The basis of functional imaging lies with the targeted detection of specific cell targets or receptors, allowing precise localization of lesions. In the context of diagnostic imaging, the concentration of receptor molecules in target tissues may be hard to differentiate from background non-specific binding. As such, molecular imaging techniques have often been confined to nuclear-based modalities such as positron emission tomography (PET) or single photon emission CT (SPECT), which are able to generate images with micromolar to picomolar concentrations of imaging probes.
The Delphi consensus with regard to the diagnostic imaging of NETs, has acknowledged that functional imaging in the form of somatostatin receptor scintigraphy (SRS) plays a central role in the diagnosis of NETs, and we will explore this and various other functional imaging modalities and techniques in relation to their clinical utility in the diagnosis of NETs. Our discussion will focus largely on the gastroenteropancreatic system, but general principles are likely applicable to NETs in other parts of the body.
Somatostatin receptor scintigraphy
Somatostatin receptors are widely distributed in the human nervous system and tissues in the body, including the adrenals, kidneys, pancreas and prostate. Currently, 5 subtypes of somatostatin receptors have been identified in humans (SSRT1, SSRT2, SSRT3, SSRT4, SSRT5), with SSRT2 further classified into subtypes 2A and 2B.
Of particular interest, somatostatin receptor expression has been found in a large number of tumors, of which NETs are the archetypical class, and this forms the basis for the molecular imaging of NETs.
The half-life of somatostatin itself is too short (< 2 min) for use in either diagnosis or therapy. As a result, synthetic somatostatin analogues with sufficiently long half-lives have been developed for use in diagnostic imaging or therapeutics.
The first commercially available somatostatin analogue was Octreotide (Sandostatin, Novartis Pharmaceutical Corp), with an approximate half-life of 2 h, and a radiolabeled analogue of octreotide, Octreoscan® (111In-DTPA-Octreotide, D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr[ol]), was successfully used to visualize somatostatin receptor positive tumors by gamma camera scintigraphy in the early 1990s[48-50].
Normal physiological uptake is seen in the thyroid, spleen, liver and pituitary due to receptor binding of the peptides, while tracer uptake in the kidneys is predominantly secondary to reabsorption of filtered peptides, and bowel uptake is presumably secondary to hepatobiliary clearance (Figures 6 and 7).
Figure 6 Gallium 68 DOTATATE positron emission tomography from the skull vertex to mid-thigh.
The coronal maximum intensity projection image demonstrates physiological areas of tracer uptake in the pituitary (black arrowhead), kidneys (black arrows), liver and spleen (curved arrows).
Figure 7 Indium 111 Octreotide Planar whole-body images.
A: Indium 111 Octreotide 24-h delayed anterior and posterior planar whole body images in a patient with prior resected pancreatic neuroendocrine carcinoma. Several abnormal tracer foci (white arrows) are seen in the peri-hepatic region, suspicious for somatostatin receptor expressing lesions. These were later confirmed as neuroendocrine nodal metastasis in the peri-hepatic and peri-gastric nodes; B: Indium 111 Octreotide 24-h delayed anterior and posterior planar whole body images in a patient with histologically confirmed neuroendocrine carcinoma of the pancreatic body. Increased tracer focus in the region of the pancreas (curved white arrow) corresponds to the primary pancreatic lesion, while multiple abnormal foci of uptake in the liver (white arrows) are in keeping with hepatic metastasis.
Gamma-based SRS (Octreoscan®) has proved to be a safe, sensitive imaging agent in the detection of GEP NETs, with an overall sensitivity of approximately 80%-90% in patients with gastrointestinal neuroendocrine neoplasms[51-54]. However, limitations include false negative results in organs with significant physiological uptake (e.g. liver) where background uptake may mask lesions, and small volume diseases that may be below the intrinsic spatial resolution of gamma imaging. Additionally, false positives can occur with a variety of lesions, such as the thyroid gland, accessory spleens, granulomatous or inflammatory tissue, and benign or malignant breast lesions. Other types of neoplasms that demonstrate somatostatin receptor expression include meningiomas and lymphomas.
Nonetheless, SRS is considered the “gold standard” in the diagnosis, staging and follow-up of patients with NETs (Figure 8).
Figure 8 Patient with prior history of neuroendocrine carcinoma in the pancreatic tail, status post partial pancreatectomy and splenectomy.
