Gastroenteropancreatic neuroendocrine tumors (GEP-NET) are a relatively rare group of tumors originating from the diffuse nervous system. Due to its low incidence, it has not received much attention. In the past few decades, the diagnostic rate of these tumors has improved significantly due to the development of pathological diagnostic techniques, endoscopic techniques and imaging techniques. The diagnosis of gastroenteropancreatic neuroendocrine tumors is mainly based on clinical symptoms, hormone levels, various imaging examinations, and pathological examinations. However, due to the lack of specificity in the clinical manifestations of most gastroenteropancreatic neuroendocrine tumors, early diagnosis is difficult.
Marker detection of gastroenteropancreatic neuroendocrine tumors Common markers
In addition to clinical manifestations and pathological features, the diagnosis of GEP-NET relies heavily on its biochemical markers. Some markers are only expressed in certain specific types of tumors, while others such as chromogranin A (CgA) and 5-hydroxyindoleacetic acid (5-HIAA), the most commonly used biomarkers, are common markers of GEP-NET.
It is an acidic glycoprotein particle secreted by many normal neuroendocrine cells and many neuroendocrine tumor cells, with a molecular weight of 49 kDa and composed of 439 amino acids. The sensitivity of CgA may be affected by different specimens and assay methods, its level in plasma is higher than that in serum in the same patient, and it is more sensitive to enzyme-linked immunosorbent assay (ELISA) than radioimmunoassay (RIA). the sensitivity of CgA for the diagnosis of neuroendocrine tumors is 77.8-84.0%, and the specificity is 71.3-85.3%.
About 60-80% of GEP-NET patients have elevated blood CgA levels, but some factors such as renal insufficiency, use of proton pump inhibitors or chronic atrophic gastritis should be noted. Blood CgA levels are associated with tumor size, prognosis of tumor patients and their malignant progression, and smaller NETs may have normal blood CgA levels.
Although the detection of circulating CgA levels is important for the diagnosis of GEP-NET, it is often limited in clinical practice. Neither ELISA nor RIA can be used to fully achieve real-time detection. The reason for this is probably related to the fact that all existing assays require bulk testing of specimens (the cost of individual testing for a single patient is too high).
Recently, the human disease-associated peptide research laboratory of the author has overcome this problem by using the biomolecular interaction system (BIAcore), which uses surface plasmon resonance technology to first label CgA antibodies on a specially designed chip, and the biomechanical changes generated by the antigen-antibody binding reaction are transmitted to the detection system via the sensor to achieve the detection of plasma CgA levels. The purpose is to detect plasma CgA levels. The labeled chip can be used repeatedly within a certain period of time, which can solve the clinical problem of real-time detection.
5-Hydroxytryptamine (5-HT) originates mainly from neuroendocrine tumors in intestinal chromophores and can cause symptoms related to carcinoid syndrome. 5-HIAA is a metabolite of 5-hydroxytryptamine and its elevation is commonly found in ileal neuroendocrine tumors. It is worth noting that urine 5-HIAA measurement can be affected by food and drugs. Bananas, eggplant, pineapple, coffee, acetaminophen, and anti-hypertensives can cause false-positive problems, while aspirin and alcohol can cause false-negative problems.
2) Hormone level testing
GEP-NET produces a variety of gastrointestinal hormones, and testing the levels of these hormones can help determine the type of tumor. For example, patients with gastrinoma have increased levels of serum gastrin (Gastrin), patients with insulinoma have hyperinsulinemia (Insulin), and high levels of vasoactive intestinal peptide (VIP) can be detected in the blood of patients with vasoactive intestinal peptide tumor. In addition, tests for 5-HT, pancreatic polypeptide, glucagon, and growth inhibitory hormone are also helpful in differentiating the types of neuroendocrine tumors. Most of these hormone levels are examined by RIA, and thus are clinically limited.
Imaging tests
Various imaging techniques have made great progress in the past decades, which facilitate the diagnosis of GEP-NET. Nevertheless, the primary focus cannot be found in 20-50% of GEP-NET. In particular, gastrinomas and carcinoid tumors originating from the midgut are even more elusive, and people often go back to look for the primary site only when metastases are found.
1) CT/MRI examination
CT and magnetic resonance imaging (MRI) examinations are helpful for localized diagnosis of gastroenteropancreatic neuroendocrine tumors, but it is more difficult to diagnose tumors smaller than 1 cm. Dynamic scanning and multi-phase scanning can improve the diagnosis rate of GEP-NET. The detection rate of conventional CT scan for GEP-NET ranges from 22% to 45%, and the sensitivity of thin-layer scan for GEP-NET diagnosis can be as high as 80%. Similarly, the use of dynamic contrast-enhanced MRI contrast agents (ultra-small superparamagnetic iron oxide particles) can detect vascular permeability and evaluate lymph nodes. Molecular MRI using antibodies or gadolinium-labeled peptides can detect receptors on tumor cells and identify tumor antigens such as Erb-B2, and is useful for evaluating the efficacy of antitumor drugs.
