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, lambda(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 lambda(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-lambda(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: U0(ther)=0.88U0(dos) (r=0.97,p<0.01), lambda(eff)ther = 1.01 lambda(eff)dos (r=0.85,p<0.01), and D(estimated)= 0.85D(planned) (r=0.88, p<0.01). The percent difference between U0(ther) and U0(dos) ranged from -44 to 32% and between lambda(eff)ther and lambda(eff)dos from -32 to 48%. U0(ther) was lower than U0(dos) 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, deltaD, 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-lambda(eff) method, in which deltaD 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-lambda(eff) method is less reliable.