Improved prediction of therapeutic absorbed doses of radioiodine in the treatment of thyroid carcinoma

We proposed an alternative to a monoexponential model of radioiodine kineti cs to obtain a more accurate estimate of absorbed doses to postsurgical thy roid remnants. We suggested that part of the difference between the predict ed and the actually absorbed therapeutic doses of I-131, usually explained by radiation damage of thyroid cells, can be attributed to, errors resultin g from inadequate sampling of data and oversimplified modeling. Methods: A standard monoexponential model and alternative biphasic model (incorporatin g both radioiodine uptake and clearance) were used on 2 sets of patient dat a to fit time-activity measurements after administration of diagnostic and therapeutic activities of radioiodine. One set of data consisted of 633 rec ords of routine measurements, and the second set consisted of 71 prospectiv ely 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 I -131, and the respective residence times were compared using the paired t t est. Errors of fitting and prediction of therapeutic time-activity data wer e also calculated. Results: With both models, a statistically significant d ifference (P < 0.01) was found between residence times after diagnostic adm inistration of I-131 and residence times after therapeutic administration o f I-131. However, the effects of biphasic modeling and of improved sampling substantially reduced the difference (P < 0.01). Errors of fitting and pre diction were smaller with the biphasic model than with the monoexponential model (P < 0.01): Conclusion: The biphasic model more accurately predicts I -131 kinetics when applied to measurements in the short interval after diag nostic administration of radioiodine. The minimum requirement for the bipha sic model is measurement twice a day at intervals > 6 h for at least 3 d af ter administration.