Assessment of Minimum ¹²⁴I Activity Required in Uptake Measurements Before Radioiodine Therapy for Benign Thyroid Diseases

Abstract

This study aimed to assess a hypothetical minimum administered activity of 124 I required to achieve comparability between pretherapeutic radioiodine uptake (RAIU) measurements by 124 I PET/CT and by 131 I RAIU probe, the clinical standard. In addition, the impact of different reconstruction algorithms on 124 I RAIU and the evaluation of pixel noise as a parameter for image quality were investigated. Methods: Different scan durations were simulated by different reconstruction intervals of 600-s list-mode PET datasets (including 15 intervals up to 600 s and 5 different reconstruction algorithms: filtered-backprojection and 4 iterative techniques) acquired 30 h after administration of 1 MBq of 124 I. The Bland-Altman method was used to compare mean 124 I RAIU levels versus mean 3-MBq 131 I RAIU levels (clinical standard). The data of 37 patients with benign thyroid diseases were assessed. The impact of different reconstruction lengths on pixel noise was investigated for all 5 of the 124 I PET reconstruction algorithms. A hypothetical minimum activity was sought by means of a proportion equation, considering that the length of a reconstruction interval equates to a hypothetical activity. Results: Mean 124 I RAIU and 131 I RAIU already showed high levels of agreement for reconstruction intervals of as short as 10 s, corresponding to a hypothetical minimum activity of 0.017 MBq of 124 I. The iterative algorithms proved generally superior to the filteredbackprojection algorithm. 124 I RAIU showed a trend toward higher levels than 131 I RAIU if the influence of retrosternal tissue was not considered, which was proven to be the cause of a slight overestimation by 124 I RAIU measurement. A hypothetical minimum activity of 0.5 MBq of 124 I obtained with iterative reconstruction appeared sufficient both visually and with regard to pixel noise. Conclusion: This study confirms the potential of 124 I RAIU measurement as an alternative method for 131 I RAIU measurement in benign thyroid disease and suggests that reducing the administered activity is an option. CT information is particularly important in cases of retrosternal expansion. The results are relevant because 124 I PET/CT allows additional diagnostic means, that is, the possibility of performing fusion imaging with ultrasound. 124 I PET/CT might be an alternative, especially when hybrid 123 I SPECT/CT is not available.

Systematic review and meta-analysis of the diagnostic value of radionuclide imaging for thyroid nodules

“…The radiometric distribution of cold nodules decreases significantly, and radiometric defects often appear. The difference between cold and cool nodules is the degree of reduction in iodine uptake at the nodular site (21). Several studies have suggested that radionuclide scanning (ECT) can help clinicians understand the functional status of the thyroid.…”