Conventional imaging (CI) methods, including contrast-enhanced CT scans and MRI, are usually used as the primary imaging methods for patients presenting with signs or symptoms suggestive of GEP-NENs, both for detecting the site of primary tumor and possible metastatic sites. These modalities are most useful for staging and surgical planning, due to the great anatomic detail of the tumors and their surrounding structures. Most GEP-NENs are shown as contrast-enhancing lesions during the arterial phase on CT or MRI. MRI is an ideal imaging modality for differentiating between benign and malignant hepatic lesions. MRI seems to have higher sensitivity than CT in the diagnosis of liver and bone-marrow metastases, but both types of imaging techniques may miss small lymph node metastases. The sensitivity of techniques can also be variable for either recurrent or metastatic malignancy. This is due to the fact that these techniques primarily detect change in anatomy rather than function, and, therefore, they are considered to have moderate specificity [11].

According to diagnostic and therapeutic guidelines for GEP-NENs, the sensitivity of CT imaging in the diagnosis of pancreatic tumors is approximately 70%, and specificity is approximately 90%. In addition, CT has high sensitivity and specificity in the detection of metastases. The sensitivity is 82% and the specificity is 92% in liver metastases, while the sensitivity is 60–70% and specificity is 87–100% for lymph nodes metastases [12,13]. CT is preferable for imaging the lungs as compared to MRI, because it has a better spatial resolution. It is considered as the most effective imaging method for the detection of small bowel NENs. The disadvantage of contrast-enhanced CT is its poor sensitivity to detect small lesions less than 1 cm and for bone metastases (sensitivity about 50%). Recent studies have reported that MRI has more than 80% sensitivity to detect hepatic metastases [14,15,16].

Functional imaging studies based on the expression of SSTRs play an important role in evaluating patients with suspected GEP-NENs and high density of SSTRs. CT and MRI allow for anatomical characterization, but they cannot provide accurate functional information and are often insufficient for the diagnosis of GEP-NENs if the lesions are small and cover various anatomical locations. Somatostatin receptor scintigraphy (SRS) can provide important functional data. Thus, SRS shows good accuracy for whole-body imaging and has been routinely used for the diagnosis and follow-up of GEP-NENs [17].

SRS using the somatostatin analog ¹¹¹In-pentetreotide (¹¹¹In-DTPA-D-Phe-conjugate of octreotide; Octreoscan^®) was the first radiotracer agent to be approved for NENs in 1989 [18]. The radiotracer preferentially binds to SSTR subtypes 2, 3 and 5, especially 2, and has proven to be an essential diagnostic functional imaging for the diagnosis, staging and follow-up of NENs, both pulmonary and, in particular, GEP-NENs. The added value of a whole body assessment in one setting is particularly advantageous.

¹¹¹In-Pentetreotide scans are routinely obtained at 4 and 24 h after the intravenous administration of the radiotracer, and image acquisition is ideally performed with a double-headed gamma camera equipped with a medium-energy collimator [18]. However, in spite of the advantage over CI, the principal limitation of using ¹¹¹In-Pentetreotide is the low spatial resolution of the gamma camera that significantly affects the assessment of small lesions, particularly in organs with a high physiological uptake (such as the liver). The use of SPECT/CT imaging has markedly improved the accuracy of ¹¹¹In-Pentetreotide, increasing its spatial resolution and the anatomic localization of pathologic sites of increased uptake due to CT co-registration, with decreased false positive results and its capacity to provide cross-sectional scintigraphic images [19,20,21,22,23].

¹¹¹In-Pentetreotide, acquired as SPECT/CT, shows higher sensitivity and accuracy values than CI and can be still used for routine diagnostic imaging of non-functioning NEN-GEPs, with significant impact on treatment planning. The added value of SPECT/CT over CI was 35.6%. SPECT/CT corrected CI classification and patient management in 27.9% of cases, while it down-staged the disease better than CI in 9.6% of cases. However, the combined use of CI and functional imaging, using ¹¹¹In-Pentetreotide SPECT/CT, still leaves a big gap in the diagnostic pathway [24]. Therefore, newer, higher-affinity SST analogs, labeled with radioisotopes with better resolution and dosimetry, such as ⁶⁸Ga, which is a positron emitter, have been considered as promising SSTR imaging agents.

⁶⁸Ga-DOTA-conjugated peptide-binding SSTR-based imaging techniques are acknowledged as a new category of radiopharmaceuticals that has recently been instructed into routine clinical practice, including DOTA-D Phe1-Tyr3-octreotide (DOTATOC), DOTA 1-Nal3-octreotide (DOTANOC), and DOTA-D Phe1-Tyr3-Thr8-octreotide (DOTATATE) [25]. Although all of these radiopeptides bind to SSTR2, each has various affinities for other SSTR subtypes. In addition to the greatest affinity to SSTR2, ⁶⁸Ga-DOTA-NOC also reveals a good affinity for SSTR3 and SSTR5; ⁶⁸Ga-DOTA-TOC also shows a high affinity for SSTR5, but less affinity compared to DOTA-NOC, whereas ⁶⁸Ga DOTA-TATE predominantly binds to SSTR2 [26].

Table 1 lists the frequency of overexpression of different SSTR subtypes in different clinical presentations [27].

