Neuroendocrine tumors are a group of neoplasms that can arise in a variety of locations throughout the body and often metastasize early. A patient’s only chance for cure is surgical removal of the primary tumor and all associated metastases, although even when surgical cure is unlikely, patients can benefit from surgical debulking. A thorough preoperative workup will often require multiple clinical tests and imaging studies to locate the primary tumor, delineate the extent of the disease, and assess tumor functionality. This review discusses the biomarkers important for the diagnosis of these tumors and the imaging modalities needed.
Key points
- •
Many neuroendocrine tumors (NETs) secrete substances that can cause symptoms, but also aid biochemical diagnosis and localization of the primary tumor.
- •
There are many foods and medications that can interfere with biomarker assays.
- •
In cases where a pancreas neuroendocrine tumor is suspected, pancreatic polypeptide, chromogranin A (CgA), calcitonin, parathyroid hormone-related peptide, and growth hormone releasing hormone should be drawn during the patient’s initial visit.
- •
When a gastrointestinal NET is suspected, CgA and serotonin levels should be obtained.
- •
Molecular testing may be used to identify an unknown metastasis as a NET and can be more accurate than traditional histologic procedures in differentiating between primary tumor sites.
Introduction
There has been a marked increase in the incidence of neuroendocrine tumors (NETs) over the past several decades, from approximately 1 case per 100,000 in 1973 to 5 cases per 100,000 in 2004. The reasons for this increase are unclear and could be due to increased environmental exposures, a greater understanding and awareness of these tumors, and the parallel, marked increased use of anatomic imaging studies over this period. Regardless of the cause, these tumors have gone from rare to commonplace, and clinicians need tools to help differentiate NETs from other neoplasms. Furthermore, 30% of patients with small bowel (SBNETs) and 64% of pancreatic NETs (PNETs) present with metastatic disease, and determining the primary NET site of origin is critical for guiding future surgical and medical therapy. This review describes the different modalities commonly used in the diagnosis and follow-up of gastroenteropancreatic (GEP) NETs, including biochemical markers, gene expression tests, and radiologic and nuclear medicine imaging.
Introduction
There has been a marked increase in the incidence of neuroendocrine tumors (NETs) over the past several decades, from approximately 1 case per 100,000 in 1973 to 5 cases per 100,000 in 2004. The reasons for this increase are unclear and could be due to increased environmental exposures, a greater understanding and awareness of these tumors, and the parallel, marked increased use of anatomic imaging studies over this period. Regardless of the cause, these tumors have gone from rare to commonplace, and clinicians need tools to help differentiate NETs from other neoplasms. Furthermore, 30% of patients with small bowel (SBNETs) and 64% of pancreatic NETs (PNETs) present with metastatic disease, and determining the primary NET site of origin is critical for guiding future surgical and medical therapy. This review describes the different modalities commonly used in the diagnosis and follow-up of gastroenteropancreatic (GEP) NETs, including biochemical markers, gene expression tests, and radiologic and nuclear medicine imaging.
Biochemical markers for gastroenteropancreatic neuroendocrine tumors
Approximately 50 years ago, Pearse proposed that all peptide-producing cells of the gut, pancreas, and to a lesser extent, the anterior pituitary gland, belonged to a larger system that shared similar chemical, ultrastructural, and functional characteristics. This system was called the diffuse neuroendocrine cell system, and Pearse held that all of these cells were of neural crest origin. GEP NETs were postulated to derive from a common endocrine progenitor termed amine precursor uptake and decarboxylation cell. Neoplasms arising from this system are defined as epithelial neoplasms with predominant neuroendocrine differentiation. One property shared by these cells and their respective tumors is staining with neuroendocrine immunohistochemical (IHC) markers CgA and synaptophysin. Another property is that approximately 80% of NETs express the somatostatin subtype 2 receptors (SSTR2), allowing for the use of synthetic somatostatin (congeners) in the diagnosis and management of these NETs. It has been suggested that the long latency period of NETs (up to 9 years for midgut carcinoids) may be related to the inhibitory and antiproliferative action of native somatostatin and its congeners via membrane receptor coupling.
