Neuroendocrine tumor imaging: can 18-FDG Positron Emission Tomography detect tumors with poor prognosis and aggressive behavior?

We evaluated the clinical value of positron emission tomography (PET) using 18F-fluorodeoxyglucose (FDG) for neuroendocrine tumor (NET) detection. Sixteen patients with cytologically or histologically proved NETs were investigated. Patients were divided in two groups of eight patients each according to the clinicopathologic features related to prognosis: slow-growing NETs and aggressive NETs. Results of FDG tumor uptake as detected by PET were compared with computed tomography (CT) scans and with scans obtained with 111In-octreotide scintigraphy (n = 13). Tumor FDG uptake was increased in the primary lesion of all eight aggressive NETs; the tracer was shown also in lymph nodes, liver metastases, or both in five of six of them (83%). In four cases, additional unknown tumor sites undetected by CT scan were identified. A slight positivity was found in only one of eight cases with a slow-growing NET. The overall octreotide scintiscan sensitivity was 85%, but in the aggressive NETs it failed to detect the primary lesion in two of seven cases. Uptake of the tracer in some but not all tumor lesions in the same patient was seen by both FDG-PET and octreotide scintiscans. From our limited experience 18F-FDG PET seems to be useful for identifying NETs characterized by rapid growth or aggressive behavior. Uptake of the FDG tracer by the tumor may be related to a worse prognosis. Despite the heterogeneity of tracer uptake in the various lesions of NETs with multiple tumor sites, FDG-PET was able to detect unsuspected distant metastases, contributing to better staging of advanced disease.