A: Gallium 68 DOTATATE positron emission tomography/computed tomography (PET/CT). Axial CT and fused PET/CT images of the abdomen shows a mass in the left upper abdomen demonstrating significant DOTATATE tracer avidity (white arrow). Considerations included tumor recurrence or splenunculus; B: Technetium 99m Sulfur Colloid scintigraphy. Anterior and posterior planar spot views of the upper abdomen demonstrates a focus of uptake (black arrows) in the left upper abdomen, corresponding to the area of uptake seen on the previous Gallium 68 DOTATATE scan, confirming the mass to be a splenunculus.
Newer generation somatostatin analogues have since been developed, allowing radiolabeling with positron emitting tracers. Together with the development and adoption of hybrid PET/CT modalities, this potentially addresses several limitations faced with first generation SRS, largely related to the poorer spatial resolution of gamma-based probes and the issue of precise anatomical localization (Figures 9 and 10).
Figure 9 Gallium 68 DOTATATE positron emission tomography/computed tomography of a patient with right hepatic lobe neuroendocrine tumor metastasis, status post resection, but of unknown primary.
A: Axial computed tomography (CT) and fused positron emission tomography/CT (PET/CT) image of the pelvis shows an intensely DOTATATE tracer avid eccentric thickening of the rectum (white arrow), suspicious for a rectal primary. This was histologically confirmed as a neuroendocrine carcinoma; B: Axial CT and fused PET/CT image of the abdomen. Surgical clips are seen along the right liver margin, in keeping with previous surgery. There is an intensely tracer avid focus seen in segment 3 (white arrow). Although there were no obvious findings on the correlative non-contrast CT, this is suspicious for an additional liver metastasis. Note the normal physiological uptake in the kidneys, spleen and liver.
Figure 10 Gallium 68 DOTATATE positron emission tomography/computed tomography of a patient with histologically proven abdominal paraganglioma.
For pre-therapy staging. A: Axial computed tomography (CT) and fused positron emission tomography/CT (PET/CT) image of the abdomen demonstrates an intensely DOTATATE avid mass in the inter aorto-caval region (white arrow), correlating with the primary tumor; B: Axial CT and fused PET/CT image of the abdomen shows a peritoneal mass adjacent to the spleen that shows avid tracer uptake (white arrow). This is compatible with a peritoneal deposit.
PET-based SRS has shown high sensitivities, specificities and accuracies in the evaluation of NETs. Initial evaluations using PET-based SRS were encouraging. Hofmann et al found higher tumor to non-tumor contrast ratios with significantly higher detection ratios for PET-based SRS, and Kowalski et al also concluded that PET-based SRS was able to detect more lesions and was superior in detecting smaller lesions.
A larger prospective study by Gabriel et al evaluating the diagnostic value of 68Ga-DOTATOC PET in 84 patients with known or suspected NETs demonstrated a sensitivity of 97%, specificity of 92% and an overall accuracy of 96%, showing significantly higher diagnostic efficacy as compared with SPECT imaging using gamma-based SRS and normal diagnostic CT. In addition, PET-based SRS detected more tumor sites in the liver, nodes and bone as compared with the other modalities, and provided further clinically relevant information in 14% of patients compared with gamma-based scintigraphy and 21% as compared with CT.
This was substantiated by Putzer et al, who evaluated 51 patients with histologically proven NETs with 68Ga-DOTATOC PET/CT. Reported sensitivity and specificity were 97% and 92%, respectively, higher than CT or bone scan, and detected bone metastasis in patients at a significantly higher rate. This is particularly important as osseous metastasis has a negative prognostic implication on clinical outcomes.
Furthermore, the increased diagnostic accuracy of PET-based SRS has been shown in publications to impact on actual clinical management. Ambrosini et al evaluated the clinical impact of 68Ga-DOTANOC PET/CT imaging in 90 patients with histologically proven NETs. In the subgroup of patients with concordant PET and CT findings (n = 47), PET resulted in a modification of therapeutic management in 36.2% of patients. In the subgroup of patients with discordant PET and CT findings (n = 42), PET resulted in stage modification in 28.6% of patients and a change in management in 76.2% of patients. Overall, PET imaging affected either staging or therapy in 55.5% of patients imaged, with the most frequent management impact being initiation or continuance of peptide receptor radionuclide therapy, initiation or continuance of somatostatin analogue treatment, or referral for surgery. The author also reported that PET prevented unnecessary surgery in 6 patients, and excluded 2 patients with peptide receptor radionuclide treatment who did not show significant somatostatin analogue avidity.
