Verification of the agreement of two dosimetric methods with radioiodine therapy in hyperthyroid patients
- Departments of Nuclear Medicine and Health Physics, Ospedale Maggiore Policlinico, Mangiagalli e Regina Elena, Milan (Italy)
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, U{sub 0}, and effective clearance rate, {lambda}{sub eff}) and absorbed dose after oral therapeutic {sup 131}I administration. The method is based on {sup 123}I 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 {sup 123}I 24-h uptake and a fixed {lambda}{sub 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{sup -1} was chosen in the fixed-{lambda}{sub 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: U{sub 0}ther=0.88U{sub 0}dos (r=0.97,p<0.01), {lambda}{sub eff}ther=1.01{lambda}{sub eff}dos (r=0.85,p<0.01), and D{sub estimated}=0.85D{sub planned} (r=0.88,p<0.01). The percent difference between U{sub 0}ther and U{sub 0}dos ranged from -44 to 32% and between {lambda}{sub eff}ther and {lambda}{sub eff}dos from -32 to 48%. U{sub 0}ther was lower than U{sub 0}dos in 74% of cases: this can be explained by the self-stunning effect of {sup 131}I 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, {delta}D, 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}{sub eff} method, in which {delta}D 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}{sub eff} method is less reliable.
- OSTI ID:
- 20853386
- Journal Information:
- Medical Physics, Vol. 33, Issue 8; Other Information: DOI: 10.1118/1.2210564; (c) 2006 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
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