The aim of this study was to assess the agreement between predicted blood uptake values using 124I and actually measured 131I blood uptake values (reference) in patients with differentiated thyroid carcinoma receiving largely high therapeutic activities.Patients and methods
Fourteen patients were analyzed retrospectively, who underwent a series of both pretherapeutic and intratherapeutic blood sampling using median 124I activities of 23 MBq and median therapy 131I activities of 10 GBq. Data of five blood samples from each patient were analyzed. Lin’s concordance correlation coefficient analysis was carried out to assess the kinetic agreement. The time-integrated 131I activity coefficient (TIAC) for the blood compartment and the effective 131I clearance time (ECT), expressed as effective 131I half-life on the basis of a monoexponential model, were ascertained. For each patient, the (intrapatient) percentage differences between pretherapeutic and intratherapeutic TIACs and ECTs were calculated. The (interpatient) difference in TIACs and ECTs between pretherapy and intratherapy groups was evaluated using the Mann–Whitney U-test.Results
Lin’s concordance correlation coefficient was at least 0.97, indicating substantial kinetic agreement between pretherapeutic and intratherapeutic radioiodine kinetics. The mean (median)±SD (range) of the absolute percentage difference was 9% (11%)±7% (0.33–20%) for the TIAC and 11% (10%)±10% (0–23%) for the ECT. A slightly higher median TIAC was observed in intratherapy (2.8 vs. 3.3 h), but this was not statistically significant (P=0.15), whereas no remarkable ECT difference (P=0.62) was found.Conclusion
The pretherapeutic blood kinetics derived from diagnostic 124I activities provides a reliable estimation of the intratherapeutic 131I blood kinetics in patients receiving largely high therapy activities, showing its potential for radioiodine treatment planning.