Validation for performing ¹¹C-methionine and ¹⁸F-FDG-PET studies on the same day

The performance of two PET examinations, one using L-[methyl-¹¹C] methionine (MET) and one using 2-[¹⁸F] fluoro-2-deoxy-D-glucose (FDG), on the same day may offer a clinical advantage for the investigation of brain tumors or other lesions. The purpose of this study was to investigate the effect of positron cross-talk (PCT) and to determine the optimal protocol for using MET and FDG on the same day.

The participants comprised 62 patients with head and neck cancer. We focused on the high physiological uptake of MET in the liver and evaluated the effect of PCT with MET on FDG uptake in the liver and muscle. Three FDG-PET scans [one: whole body (early image), two: head and neck, and three: one-bed-position scan of the liver (delayed image)] were performed after completing a MET-PET scan (head and neck) at varying injection intervals. Standard uptake value mean variations in the liver and muscle were calculated, assuming that the differences between the early and the delayed images reflected the PCT from carbon-11 on fluorine-18, on the basis of the results of a phantom study and a study in volunteers. The participants were categorized into four groups (G) according to the injection interval: G1 (n=15, 30-49 min), G2 (n=16, 50-69 min), G3 (n=17, 70-89 min), and G4 (n=14, ≥ 90 min).

The PCT level decreased from the G1 group through to the G3 group (analysis of variance, P<0.001) but was stable, with no further decrease in the G4 group. The PCT level in the muscle was not significantly different among the G1, G2, G3, and G4 groups (analysis of variance, P=0.693). Thus, PCT in the liver decreased at longer injection intervals, and PCT was no longer observed at injection intervals of more than 90 min.

MET and FDG-PET examinations can be successfully performed on the same day without PCT between the studies if the injection interval is longer than 90 min. This method reduces the examination burden of patients and may be useful for performing multiple PET examinations while the patient's condition remains almost the same.