NETs may occur throughout the body, including the lung (bronchial carcinoids), thyroid (medullary thyroid cancer), adrenal gland (pheochromocytoma), gastrointestinal (GI) tract (stomach, duodenum, jejunum, ileum, colon, and rectum), pancreas, and the skin (Merkel cell carcinoma). This occurrence throughout the body is not surprising because the cells of the diffuse neuroendocrine system have come to reside normally in these various organs and tissues. These tumors produce amines and peptides that can be exploited for diagnosis and followed for response to therapy ( Table 1 ). These secreted substances may cause symptoms that give clues as to tumor location and are ideal markers to be selected for biochemical testing. This review focuses on NETs of the GEP system, which may be functional (cause symptoms) or nonfunctional. The most frequently encountered GEP NETs are of the small bowel (SBNETs, or carcinoid tumors) and pancreas (PNETs), which account for approximately 70% to 75% of all tumors of the diffuse neuroendocrine system in humans.
| (Neuro) Peptide/Amine | Tumor | Value | Interfered with by |
|---|---|---|---|
| Urine 5-HIAA | GI NETs | Elevated in 88% midguts, 30% foreguts, rare in hindguts | Tryptophan-rich foods, caffeine, wine, several medications (see text) |
| Serotonin | GI NETs Some PNETs | Elevated in 96% midgut, 43% foregut, 20% hindgut | Lithium, MAO inhibitors, morphine, methyldopa, reserpine |
| CgA | GI NETs PNETs | 80%–90% midgut and foregut, most hindgut Useful to follow debulking, recurrence, progression | Somatostatin analogues, PPIs, renal insufficiency, cirrhosis, CHF May also be elevated in HCC and MTC |
| Pancreastatin | GI NETs PNETs | Elevated in 80% GI NETs Useful to follow debulking, recurrence, progression | Renal insufficiency Medications affecting insulin levels |
| NKA | GI NETs | Elevated in 21%–70% of midgut carcinoids Indicates poor prognosis if elevated | Medications for hypertension, pain, and GI function |
| Gastrin | Gastrinoma | Elevated in 98% Should also have hyperchlorhydria, high basal acid output | PPIs; atrophic gastritis/pernicious anemia, diabetic gastroparesis, gastric outlet obstruction, short bowel syndrome, retained antrum, H pylori infection |
| Insulin | Insulinoma | Elevated in 98% Hypoglycemia with 72-h fast | Exogenous recombinant insulin |
| Glucagon | Glucagonoma | Useful when syndrome is present | DM, acute burns and trauma, cirrhosis, renal failure, Cushing syndrome, bacteremia |
| VIP | VIPoma | Useful when syndrome is present | Recent radioisotope administration |
| Somatostatin | Somatastatinoma | Useful when syndrome is present | MTC, small cell lung cancer, pheochromocytoma |
| PP | PPoma | Good marker for nonfunctional PNETs and cosecreted with hormone in many functional PNETs | Other PNETS, nesidioblastosis, PP cell hyperplasia, renal dysfunction |
Gastrointestinal Neuroendocrine Tumors
The derivation of the term “carcinoid” (carcinoma-like, karzinoide) is credited to Oberndorfer, whose series of 6 cases published in 1907 identified what was thought to be a form of benign neoplasia. Carcinoid tumors of the small intestine account for approximately 55% of all adult NETs, and 28% to 44% of all malignant tumors of the small bowel. Its incidence has increased 4-fold between 1973 and 2004 (from 2.1 to 9.3 cases per million), and it has transcended adenocarcinoma as the most common cancer type of the small bowel in 2000. The neuroendocrine cell giving rise to small bowel carcinoids of the jejunum and ileum is the Kulchitsky-enterochromaffin (EC) cell, which is a gut epithelial cell that contains secretory granules that store and release serotonin (5-hydroxytryptamine) and other peptides (such as CgA, synaptophysin, and substance P). They are actually derived from enterocyte stem cells, rather than from neural crest cells, as first proposed by Pearse.