With regard to post-therapy response assessment of NETs following peptide receptor radionuclide therapy, findings are controversial.
Gabriel et al evaluated 46 patients with advanced NETs who underwent peptide receptor radionuclide therapy. 68Ga-DOTATOC PET (dedicated PET) and conventional CT was performed pre- and post-therapy for all patients. RECIST criteria were used to evaluate therapy response, with a reported 30% response rate, 48% stable disease and 22% progressive disease. Concordant findings were noted in 70% of cases. In the 30% discrepant group (n = 14), PET-based SRS outperformed CT in 10 patients, was able to detect lesions not seen on CT in 5 patients and accurately determined disease response in 5 patients. In contrast, CT was able to detect small pulmonary lesions in 1 patient not seen on PET, and in the remaining 3 patients, PET-based SRS showed decreased tracer uptake in the lesions, but these were due to tumor dedifferentiation rather than therapy response, while CT clearly showed tumor size and extent of progression.
The author concluded that PET-based SRS showed no advantages over conventional imaging in response assessment, but several limitations in the study have to be noted. Firstly, the study utilized a dedicated PET scanner, while the majority of newer installations are hybrid PET/CT scanners, and the intrinsic limitations of dedicated PET imaging is accounted. Indeed, based on the 4 discrepant findings reported in the study, if a hybrid PET/CT scanner was utilized, it is expected that such discrepancies would not exist. Secondly, the emergence of non-somatostatin analogue avid lesions on post -therapy assessment scans is of clinical use, as it indicates dedifferentiation of tumor, suggesting the need for alternative treatment from peptide receptor radionuclide therapy or somatostatin analogues (Figures 9 and 10).
Overall, PET-based SRS has been routinely found to demonstrate high diagnostic sensitivity, specificity and accuracy, with positive clinical impact during pre-therapy staging. The use of SRS for post-therapy assessment is more indeterminate, and further evaluation needs to be carried out.
Fluorodeoxyglucose (FDG) PET imaging is a molecular imaging technique that addresses the glucose metabolism of tissue. As a rule of thumb, malignant tumors tend to demonstrate significantly higher levels of glucose metabolism as compared with normal physiological tissue, and this has proven true across a wide range of tumor types.
The molecular basis of increased glucose metabolism in tumor cells is complex, and there appears a multitude of factors controlling aerobic glycolysis in tumors. However, 2 major factors have been implicated with increased FDG tumor uptake. Firstly, the overexpression of glucose transporters and activity in tumor cells (predominantly GLUT-1, 3 and 5) which actively drive glucose into the cells, and secondly, the overexpression of hexokinase enzymes (predominantly hexokinase-2) that increase glucose metabolism[65,66].
The use of FDG PET in NETs is currently controversial. There are limited sensitivities overall, but there is emerging evidence that the presence of increased glucose metabolism in tumors highlights an increased propensity for invasion and metastasis, and overall poorer prognosis. This correlates with mathematically-based telogenic models and empiric data reviewed by Gillies et al, where such increased glucose metabolism confers an “evolutionary advantage” in cancer cells over normal parenchymal tissue.
An early study performed by Adams et al found that FDG PET only demonstrated increased glucose uptake in less differentiated tumors with high proliferative activity. Another small study performed by Pasquali et al evaluated the clinical use of FDG PET against conventional gamma-based SRS and CT, and again found that FDG PET was able to detect NETs characterized by rapid growth or aggressive behavior. Garin et al performed a prospective study evaluating the clinical outcomes of 38 patients with metastatic NETs. FDG PET, SRS and conventional CT were performed for these patients, and patients were tracked to determine progression-free survival and overall survival. Overall 2 year survival and progression-free survival was 73% and 45%, respectively, and it was found that most patients with FDG PET positive lesions had early progressive disease (14/15 for FDG PET positive as compared with 2/23 for FDG PET negative). Furthermore, when only patients with low-grade tumors were considered, FDG PET was able to predict those with early progression. Progression-free survival was 87% ± 7% and 75% ± 10% at 1 and 2 years, respectively, for FDG PET negative lesions, as compared with 7% ± 6% and 0% at 1 and 2 years, respectively, for FDG PET positive patients. Overall, the relative risk of early progression with FDG PET positive lesions was 10.7 (95% CI: 2.8-40.6).
