiated Thyroid Cancer

Abstract

Differentiated thyroid carcinoma (DTC) in children and young adults is rare, can be aggressive, and often presents at advanced stages. In a population of young Belarusian patients with advanced DTC after the nuclear reactor accident at Chernobyl, we determined the activities that are as high as safely administrable (AHASA). In 180 children and adolescents, we studied 133 courses of I-131 thyroid remnant ablation (median age at ablation, 14.3 yr) and 250 courses of I-131 therapy (median age at therapy, 15.7 yr). Remnant ablation was performed with weight-adapted I-131 activities of a median of 51.8 MBq/kg (range, 23.9-73.8 MBq/kg); and residual disease therapy was performed with a median activity of 98.0 MBq/kg (range, 56.7-164.7 MBq/kg). The radiation absorbed dose to the blood (BD) per unit of activity administered for each treatment was deduced from whole-body retention data measured twice daily using ceiling probes. The AHASA activity was calculated assuming an upper limit of 2 Gy BD. For I-131 ablation, the median weight-adjusted AHASA activity leading to a BD of 2 Gy was 407 MBq/kg (range, 137-661 MBq/kg). In three patients with extensive diffuse pulmonary metastases, the AHASA was lower than 200 MBq/kg. For patients receiving additional I-131 treatments after ablation, a median body weight-adapted AHASA activity of 406 MBq/kg (range, 210-775 MBq/kg) was calculated. Children and adolescents with advanced DTC can be treated with I-131 activities of at least 200 MBq/kg. For children with extensive pulmonary metastases, pretherapeutic dosimetry is needed to determine the AHASA.