Most serotonin in the body (>90%) is produced in the GI tract, which is metabolized by monoamine oxidase (MAO) into its breakdown product 5-hydroxyindole acetic acid (5-HIAA) in the liver and lung, and then excreted into the urine. When SBNETs are metastatic to the liver, serotonin may not be metabolized before its release into the systemic circulation. Sustained serotonin elevation results in the carcinoid syndrome. Serotonin is normally released by EC cells in response to pressure (food), certain nutrients, and bacteria. It acts on the neurons within the gut to stimulate peristalsis. Only 5% to 10% of persons with carcinoid tumors will have carcinoid syndrome, and 76% of these patients will have diarrhea, which may be secretory, hypermotile, malabsorptive, or obstructive. Facial flushing may affect 80% of persons with carcinoid syndrome and is usually episodic. It may be precipitated by catecholamine-driven emotion, excitability, exercise, decongestants, and eating. This symptom is generally mediated by kallikrein, bradykinins, substance P, histamines, and other peptides, rather than serotonin. The episodic spikes of serotonin levels are often associated with hypotension, which worsens the catecholamine-driven serotonin release. Serotonin (and possibly the tachykinin, substance P) is associated with severe bronchial wheezing, which occurs in about 20% of patients with carcinoid syndrome. Cardiac fibrosis with right-sided valvular disease is seen in as many as 50% of patients. The primary mediator is thought to be serotonin, but substance P may contribute as well. Some patients may also develop pellagra, due to niacin depletion resulting from tryptophan being shunted to serotonin synthesis rather than nicotinic acid.
Laboratory tests and biomarkers for gastrointestinal neuroendocrine tumors
In the past, the gold standard for the biochemical diagnosis of carcinoid tumors was the measurement of the serotonin metabolite 5-HIAA in a 24-hour urine collection. This test remains useful, with elevation found in 88% of carcinoid patients, but can be falsely elevated by a variety of tryptophan-rich foods (cheese, bananas, kiwis, walnuts, tomatoes, pineapples, spinach, eggplant, avocados), wine, caffeine, and various medications (acetaminophen, MOA inhibitors, isoniazid, phenothiazines, iodine, 5-fluorouracil). It is less commonly elevated in those with foregut tumors and hindgut tumors. It has a high sensitivity (approaching 100%), but low specificity (35%). Limitations of the test are its inconvenience for the patient and that it may be negative in those with low volume disease (such as a patient with a small bowel primary without nodal or liver metastases). Plasma assays for 5-HIAA are now available and may become another useful tool in the management of these patients.
Most serotonin in the blood is stored within platelets, and measurement of whole blood serotonin has been improved by performing liquid chromatography from platelet-rich plasma. The plasma serotonin assay is now considered very reliable for the diagnosis of carcinoids when performed by Clinical Laboratory Improvement Amendment (CLIA) -licensed and College of American Pathology -approved commercial laboratories in the United States. This test has a positive predictive value of 89% and negative predictive value of 93% in patients with midgut carcinoids, but is less accurate in those with foregut and hindgut carcinoids. It may not correlate as well with the tumor burden as other laboratory assays (eg, chromogranin, pancreastatin) because platelets become saturated at high levels of serotonin. Excess serotonin remains unbound and continues to circulate in the blood. This assay can also be falsely elevated in patients taking lithium, MAO inhibitors, morphine, methyldopa, and reserpine.
CgA is a 457-amino-acid peptide that is widely distributed in endocrine and neuroendocrine tissues, is present in normal islet cells, and is cosecreted from EC cells with serotonin. Its normal function is to promote formation of secretory granules and it serves as the precursor protein for several negative regulators of neuroendocrine cells (pancreastatin, vasostatin, catestatin). Serum CgA levels are considered one of the most useful markers for diagnosis and surveillance of patients with GI NETs, including hindgut and foregut tumors, where 5-HIAA and serotonin levels are often within normal limits. The sensitivity depends on the specific assay used, but ranges from 67% to 93%, and the specificity from 85% to 96%. Levels of CgA may also be useful for determining prognosis, as patients with CgA greater than 200 U/L have a lower median survival than those less than 200 U/L (2.1 vs 7 years, respectively). CgA is also a useful marker for determining the efficacy of debulking procedures, disease recurrence, and progression. Unlike serotonin, CgA levels maintain a good relationship with overall tumor burden, even when circulating levels are high. CgA levels are increased by somatostatin analogues, use of proton pump inhibitors (PPIs; but not H2 blockers), atrophic gastritis/pernicious anemia, and renal insufficiency.