2) Ultrasonography
Ultrasound endoscopy is helpful in the diagnosis of neuroendocrine tumors suspected to originate from the pancreas. While general ultrasound is still related to the experience of the operator, techniques such as endoscopic ultrasound (EUS), intraoperative ultrasound (IOUS) and laparoscopic ultrasound have improved the detection rate of GEP-NET. EUS combined with fine-needle aspiration biopsy can detect 45% to 60% of duodenal-derived lesions and 90% to 100% of pancreatic-derived lesions, while laparoscopic ultrasound combined with fine-needle liver biopsy is useful for determining the nature of liver metastases.
(3) Growth inhibitory receptor imaging (SRS)
The cell surface of most gastroenteropancreatic neuroendocrine tumors is rich in growth inhibitory receptors, and about 70%-90% of GEP-NETs express multiple growth inhibitory receptor subtypes, mainly type 2 and type 5 receptors. Therefore, the use of synthetic growth inhibitory short peptides [octreotide or pentetreotide] in combination with radionuclides [111 indium (In)] has greatly improved the localization of tumors. SRS has a sensitivity of 81%-96% (50%-90% for angiography, 55%-70% for MRI, and 14%-63% for ultrasound), especially for non-functional GEP-NET (one of the most effective means). the sensitivity of SRS for the diagnosis of non-insulinomas is 55%-77%, and for the diagnosis of insulinomas is only 25%. the prognosis of SRS-negative patients is worse, probably related to the fact that this group of patients does not benefit from treatment with growth inhibitors and their analogues.
Although SRS is a highly effective imaging technique for the diagnosis of most GEP-NETs, it is influenced by a number of factors. For example, patients with Crohn’s (Crohn’s) disease may also present with elevated growth suppressor receptor (SSTR) expression. In addition, some GEP-NETs may be too small or fail to express sufficient levels of SSTR to be detected. Other reasons for failure to detect lesions include weak signals in high background areas (e.g., liver), technical problems (too short a scan time), etc.
Positron Emission Tomography (PET)
Functional imaging is a relatively new imaging technique based on the metabolic activity of tumors, often using multiple radioactive substrates for detection. Although 18F-deoxyglucose (FDG)-PET is an accepted technique for tumor imaging, it is of little value for most GEP-NETs except for aggressive tumors. Recently, the use of 68Ga marker [(68Ga-DOTA)-D-Phe(1)-Tyr(3)-octreotide] is effective in detecting metastatic GEP-NET. clinical studies have found that 68Ga marker PET imaging has a higher detection rate and sensitivity for NET than SRS.
Gastrointestinal endoscopy
The use of endoscopy as a common examination of the gastrointestinal tract has become increasingly common, which is beneficial to improve the detection rate of NET in the gastrointestinal tract. Although endoscopy cannot directly determine NET, it can be combined with biopsy to detect it preoperatively rather than waiting for postoperative pathological testing. More than half of the GEP-NET cases identified in the gastroenterology department of the author’s hospital were first detected by endoscopy, and this group of patients may not have typical clinical symptoms.
Double balloon small bowel microscopy provides limited visual inspection of small bowel lesions, but has the advantage of identifying the location of small bowel NET and the histologic origin of the tumor by biopsy. Although the diagnostic sensitivity of this technique is only between 21% and 52%, it has some application in identifying small bowel bleeding due to NET. Compared to small bowel microscopy, capsule endoscopy has the advantage of being painless and safer, with the disadvantage of less accurate localization and the inability to take a biopsy. Therefore, it is advocated that patients suspected of small bowel NET should first undergo capsule endoscopy to initially determine the site of the lesion, and then undergo targeted small bowel microscopy and biopsy.
Angiographic techniques
Although simple angiography has been largely replaced by MRI angiography or CT 3D revascularization, selective or super-selective angiography is still valuable in determining the blood supply to the tumor, identifying the source of blood supply and the relationship between the tumor and adjacent vessels. Overall, a more accurate morphological outline of the tumor or tumor-associated vessels can be performed with the help of angiography, which is beneficial in determining the surgical approach and lesion resection. Selective excitation angiography can be performed by selective injection of pancreatin (suspected gastrinoma) or calcium (suspected insulinoma) into specific mesenteric arteries, while gastrin or insulin levels can be measured in liver and peripheral venous blood samples. Selective angiography is an invasive test, but is more valuable in cases where other tests are difficult.
Our diagnosis of GEP-NET requires early recognition of its clinical signs followed by biochemical tests such as CgA. In general, endoscopy, EUS, CT/MRI, SRS and PET-CT are effective examinations, but final confirmation of diagnosis still relies on pathology and immunohistochemistry.
SRS can identify SSTR on the surface of tumor cells and is also one of the most effective techniques to diagnose non-functional GEP-NET. In conclusion, the diagnosis of GEP-NET requires the organic combination of clinical, biochemical, imaging and pathology in order to truly achieve early detection and improve the diagnosis rate.