A method to measure the absorbed dose of the thyroid during I-131 therapy, using a collar detector system and a SPECT acquisition.

PURPOSE: Due to variations in biological half-life, accurate thyroid dosimetry for I-131 therapy is not trivial in clinical practice. In recent publications, systems are described to measure the uptake of I-131 in the thyroid repeatedly over time. In this work, we present a method to calculate patient specific pharmacokinetics and absorbed dose using such a collar detector system (CoTI) in combination with a SPECT acquisition and a two-compartment model fit.

METHODS: For three patients receiving I-131 therapy for benign thyroid conditions, the complete uptake profile is measured over a period of 15 to 25 days after administration. A SPECT measurement is performed to assess the functional volume of the thyroid and the amount of I-131 in the thyroid. The uptake profile measured in counts-per-second is converted to absolute activity in MBq using the absolute quantification of the SPECT. A two-compartment model is used as a fit to the uptake data of the thyroid and to estimate the activity in the blood-pool. The estimated absorbed dose to the thyroid is then calculated from the integral of the activity. The assessed parameters from the method (6- and 24-h uptake, thyroid volume and I-131 uptake concentration) are compared with the values as determined in clinical practice. Furthermore, the convergence of the calculated absorbed dose as a function of measurement series duration is determined to assess the required measurement duration of the uptake profile.

RESULTS: The two-compartment model fit shows a good agreement with the measured data points. Resulting dynamic uptake profiles of the three patients differ from each other. The uptake percentages differ from the pretherapy I-123 uptake measurements that are used in usual clinical practice, which shows the potential added value of the proposed method. The duration of the required measurement series appears to be patient dependent and therefore needs to be determined for each patient individually. The proposed method allows for a basic investigation of the individual dynamic uptake profile of I-131 in the thyroid and the calculation of the absorbed dose.

CONCLUSIONS: The proposed measurement method is feasible and easily implementable given a system that can measure the uptake of I-131 in the thyroid repeatedly over time. The observed differences in dynamic uptake profiles and the differences in the absorbed thyroid dose as calculated with our method and the parameters of the usual clinical care support the relevance of the proposed method. In future studies, this approach may possibly be used for outcome prediction and therapeutic activity optimization.