Pancreastatin is a 52-amino-acid derivative of CgA. Its primary function is to decrease cellular glucose uptake. It is a useful marker for diagnosis, the effect of debulking, and tumor progression. It is elevated in as many as 80% of patients with GI NETs. In contrast to the CgA assay, it is not affected by PPI or somatostatin analogue use. The pancreastatin assay does not cross-react with CgA. Pancreastatin is a useful marker for GI NET prognostication and more accurately predicted patient outcome in SBNET and PNET patients than did serial measurements of CgA, serotonin, and neurokinin A (NKA) in one recent study. Patients with SBNETs and preoperative elevation of pancreastatin had a median progression-free survival (PFS) of 1.7 years versus 6.5 years when this was normal. If pancreastatin normalized after surgery, PFS improved to 4.2 years (compared with 1.6 years if this remained high postoperatively).
NKA is a tachykinin and bronchoconstrictor that represents an alternatively spliced isoform of substance P. One study examining 73 patients with midgut NETs (80% with metastases) found elevated levels in 70% and that levels seemed to correlate with metastatic tumor burden. Unfortunately, only a minority of patients had both preoperative and postoperative levels drawn. Diebold and colleagues demonstrated that in patients with metastatic midgut NETs (40% with liver metastases), serum NKA levels of less than 50 pg/mL correlated with improved 2-year survival (93% vs 49%) compared with those with more elevated levels. They suggested that when NKA levels normalized after surgery, patient outcomes improved, but survival statistics were not given. Sherman and colleagues did not find a correlation between preoperative NKA levels and PFS or overall survival in 52 midgut patients treated with surgery, and thus, more data are needed to determine the prognostic value of NKA in midgut carcinoid patients.
Current recommendations for biochemical testing in gastrointestinal neuroendocrine tumors
The National Comprehensive Cancer Network guidelines for GI NETs suggest testing for CgA and collecting a 24-hour urine for 5-HIAA, but do not give specific recommendations for follow-up. The European Neuroendocrine Tumor Society (ENETS) also recommends that the minimal testing performed for GI NETs should include serum CgA and urine 5-HIAA. They also recommend using these assays in follow-up for tumor recurrence and progression. They further suggest that serotonin assays are insensitive and not recommended for either diagnosis or follow-up, but state this biomarker’s utility may be improved using the platelet-based assays. They do not comment on pancreastatin and NKA (they suggest that further validation is warranted for newer markers), but do comment that neuron-specific enolase should not be used. The North American Neuroendocrine Tumor Society (NANETS) again only mentions serum CgA and urinary 5-HIAA levels as potentially valuable for measuring response to therapy or progression. They suggest that 5-HIAA may be less useful in foregut (including PNETs) and hindgut NETs, because these tumors tend to not make high levels of serotonin. In patients with midgut and other GI NETs, it is the authors’ practice to measure to serum serotonin, CgA, pancreastatin, and less commonly, NKA, preoperatively and at each follow-up visit. These biomarkers are readily available and measurable by many CLIA-certified, American College of Pathologists sanctioned laboratories in the United States. Ideally, serial measurements should be from the same laboratory, recognizing that standards and quality control vary.
Pancreatic Neuroendocrine Tumors
PNETs, previously known as islet cell tumors, account for about 1% to 2% of all pancreatic neoplasms and for about 6% of all NETs. Their incidence has increased from 1.4 cases per million in 1973 to 3.0 cases per million in 2004. According to the National Cancer Database (NCDB), 85% were classified as nonfunctional, meaning there is no clinical syndrome associated with hormone excess. Because hormone levels are not collected in the database, and tumors classified as pathologically benign (like many insulinomas) are also not included, numbers derived from the NCDB may be overestimated. However, other recent studies suggest that PNETs are nonfunctional in 68% to 90% of cases.
Human adult islet cells produce the hormones insulin, glucagon, somatostatin, vasoactive intestinal peptide (VIP), pancreatic polypeptide (PP), and serotonin; fetal islet cells can produce gastrin. PNETs may secrete any of these hormones, and in addition, rare tumors may secrete adrenocorticotropic hormone, parathyroid hormone-related peptide (PTH-rP), calcitonin, and growth hormone releasing factor (GHRH). About 5% of patients with PNETs have an inherited predisposition, which includes members of multiple endocrine neoplasia type I (MEN1), von Hippel-Lindau, tuberous sclerosis, and neurofibromatosis type I families. Features of each of these more common PNET subtypes and how their biochemical diagnoses are made are covered in the sections that follow.