⁶⁸Ga DOTA conjugated peptide-binding SSTR is generally used for primary tumor localization, including metastatic detection (staging) and evaluation of residual, recurrent, or progressive disease (restaging) [28,29] all the way through to the selection of patients for PRRT [30]. PRRT can successfully control symptoms due to hypersecretion of hormones and has also been displayed to improve OS in progressive or symptomatic NETs patients [31]. The mechanism of uptake is based on increased SSTR expression in neuroendocrine cells and can be detected on SSTR scanning [32]. The previous studies revealed statistically significant decreased tumor uptake of ⁶⁸Ga-DOTA-peptides PET in poorly differentiated NETs, and its uptake might be related to aggressive behavior and might lead to poorer prognosis [33]. However, some cases in this study were low-grade NENs that showed low tumor uptake on ⁶⁸Ga-DOTA-peptides PET/CT scans. It is possible that the FNA site may not fully reflect the true pathological grade of a patient with heterogeneity of cellular differentiation within the same tumor mass, and this may also reflect the potential ability of ⁶⁸Ga-labeled SSTR PET/CT to map these cellular characteristics (Figure 1) [33,34].

The recommended indications for patients with NENs according to EANM Guidelines for ⁶⁸Ga-DOTA-conjugated somatostatin PET/CT imaging [35] are as follows:

• Localized primary tumors and detect sites of metastasis (staging);
• Monitoring patients with known disease in the detection of residual, recurrent or progressive disease (re-staging);
• Determine SSTR status (patients with SSTR positive tumors are more likely to respond to targeted therapy with SSA);
• Select patients with metastasis for PRRT (¹⁷⁷Lu or ⁹⁰Y–DOTA-labelled peptides).

⁶⁸Ga-DOTA-conjugated peptide-binding SSTR is significantly superior to ¹¹¹In-Pentetreotide in both diagnostic accuracy and its therapeutic impact. Available evidence also supports the concept that ⁶⁸Ga-DOTA-conjugated peptide-binding SSTR imaging often demonstrates tumor uptake in some patients with negative or equivocal ¹¹¹In-Pentetreotide scans, and it identifies patients who may benefit from PRRT [36].

A prospective study of 131 GEP-NENs patients determined the superiority of ⁶⁸Ga-DOTATATE PET/CT over ¹¹¹In-pentetreotide SPECT/CT, CT, and/or MRI. ⁶⁸Ga-DOTATATE PET/CT detected 95.1% of lesions, while CI detected 45.3% of lesions, and ¹¹¹In-pentetreotide SPECT/CT detected only 30.9% of lesions with statistically significant differences between all imaging modalities. In the subgroup of patients with carcinoid symptoms but negative biochemical testing, ⁶⁸Ga-DOTATATE PET/CT detected lesions in 65.2% of patients. Of note, 40% of the lesions detected by ⁶⁸Ga-DOTATATE PET/CT were missed on CI or ¹¹¹In-pentetreotide SPECT/CT. Additionally, based on the findings from ⁶⁸Ga-DOTATATE PET/CT, there was a significant change in the patient management in approximately a third of the patients (32.8%) [37].

Binnebeek et al. revealed that significantly more lesions were detected on ⁶⁸Ga-DOTATOC-PET/CT as compared to ¹¹¹In-pentetreotide scans. The sensitivity for PET/CT was 99.9% (95% CI, 99.3–100.0), and for SPECT, it was 60.0% (95% CI, 48.5–70.2). The organ-by-organ analysis showed that the PET was most frequently visualized in the liver and skeleton. They concluded that ⁶⁸Ga-DOTATOC-PET/CT is superior for detecting NENs compared to ¹¹¹In-pentetreotide SPECT [38]. Additionally, treatment was changed in more than one-third of patients with SSTR PET/CT rather than ¹¹¹In-pentetreotide scan [39].

¹⁸F-FDG is used to evaluate glucose metabolism, and it is the most commonly used radiotracer in PET imaging. It is not currently routinely used for NENs imaging, but recent experience has suggested that it can provide complementary information. ¹⁸F-FDG PET/CT has a limited advantage in well-differentiated NETs, since most NENs frequently have near normal glucose metabolism [40]. ¹⁸F-FDG exploits the increased glycolytic activity of tumor cells, as glycolytic activity seems to be low in most NENs. In fact, it can differentiate between slowly proliferative and aggressive tumors [41,42]. Additionally, ¹⁸F-FDG PET/CT scans can allow prognostic prediction, including overall survival (OS) and progression free survival (PFS). Finally, it could potentially identify those who are not responding earlier, characterize early progression, and change therapy if required.

Assessment of glucose metabolism by ¹⁸F-FDG may be helpful in the diagnosis of high-risk aggressive disease associated with poor outcomes. ¹⁸F-FDG PET may still an essential modality in the management of NENs patients, due to its better prognostic value and greater sensitivity to identify the extent of disease, particularly in aggressive high-grade tumors [43] Bahri H et al. revealed that the median PFS and OS are significantly greater in patients with a negative ¹⁸F-FDG PET finding (OS nearly 120 months) than in patients with a positive ¹⁸F-FDG PET finding (only 15 months) [44]. Ezziddin et al. [45] retrospectively reviewed data from 89 patients with metastatic GEP-NENs and classified three different prognostic groups according to FDG uptake: mG1, tumor-to-liver ratio of maximum SUV ≤ 1.0; mG2, 1.0–2.3; and mG3, >2.3. These groups were correlated with significantly different OS (not more than 114 vs. 55 vs. 13 months, respectively). As mentioned, ¹⁸F-FDG PET must not only be used in staging but also for grading whole-body imaging modality; positive ¹⁸F-FDG PET lesions have a significant correlation with prognosis, irrespective of SSTR expression.

The recommended indications for ¹⁸F-FDG PET/CT in patients with NENs according to EANM Guideline for ⁶⁸Ga-DOTA-conjugated somatostatin PET/CT imaging [35] are as follows:

• Localization of NECs and high-grade poorly differentiated NETs with aggressive behavior;
• Prediction of prognosis;
• Localization of synchrone/metachrone non-NEN malignancy.