In terms of survival, FDG PET negative patients fared better than patients with FDG avid lesions. Overall survival was 95% ± 5% at both 1 and 2 years, respectively, for FDG PET negative patients, vs 72% ± 12% and 42% ± 13% at 1 and 2 years, respectively, for 18F-FDG PET positive patients.
Overall, the use of FDG PET appears promising in disease prognostication, possibly influencing aggressiveness of management. In addition, dual tracer imaging using both FDG and SRS PET might possibly be used in post-therapy assessment following peptide receptor radionuclide therapy to evaluate for tumor dedifferentiation or the “flip-flop” phenomenon (Figure 11).
Figure 11 Gallium 68 DOTATATE and Fluorine 18 fluorodeoxyglucose positron emission tomography/computed tomography of a patient with metastatic neuroendocrine carcinoma.
A: Axial computed tomography (CT), fused DOTATATE positron emission tomography//CT (PET/CT) and fused fluorodeoxyglucose (FDG) PET/CT images of the brain. There is a focus of moderately increased DOTATATE uptake seen in the right occipital lobe (black arrow) associated with adjacent vasogenic edema, but no definite corresponding FDG uptake is seen; B: Axial CT, fused DOTATATE PET/CT and fused FDG PET/CT images of the thorax. There is an intensely FDG avid mass in the left upper lobe, but no corresponding DOTATATE tracer avidity is seen (white arrows), demonstrating an example of the “flip-flop” phenomenon secondary to tumor dedifferentiation; C: Axial CT, fused DOTATATE PET/CT and fused FDG PET/CT images of the abdomen. Again, there are intensely FDG avid lesions in the right liver lobe (white arrowheads) that do not demonstrate significant DOTATATE uptake, indicative of neuroendocrine tumor dedifferentiation.
The APUD by Everson Pearse describes the ability of neuroendocrine type cells to take up and decarboxylate amino acid precursors, and there have been various efforts to evaluate the utility of radiolabeled amine precursors to image NETs. Examples of such precursors include hydroxytryptophan, hydroxyephedrine, dopamine and dihydroxyphenylalanine (DOPA). The radiolabeled DOPA analogues are transported into NETs via the sodium independent system L, and the activity of amino acid decarboxylase in the cells is important for intracellular retention of the metabolized radiolabeled DOPA analogue. Becherer et al in the evaluation of 23 patients with histologically proven NETs concluded that 18F-DOPA PET performed better that gamma-based SRS in visualizing lesions, with the highest sensitivity in visualizing skeletal and mediastinal lesions. Reported sensitivities were 81.3% for the liver, 90.9% for the skeleton and 100% for the mediastinum and lymph nodes.
Koopmans et al evaluated 53 patients in a prospective single-center diagnostic accuracy study using 18F-DOPA PET, conventional CT and SRS without any CT correlation, and reported that 18F-DOPA PET detected more lesions, more positive regions and more lesions per region as compared with the other modalities. Reported sensitivities at the patient, region and lesion levels were 100%, 95% and 96%, respectively.
Kauhanen et al evaluated 82 patients with suspected/known NETs using 18F-DOPA PET, comparing the diagnostic accuracy with histological findings and clinical follow-up. 32 patients were for primary diagnosis and staging, while 61 patients were for restaging. Overall accuracy for gastrointestinal NETs was approximately 89%.
Overall, based on a meta-analysis by Jager et al, the radiolabeled DOPA analogues have reported sensitivities in the range of 65%-96% for the detection of individual lesions, with most of the values in the upper half of this range.
The advantages of DOPA PET over conventional anatomic imaging or gamma-based SRS are fairly conclusive, but the role in comparison with PET-based SRS techniques is still uncertain. Accuracies for PET-based SRS as discussed earlier appear to be comparable or better than DOPA PET. In a head to head comparison between 68Ga-DOTATATE and 18F-DOPA PET in the diagnosis of differentiated metastatic NETs, patient-based sensitivities were 96% for 68Ga-DOTATATE compared with 56% for 18F-DOPA, with 68Ga-DOTATATE PET proving clearly superior for detection and staging of NETs.
In addition, the clinical impact of PET-based SRS has been established in several publications, and the added advantage of SRS is that it allows the suitability assessment for peptide receptor radionuclide therapy, something that DOPA PET does not allow.