Late 124I PET/CT Uptake Measurement—Assessment of Appropriate Examination Protocol in Benign Thyroid Diseases

Purpose: This study aimed at investigating the performance of late 124I PET/CT for radioiodine uptake (RAIU) measurement at 336 hours after administration in patients with benign thyroid diseases requiring radioiodine therapy. Special attention was paid to the comparability of 124I uptake (124I-RAIU) to the clinical standard (131I-RAIU, probe measurement). Considering cost aspects, we sought to establish an economically reasonable examination protocol based on scan duration and administered activity. Methods: List-mode PET data sets of 40-minute acquisition time were acquired 336 hours after administration of 1 MBq 124I in 18 patients. Different scan durations were simulated by different reconstruction intervals (RIs) ranging from 5 seconds to 40 minutes, and total thyroid activity was measured. Mean 124I-RAIU levels of each RI were compared with mean 131I-RAIU levels (3 MBq). A hypothetical scan duration or hypothetical activity, respectively, was sought by means of a proportion equation, considering that the length of an RI is equitable to a hypothetical activity. Results: After 336 hours, the mean total thyroid activity was 254 ± 7.7 kBq for 131I and 26.9 ± 8.7 kBq for 124I. The mean 124I-RAIU and 131I-RAIU showed high levels of agreement for RI from 2 minutes to 40 minutes. Reconstruction interval shorter than 2 minutes did not result in sufficient agreement. Conclusions: The present study confirmed the feasibility of late 124I PET/CT as alternative method for RAIU measurement in patients with benign thyroid diseases; 1 MBq 124I PET/CT scans as short as 2 minutes resulted in RAIU levels comparable to those of standard 131I-RAIU. The parameter "appropriate scan-duration activity product" is proposed to enable an economically reasonable examination protocol.

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