Optimization of 131I activity value required for thyroid remnants and metastatic lesions ablation in children with differentiated thyroid cancer

66 children with differentiated thyroid cancer have been treated with I'll after thyroidectomy. Activities administered amounted from 370 to 5600 MBq. Patients were divided into three groups according to scintigraphy data: Group 1-with thyroid remnants only (25 patients), Group 2-with metastatic lesions in lymph nodes (15 patients), Group 3-with metastatic lesions in lungs (26 patients). The aim of the study was to evaluate parameters of radioiodine accumulation and excretion in remnants during radioiodine treatment in children after thyroidectomy, to determine the influence of I'll activity value on the result of the first course of radioiodine treatment. The fraction (f) of patients with thyroid remnants ablation after first course of the treatment within each specified activity range has been calculated and plotted against each interval of I-131 activity. In Group 3 number of courses required for total ablation of thyroid tissue was evaluated as a function of activities administered. In Group 1 f has a constant value (about 0.55) within the range of 1000-2500 MBq followed by the abrupt increase (up to 1) at 2500 MBq remaining at this plateau value with further increase of activity. In Group 3 number of courses of radioiodine treatment largely depends first activity value, namely the greater is first activity value the less is number of courses required for total ablation of remnants and metastatic lesions. The activity 2500 MBq could be considered as an optimal value for thyroid remnants ablation after first course of radioiodine treatment in children without metastatic lesions accumulating radioiodine, while in groups with metastatic lesions, for minimizing number of courses, first activity should be not less then 4000 MBq.