We conducted a prospective multicenter trial to compare the usefulness of C-methioinine (MET) and F-fluorodeoxyglucose (FDG) positron emission tomography (PET) for identifying tumor recurrence. Patients with clinically suspected tumor recurrence after radiotherapy underwent both C-MET and F-FDG PET. When a lesion showed a visually detected uptake of either tracer, it was surgically resected for histopathological analysis. Patients with a lesion negative to both tracers were revaluated by magnetic resonance imaging (MRI) at three months after the PET studies. The primary outcome measure was the sensitivity of each tracer in cases with histopathologically confirmed recurrence, as determined by the McNemar test. Sixty-one cases were enrolled, and 56 cases could be evaluated. The 38 cases where the lesions showed uptake of either C-MET or F-FDG underwent surgery; and 32 of these cases were confirmed to be subject to recurrence. Eighteen cases where the lesions showed uptake of neither tracer received follow-up MRI; the lesion size increased in 1 of these cases. Among the cases with histologically confirmed recurrence, the sensitivities of C-MET PET and F-FDG PET were 0.97 (32/33, 95% CI: 0.85-0.99) and 0.48 (16/33, 95% CI: 0.33-0.65), respectively, and the difference was statistically significant (p<0.0001). The diagnostic accuracy of C-MET PET was significantly better than that of F-FDG PET (87.5% vs. 69.6%, p=0.033). No examination-related adverse events were observed. The results of the study demonstrated that C-MET PET was superior to F-FDG PET for discriminating between tumor recurrence and radiation-induced necrosis.
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