Verification of the agreement of two dosimetric methods with radioiodine therapy in hyperthyroid patients

The aim of this study was to verify the capability of an MIRD formula-based dosimetric method to predict radioiodine kinetics (fraction of administered iodine transferred to the thyroid, U₀, and effective clearance rate, λ_eff) and absorbed dose after oral therapeutic ¹³¹I administration. The method is based on ¹²³I intravenous administration and five subsequent gamma camera measured uptake values determined separately on different structures within the thyroid. Another dosimetric method based on only the ¹²³I 24-h uptake and a fixed λ_eff value was also considered. Eighty-nine hyperthyroid patients (10 with Graves' disease and 79 with autonomously functioning nodules) were studied and 132 thyroidal structures were evaluated. The mean time interval between dosimetry and therapy was 20±10d. Uptake values were measured at 2, 4, 24, 48, and 120 h during dosimetry and at 2, 4, 24, 48, 96, and 168 h during therapy. The value 0.125d^-1 was chosen in the fixed-λ_eff method. The planned doses to the target ranged from 120 to 250 Gy depending on the type and severity of hyperthyroidism. The following significant correlations between therapeutic and dosimetric parameters were found: U₀ther=0.88U₀dos (r=0.97, peffther =1.01λ_effdos (r=0.85, pestimated=0.85D_planned (r=0.88, p0ther and U₀dos ranged from -44 to 32% and between λ_effther and λ_effdos 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 24-48 h. The differences, ΔD, between the estimated and the planned doses ranged from -42% (-87 Gy) to 32% (59 Gy); in 73% of cases the difference was within ±35 Gy. Greater discrepancies were found with the fixed-λ_eff method, in which ΔD ranged from -69 to 95% (-202 to 88 Gy, respectively). In hyperthyroid patients, the five uptake value dosimetric method is able to predict with a good agreement the radioiodine kinetics and the dose after the therapeutic administration in about 73% of the analyzed thyroid structures. The fixed-λ_eff method is less reliable.