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Improved Prediction of Therapeutic Absorbed Doses of Radioiodine in the Treatment of Thyroid Carcinoma

Jindika Hemanská, Miroslav Kárn, Jaroslav Zimák, Ladislav Jirsa, Martin ámal and Petr Vlek

Department of Nuclear Medicine, Faculty Hospital Motol and First Faculty of Medicine, Charles University Prague, Prague; and Institute of Information Theory and Automation, Academy of Sciences of the Czech Republic, Prague, Czech Republic

We proposed an alternative to a monoexponential model of radioiodine kinetics to obtain a more accurate estimate of absorbed doses to postsurgical thyroid remnants. We suggested that part of the difference between the predicted and the actually absorbed therapeutic doses of ¹³¹I, usually explained by radiation damage of thyroid cells, can be attributed to errors resulting from inadequate sampling of data and oversimplified modeling. Methods: A standard monoexponential model and alternative biphasic model (incorporating both radioiodine uptake and clearance) were used on 2 sets of patient data to fit time–activity measurements after administration of diagnostic and therapeutic activities of radioiodine. One set of data consisted of 633 records of routine measurements, and the second set consisted of 71 prospectively collected records with measurements performed more frequently and for a longer time. The time–activity curves derived from the 2 models were used to calculate residence times for diagnostic and therapeutic activities of ¹³¹I, and the respective residence times were compared using the paired t test. Errors of fitting and prediction of therapeutic time–activity data were also calculated. Results: With both models, a statistically significant difference (P < 0.01) was found between residence times after diagnostic administration of ¹³¹I and residence times after therapeutic administration of ¹³¹I. However, the effects of biphasic modeling and of improved sampling substantially reduced the difference (P < 0.01). Errors of fitting and prediction were smaller with the biphasic model than with the monoexponential model (P < 0.01). Conclusion: The biphasic model more accurately predicts ¹³¹I kinetics when applied to measurements in the short interval after diagnostic administration of radioiodine. The minimum requirement for the biphasic model is measurement twice a day at intervals > 6 h for at least 3 d after administration.

Key Words: thyroid cancer • radioiodine therapy • pharmacokinetics of ¹³¹I • residence time

This article has been cited by other articles:

				M. Medvedec, J. Hermanska, and L. Jirsa Improved Planning of Radioiodine Therapy for Thyroid Cancer J. Nucl. Med., May 1, 2002; 43(5): 714 - 714. [Full Text] [PDF]