Which Is the Optimal Scan Time of 18F-DOPA PET/CT in Patients With Recurrent Medullary Thyroid Carcinoma?: Results From a Dynamic Acquisition Study.

Purpose: The aim of this retrospective study was to determine, by dynamic\r\nacquisition, the optimal scan time of 18F-DOPA PET/CT in patients with recurrent\r\nmedullary thyroid carcinoma (MTC).\r\nMethods: Twenty-one patients with suspected recurrent MTC underwent\r\ndynamic 18F-DOPA PET/CT (lasting 45 minutes) followed by whole-body\r\nscan. Three different time intervals of dynamic acquisition were evaluated:\r\nultra-early phase (2–5 minutes), early phase (5–10 minutes), and late phase\r\n(40–45 minutes). The number and SUVmax of all detected lesions among\r\nthe 3 dynamic acquisition phaseswere compared on qualitative and semiquantitative\r\nanalyses. Time-activity curves, SUVmax washout rate between ultraearly\r\nor early phase and late phase, and signal-to-noise ratio (SNR) between\r\nlesion and background activity were also calculated.\r\nResults: At dynamic acquisition, 15 of 21 patients were classified as PETpositive\r\nand 6 of 21 as PET-negative, with overall 21 detected lesions. Ultraearly\r\nand early imaging provided a better lesion visualization than late phase\r\nin more than 70% of cases, as also reflected by SNR (mean SNR reduction between2and45minutes,\r\n−45%± 19%). Time-activity curves showed a rapid tracer\r\naccumulation inMTClesions,with an average maximum uptake at 2 minutes after\r\ninjection. Mean lesion SUVmax was 2-fold higher in ultra-early frames compared\r\nwith last frames (mean washout rate, −44% ± 33%). Finally, compared with\r\nwhole-body imaging in the same field of view, dynamic acquisition identified 1\r\nadditional positive patient and 3 additional lesions in 2 patients.\r\nConclusions: Our study, showing a very fast 18F-DOPA uptake in MTC lesions,\r\nsuggests the utility to obtain early PET/CT images, already at 2 to 5 minutes\r\nafter tracer injection, when maximum lesion tracer uptake is reached.