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 acquisition, the optimal scan time of 18F-DOPA PET/CT in patients with recurrent medullary thyroid carcinoma (MTC). Methods: Twenty-one patients with suspected recurrent MTC underwent dynamic 18F-DOPA PET/CT (lasting 45 minutes) followed by whole-body scan. Three different time intervals of dynamic acquisition were evaluated: ultra-early phase (2–5 minutes), early phase (5–10 minutes), and late phase (40–45 minutes). The number and SUVmax of all detected lesions among the 3 dynamic acquisition phaseswere compared on qualitative and semiquantitative analyses. Time-activity curves, SUVmax washout rate between ultraearly or early phase and late phase, and signal-to-noise ratio (SNR) between lesion and background activity were also calculated. Results: At dynamic acquisition, 15 of 21 patients were classified as PETpositive and 6 of 21 as PET-negative, with overall 21 detected lesions. Ultraearly and early imaging provided a better lesion visualization than late phase in more than 70% of cases, as also reflected by SNR (mean SNR reduction between2and45minutes, −45%± 19%). Time-activity curves showed a rapid tracer accumulation inMTClesions,with an average maximum uptake at 2 minutes after injection. Mean lesion SUVmax was 2-fold higher in ultra-early frames compared with last frames (mean washout rate, −44% ± 33%). Finally, compared with whole-body imaging in the same field of view, dynamic acquisition identified 1 additional positive patient and 3 additional lesions in 2 patients. Conclusions: Our study, showing a very fast 18F-DOPA uptake in MTC lesions, suggests the utility to obtain early PET/CT images, already at 2 to 5 minutes after tracer injection, when maximum lesion tracer uptake is reached.