Gastrinomas
Gastrinomas, which cause Zollinger-Ellison syndrome (ZES), comprise about 15% of all functional PNETs and are the most common PNET associated with MEN1. Approximately 90% of gastrinomas are found in the gastrinoma triangle, an area bordered by the confluence of the cystic and common duct superiorly, the pancreatic neck/body junction medially, and the second and third portions of the duodenum laterally. In patients with sporadic ZES, 50% to 88% of gastrinomas are duodenal in origin versus 70% to 100% of those in MEN1 patients. Approximately 22% to 35% of patients with pancreatic gastrinomas will have liver metastases. The mean size of these tumors is 3.8 cm.
The predominant functional abnormality seen is inappropriately high circulating gastrin causing irreversible hyperchlorhydria (gastric acid overproduction) with subsequent typical or atypical ulcer formation, hemorrhage, and excess acid-induced malabsorptive diarrhea. The hallmarks of a gastrinoma are very elevated gastrin levels as well as gastric acid hypersecretion. More than 98% of gastrinoma patients have elevation of fasting gastrin, but this alone is nondiagnostic. The finding of hyperchlorhydria and basal gastric acid output greater than 15 mEq/h will help to confirm the diagnosis, and a gastric pH of less than 2 is also helpful. In the past, secretin infusion resulting in a paradoxic increase of gastrin was a useful way to make the diagnosis of marked hypergastrinemia, but is performed less commonly due to limited availability of this secretogogue.
There are other conditions associated with high levels of gastrin. These conditions include atrophic gastritis/pernicious anemia, gastric outlet syndrome, retained gastric antrum, Helicobacter pylori infection, short bowel syndrome, and diabetic gastroparesis. PPIs will also cause significantly elevated gastrin levels through their significant suppression of gastric acid, with subsequent sustained hypergastrinemia and stimulation of CgA from gastric enterochromaffin-like (ECL) cells. Over time, ECL cell nodular hyperplasia can develop with secondary formation of small NETs (usually <1 cm in size). EC cells in the stomach can also be stimulated by high gastrin and result in CgA elevation along with modest levels of serotonin. In MEN1 patients with ZES, hypercalcemia can increase gastrin levels and basal acid secretion. Parathyroidectomy can significantly improve this situation.
Insulinoma
Insulinomas are the most common functional PNET, accounting for approximately 17% of cases. Patients are usually symptomatic with episodic hypoglycemia, although some patients can live for years with subclinical disease and have no overt symptoms, or will compensate for hypoglycemia with sugar ingestion. Up to 60% of all insulinomas are found in women, and the average age at diagnosis is 45 years. About 10% are associated with MEN1 and are second to gastrinomas in terms of MEN1-associated PNET frequency. Most are benign adenomas, but malignant insulinomas occur in approximately 10% of patients.
The demonstration of Whipple triad, (1) symptoms of hypoglycemia after fasting, (2) low glucose when symptomatic, and (3) relief of symptoms with ingestion of glucose or food, is strongly suggestive of an insulinoma. Major symptoms may include headache, blurry vision, seizures, confusion, and even coma. Other commonly observed symptoms are due to peripheral catecholamine release and include tremor, diaphoresis, and tachycardia. Approximately 98% of patients with insulinoma will demonstrate inappropriate insulin secretion with symptomatic hypoglycemia within 72 hours, and therefore, a supervised fast within a hospital setting has been recommended as the gold standard for diagnosis. Blood sugars should be measured every 4 hours until symptoms occur or blood glucose drops less than 50 mg/dL, and then serum insulin, C-peptide, and glucose levels are drawn. A measurably low blood glucose, symptoms, and inappropriate elevation of insulin (usually >6 uU/ml) or an insulin-to-glucose ratio of 0.3 or greater is highly suspicious for insulinoma. When proinsulin is cleaved by signal peptidase, C-peptide and insulin are formed, which are present in equal amounts in β cells. If a patient has been administered exogenous insulin, C-peptide levels will be low, whereas these levels will be elevated (>200 pmol/L) in insulinoma. Some patients with PNETs and hypoglycemia may have elevated levels of proinsulin rather than insulin. For follow-up of metastatic insulinoma, serial measurement of CgA and pancreastatin can be useful for assessing the extent of metastatic disease.
