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Verification of the agreement of two dosimetric methods with radioiodine therapy in hyperthyroid patients
Author(s) -
Canzi Cristina,
Zito Felicia,
Voltini Franco,
Reschini Eugenio,
Gerundini Paolo
Publication year - 2006
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2210564
Subject(s) - dosimetry , nuclear medicine , medicine , radioiodine therapy , absorbed dose , thyroid , therapeutic index , effective dose (radiation) , thyroid cancer , pharmacology , drug
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 0 , and effective clearance rate,λ eff ) and absorbed dose after oral therapeuticI131administration. The method is based onI123intravenous administration and five subsequent gamma camera measured uptake values determined separately on different structures within the thyroid. Another dosimetric method based on only theI12324‐h uptake and a fixedλ effvalue 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 ± 10 d . 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 0.125 d − 1was chosen in the fixed‐ λ effmethod. 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 0 ther = 0.88 U 0 dos( r = 0.97 , p < 0.01 ) ,λ eff ther = 1.01 λ eff dos( r = 0.85 , p < 0.01 ) , andD estimated = 0.85 D planned( r = 0.88 , p < 0.01 ) . The percent difference betweenU 0 ther andU 0 dos ranged from − 44 to 32% and betweenλ eff ther andλ eff dos from − 32 to 48%.U 0 ther was lower thanU 0 dos in 74% of cases: this can be explained by the self‐stunning effect ofI131therapeutic 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, Δ 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‐ λ effmethod, in which Δ 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‐ λ effmethod is less reliable.