Abstract
The therapeutic activity of 131I administered to patients with Graves’ disease can be calculated by means of Marinelli’s formula. The thyroidal iodine uptake (131IUmax) needed for the calculation is usually determined with the use of 131I. The purpose of the paper was to estimate 131IUmax on the basis of technetium uptake in the thyroid at 20 min (99mTcU20min). Eighty patients suffering from Graves’ disease were qualified for radioiodine therapy with measurement of fT4, fT3, thyroid-stimulating hormone and its receptor (TRAb). Prior to the treatment, all the patients were euthyroid. 131IUmax for each patient was determined according to the levels of 131I after 24 h (131IU24h), while effective half-life (T eff) according to the measurements of 131IU24h and 131I uptake after 48 h (131IU48h). Additionally, on the day before measuring 131IU24h, 99mTcU20min was calculated for each patient. It was demonstrated that there existed a correlation, with statistical significance at p < 0.05, between the following pairs of values: TRAb and 131IU24h, TRAb and 99mTcU20min, and 99mTcU20min and 131IU24h. The interdependence between 131IU24h and 99mTcU20min at the level of significance p < 0.05 is described by the following algorithms: 131IU24h = 17.72 × ln (99mTcU20min) + 30.485, if TRAb < 10 IU/ml, and 131IU24h = 18.03 × ln (99mTcU20min) + 38.726, if TRAb > 10 IU/ml. It is possible to predict thyroid iodine uptake 131IU24h in Graves’ disease on the basis of measuring the uptake of 99mTcU20min. This shortens the time necessary for diagnosis and enables the calculation of 131I activity using Marinelli’s formula.
Highlights
Radiotherapy is, apart from pharmacotherapy and surgical intervention, one of the major methods of treatment for Graves’ disease (GD)
ÁD Teff where A—131I therapeutic activity (MBq) 25—unit conversion coefficient m—mass of thyroid gland calculated with USG (g) D—absorbed dose of 131I (Gy), with D = 150 Gy, as recommended by European Association of Nuclear Medicine (EANM) for GD [1] IU24h—24-h 131I uptake (%) Teff—effective 131I half-life in thyroid gland calculated with gamma-camera based on 131IU24h and 131IU48h
The non-linear regression function was used to establish the correlation of the IU24h, 99mTcU20min and TRAb parameters, with the level of significance at p < 0.05. It can be noticed among the parameters measured in GD patients in the course of the eligibility study, the effective 131I half-life in the thyroid gland (Teff) has a very low standard deviation (±0.04) and that its mean value equals 5.5 days, while the serum level of thyroid hormones in all the patients before treatment remains within the normal limits, thanks to an effective therapy with thyrostatic medications (Table 1)
Summary
Radiotherapy is, apart from pharmacotherapy and surgical intervention, one of the major methods of treatment for Graves’ disease (GD). Effective radioiodine therapy for GD consists in calculating an appropriate activity of the radioisotope in order to achieve the highest possible therapeutic efficacy, with the lowest possible radiation of the thyroid, and of the patient. The clinics that offer radioiodine therapy in Europe and around the world use various methods of selecting correct therapeutic activities of 131I. Radioiodine 131I is usually used to determine the thyroidal iodine uptake required by the formula. Some use the iodine isotope 123I, much less frequently because of the high costs of its production in a cyclotron [2, 3]. The disadvantages of using 131I for therapeutic purposes include: beta radiation during radioactive decay, relatively high gamma radiation energy (364 keV) and the long time (usually 24 h) that is required for assessing the maximum iodine uptake in the thyroid.
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