EANM practice guideline/SNMMI procedure standard for RAIU and thyroid scintigraphy.

INTRODUCTION: Scintigraphic evaluation of the thyroid gland enables determination of the iodine-123 iodide or the 99mTc-pertechnetate uptake and distribution and remains the most accurate method for the diagnosis and quantification of thyroid autonomy and the detection of ectopic thyroid tissue. In addition, thyroid scintigraphy and radioiodine uptake test are useful to discriminate hyperthyroidism from destructive thyrotoxicosis and iodine-induced hyperthyroidism, respectively. METHODS: Several radiopharmaceuticals are available to help in differentiating benign from malignant cytologically indeterminate thyroid nodules and for supporting clinical decision-making. This joint practice guideline/procedure standard from the European Association of Nuclear Medicine (EANM) and the Society of Nuclear Medicine and Molecular Imaging (SNMMI) provides recommendations based on the available evidence in the literature. CONCLUSION: The purpose of this practice guideline/procedure standard is to assist imaging specialists and clinicians in recommending, performing, and interpreting the results of thyroid scintigraphy (including positron emission tomography) with various radiopharmaceuticals and radioiodine uptake test in patients with different thyroid diseases.

The so-called stunning of thyroid tissue.

UNLABELLED: When thyroid tissues exhibited concentrations of therapeutic (131)I that appeared to be less than that predicted by data from the preceding diagnostic (131)I, the phenomenon was called stunning. We hypothesized that stunning arose from the early effects of the therapeutic dose of (131)I and that the initial uptake of (131)I, observed within the first day, was not impaired by the diagnostic dose. METHODS: The hypothesis was tested by 2 types of studies. In each type, the fractional concentrations of (131)I in residual neck thyroid tissues of patients with papillary thyroid carcinoma were quantified. In the first study, fractional concentrations of diagnostic and therapeutic (131)I were measured at 2 d, a time when stunning has been observed, and expressed as ratios of radioactivity: therapeutic/diagnostic (Rx/Dx). Three different doses of diagnostic (131)I were prescribed to assess a dose response. In the second study, patients were prospectively recruited and tested to record disappearances of radioactivity from thyroid tissues. Diagnostic doses were 1.0 mCi (37 MBq) in all; therapeutic doses were 150 and 30 mCi (5,550 and 1,110 MBq), each to half of the patients. The disappearance curves were extrapolated to the period between 0 and 1 d, an interval when maximum uptake of ingested (131)I would be expected. The fractional concentrations of (131)I at 2 d and at 0-1 d were compared in terms of Rx/Dx ratios to assess changes at each time point. RESULTS: In the first study, after diagnostic doses of 2, 1, and 0.5 mCi (74, 37, and 18.5 MBq), mean 2-d Rx/Dx values in 24, 29, and 17 patients were 0.35, 0.50, and 0.46 (P = 0.087). Of all patients, 74% exhibited Rx/Dx <0.6. In the second study, 6 of 10 patients exhibited disappearance curves of (131)I in which Rx/Dx was <0.6 at 2 d; 5 of the 6 had Rx/Dx values >0.97 at the 0- to 1-d point. In 1 patient the Rx/Dx was 0.54 at 2 d and 0.66 at the earlier time point. The other 4 patients had disappearance curves in which Rx/Dx values were >1.0 throughout or were above 0.6 and did not greatly change. CONCLUSION: Two days after the administration of (131)I, the mean fractional concentration of radioactivity in thyroid tissues after a therapeutic dose is <60% of the diagnostic dose in most patients, but no correlation of Rx/Dx with the mCi in the diagnostic dose was seen. In 5 of 6 patients in whom the Rx/Dx at 2 d was <0.6, the maximum fractional concentrations of therapeutic and diagnostic (131)I (i.e., the tissue uptakes during the first day) were similar; this pattern was most apparent after therapies with 150 mCi. These results support the hypothesis that "stunning" of thyroid tissues, often observable by 2 d, is primarily the consequence of early destructive effects from therapeutic (131)I.