Positron emission tomography (PET) scan is becoming more widely used and may be a useful localizing modality for neuroendocrine tumors as different radiolabeled substances can be used as metabolic substrate. After the development of a PET tracer for somatostatin analogs, ⁶⁸Ga-DOTA-NOC (tetra-azacyclododecane tetra-acetic acid-[1-Nal3]-octreotide) has been introduced. This compound for PET imaging has a high affinity for sst2 and sst5 and has been used for the detection of NETs in preliminary studies. The uptake of ⁶⁸Ga-DOTA-NOC is based on a receptor mechanism and although this has not yet been adequately assessed, it seems to have higher sensitivity for NETs than Octreoscan®, thereby increasing diagnostic accuracy. Additionally, it has several advantages over Octreoscan®: increased spatial resolution and the possibility of images with a short uptake time (60 min), and relatively easy synthesis [14].

The two other compounds most often used in functional imaging with PET are ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE. ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE possess similar diagnostic accuracy for detection of NET lesions. The increasing availability of ⁶⁸Ga somatostatin analogs PET-CT now offers superior accuracy for localization and functional characterization of NETs. However, studies are needed to enable imaging of NET with optimal targeting of tumor receptors.

Fluorodihydroxyphenylalanine-(¹⁸F-FDOPA) PET is a recent imaging modality used to localize neuroendocrine tumors [15]. These tumors have the ability to produce biogenic amines and polypeptide hormones, and they take up and decarboxylate their amine precursors, L-dihydroxyphenylalanine. ¹⁸FDOPA-PET appears to be a major tool for the management of carcinoid tumors with excellent diagnostic performances (65–96 %) related to these capacities to concentrate amino acids inside the vesicules of cytoplasmatic space through metabolic mechanism. ¹⁸FDOPA-PET is less sensitive and less useful for the management of noncarcinoid tumors (Fig. 1.2).

Although ¹⁸F-2-Deoxy-d-glucose (¹⁸F-FDG) PET is the most widely used and accepted type of PET in clinical oncology, it has limited use in well-differentiated tumors such as NETs due to their low expression of glucose transporters and low proliferative activity. However, several studies have evaluated ¹⁸F-FDG PET-CT in well-differentiated NET [13, 16, 17]. Garin et al. reported that ¹⁸FDG uptake is a poor prognostic factor in NETS, in relation to tumor aggressiveness and is related to a lower overall survival (Fig. 1.3).

All of these performances highlight the significant contribution of the scintigraphic procedures from a diagnostic point of view and the management of therapy of patients with NETs. PET imaging could be of major interest for the diagnosis, evaluation of progression and treatment response in NETs. ¹⁸FDG-PET even though still not validated, carries major prognostic information and may influence determination of the optimal therapeutic strategy. The role of ¹⁸F-DOPA is clearly recommended before surgery for the detection of carcinoid tumors. The different new somatostatin analogs with ⁶⁸Ga radiolabeling must be evaluated.