Is There an Optimal Scan Time for 6-[F-18]Fluoro-L-DOPA PET in Pheochromocytomas and Paragangliomas?

Abstract

To define the appropriate scan time for fluorine-18-labeled dihydroxyphenylalanine (F-18 DOPA) PET in oncological imaging of pheochromocytomas and paragangliomas. F-18 DOPA PET examinations were performed in 9 patients with 7 pheochromocytomas and 4 head and neck paragangliomas using a dedicated PET scanner. The acquisition started with a dynamic single-bed scan in the tumor region over the first 60 minutes after tracer injection followed by a late time whole-body scan at approximately 130 minutes. Standard uptake values (SUVs) were calculated in tumors, surrounding background, and adjacent normal tissues of relevance. Furthermore, kinetic analysis was performed using a 2-compartment model with rate constants for uptake (K1'), release (k2'), metabolism (k3'), and reverse reaction (k4') for region of interest and pixel-wise analysis. All tumors show a marked increased F-18 DOPA uptake, which was visually detectable and distinguishable from the surrounding tissue. The SUV is significantly lower in neck paraganglioma compared with abdominal pheochromocytomas. Mean time-activity curves of F-18 DOPA in tumors show a rapid uptake of the tracer. Already 2 minutes after the injection, the activity in the tumor is beyond that of the blood pool. The average maximum value (SUVmean = 8.2) has already been reached after 20 minutes. Afterward, a very slight decrease of the tumor SUV starts, which still amounts to 80% of the maximum value after 132 minutes. Due to the continuous decrease of activity in the background tissue, the tumor-to-background ratio of SUVs shows a constant increase within the entire period of examination. The mean values of apparent kinetic constants obtained by region of interest analysis averaged over all tumors are as follows: K1' = 2.89 ± 2.56 min(-1), k2' = 2.59 ± 2.81 min(-1), k3' = 0.301 ± 0.395 min(-1), and k4' = 0.044 ± 0.043 min(-1). Pheochromocytoma and paraganglioma take up F-18 DOPA very quickly. At best, the acquisition for static clinical PET imaging of paraganglioma with F-18 DOPA can start at 20 minutes postinjection for maximum uptake in tumors. Separation of tumor, background, and adjacent normal tissues is feasible due to their differences in SUV values and kinetics. The kinetic analysis demonstrates an F-18 DOPA accumulation within the tumor due to considerable differences between the rate constants of uptake and metabolism. Second, in contradiction to healthy brain, paraganglionic tumors show a reversible F-18 DOPA metabolism.