Evaluation of the consistency between two dosimetric methods and radioiodine therapy in hyperthyroid patients

Abstract

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Objectives: The aim of this study was to verify the capability of a dosimetric method based on the MIRD formula, 123I i.v. administration and 5 uptake values determined with a gamma camera, in prediction of radioiodine kinetics (fraction of administered iodine transferred to the thyroid, U0, and effective clearance rate, λeff) and absorbed dose after oral therapeutic 131I administration. A gamma camera was used to study separate structures within the thyroid. A dosimetric method based on the 24-h uptake and a fixed λeff value was also considered.

Methods: Ninety hyperthyroid patients (11 with Graves’ disease and 79 with autonomously functioning nodules), for a total of 133 thyroidal structures, were studied. The mean time interval between dosimetry and therapy was 20+/-10 d. Uptake values were measured at 2, 4, 24, 48, and 120h during dosimetry and at 2, 4, 24, 48, 96, and 168h during therapy. The value 0.125d-1 was chosen in the fixed-λeff method. The planned doses were from 120 to 250 Gy depending on the thyroid disease and the severity of hyperthyroidism.

Results: The following significant correlations were found: U0ther=0.87U0dos (r=0.97 p<0.01), λeffther=1.01λeffdos (r=0.85 p<0.01) and Destimated=0.85Dplanned (r=0.90 p<0.01). The percent difference between U0ther and U0dos ranged from –44 to 32% and between λeffther and λeffdos from –32 to 48%. U0ther was lower than U0dos in 74% of cases: this can be explained by the self-stunning effect of 131I therapeutic activity that produced a dose of about 20 Gy with a maximum dose rate of 0.6 Gy/h over the initial 24-48h. The differences, ΔD, between the estimated and the planned doses ranged from –42% (-87 Gy) to 32% (59 Gy); in 72% of cases the difference was within +/-35 Gy. Greater differences were found with the fixed-λeff method in which ΔD ranged from -69 to 95% (–202 to 88 Gy, respectively).

Conclusions: In hyperthyroid patients, the 5 uptake value dosimetric method predicts with a good consistency the radioiodine kinetics and the dose after the therapeutic administration in about 72% of the thyroid structures analyzed. The fixed-λeff method is less reliable.