Dosimetry prior to I-131-therapy of benign thyroid disease

The activity to be administered in I-131 therapy of benign thyroid disease is determined by the radiation absorbed dose necessary to cure the disease, the target mass, and the residence time of the I-131 in the target volume. Data from 73 patients with complete sets of uptake measurements 2, 6, 24, 48, and 96 (n=53) or 120 (n=20) hours after oral administration of 1 MBq I-131 were used to deduce residence times from subsets of 3, 2, or only 1 measurement for each individual. The values were compared to those obtained with the reference method, i.e. a fit of an uptake function based on a 2-compartment model to all 5 measurements, to quantify the errors introduced by the less demanding assessments. Deviations are less than 10% if the 2- compartment uptake function is fitted to only 3 values measured after 6, 24, and 96-120h. Use of 2, 24, and 96-120h data results in errors >20% in individual patients. The effective half-lives as determined from 2 measurements after 24 and 96-120h correlate well with those deduced from the reference method with larger deviations in individuals with slow iodine kinetics and late maximal uptake. Residence times determined from the 24h uptake, assuming linear increase during the first day, and the effective half-life limited to maximum 8days underestimate the actual values systematically in patients with long and short half-lives. These errors can be eliminated by a modification of the calculation method resulting in deviations less than 14% in all but one individual for this procedure. The accuracy of methods based on only one retention value increases with the time of measurement after the administration of I-131. While systematic errors up to a factor of two occur if the 24h uptake is used for the estimate, deviations are less than 18% for measurements after 120h. The results suggest that only one late uptake assessment warrants residence time estimates with an acceptable error. Given the high inherent uncertainties in the complete dosimetry procedure, additional measurements must be considered only if a high precision of the absorbed dose assessment is required e.g. for clinical trials.