VIPomas
VIP-secreting PNETs (VIPomas) were independently described by Priest and Alexander, and Verner and Morrison. Initially termed “pancreatic cholera,” and later, watery diarrhea, hypokalemia, achlorhydria (most often hypochlorhydria) syndrome, it is now known as watery diarrhea syndrome (WDS). VIP is a neuromodulator (not a hormone in the classical sense), which, in high sustained blood levels, acts as a powerful intestinal secretogogue, resulting in hypokalemia, metabolic acidosis, stool bicarbonate loss, and high-volume alkalotic stool. VIPoma are an uncommon functional PNET, accounting for 1% to 2% of all functional PNETs. They can be seen in MEN1 patients even when other family members may have had gastrinomas or insulinomas.
The diagnosis of VIPoma is suspected in the setting of elevated plasma VIP and severe (often life-threatening) watery diarrhea (usually >1250 cc/d) and profound hypokalemia. Initially, the severe diarrhea may be episodic (tumors may be nonautonomous). Flushing is seen in 20% of patients with WDS, also thought to be a direct action of VIP. Hypochlorhydria, not achlorhydria, is seen in 80% of VIPoma/WDS patients. Both functional and nonfunctional biomarkers from certified commercial laboratories should be measured, to include VIP and PP. In the setting of metastasis, CgA and pancreastatin may be helpful to follow for progression and response to therapy. Although most VIPoma in adults arise from the pancreas, there are other nonpancreatic sources of VIP-secreting NETs, including pheochromocytoma, neuroblastoma, ganglioneuroma, bronchogenic carcinoma, and medullary thyroid carcinoma.
Glucagonoma
Glucagonoma is a very rare functional tumor, accounting for 1% or less of all PNETs. The clinical manifestations of glucagonoma are very high circulating glucagon levels and a classic necrolytic migratory erythema skin rash, usually on the anterior lower extremity or perianal genital regions. It has come to be known as the “4D syndrome”: dementia, diarrhea, deep vein thrombosis (DVT), and depression. Other clinical stigmata include a painful glossitis, weight loss (90%), mild type II diabetes mellitus (DM; 80%), low amino acid concentrations, and DVT (50%). The high circulating glucagon levels do not seem to be the cause of the dermopathy. Most glucagonomas are large at diagnosis, although they do not often present with classic symptoms. They are more often found in the pancreatic tail and have a very high rate of metastasis at the time of diagnosis. Like many of the functional PNETs, glucagonomas may also be seen in MEN1 patients.
The clinical diagnosis can be made by the finding of a significant elevation of glucagon levels (>500 pg/mL) in the setting of symptoms listed above. Normal fasting levels are generally less than 150 pg/mL, and several conditions may cause mild elevations of glucagon (DM, acute burns and trauma, cirrhosis, renal failure, Cushing syndrome, and bacteremia). PP and insulin levels may also be elevated in association with glucagonoma. As with other metastatic PNETs, serial CgA and pancreastatin levels may be helpful to monitor for progression.
Somatostatinoma
Somatostatinomas are very rare tumors, accounting for less than 1% of PNETs. Although most functional somatostatinomas are of PNET origin (60% of cases), duodenal, ampullary, and less commonly, jejunal somatostatinomas are also recognized. In PNET somatostatinomas, the excess native somatostatin causes hyperglycemia (75% present with DM type II), atony of the gallbladder (59% have gallbladder disease), hypochlorhydria and reduced gastric acid (>80%), steatorrhea and diarrhea (very common, from inhibition of prandial pancreatic enzyme release, bicarbonate, and reduced absorption of fats), and weight loss (possibly due to diarrhea and malabsorption, seen in about 33% of patients). Somatostatinomas are large, which may lead to destruction or loss of islet cells with reduced insulin production. Approximately 80% present with metastatic disease. The diagnosis is commonly made in retrospect by IHC of the tumor, but if there is clinical suspicion, somatostatin levels should be measured. Patients with small cell and bronchogenic carcinomas of the lung have been described with elevated somatostatin levels, as well as up to 25% of patients with pheochromocytoma. PP, CgA, and pancreastatin should also be followed, the latter two for monitoring progression.
Pancreatic polypeptide-secreting tumors and nonfunctional tumors
Pancreatic polypeptide-secreting tumors (PPomas) are a group of nonfunctioning PNETs that comprise about 50% of all PNETs encountered. Although PPomas are not recognized as functional PNETs, diarrhea has been associated with very high levels of PP. One recent report suggested an association of PPomas with DM, because 5 patients with DM and PPoma had improvement or resolution after resection. For the most part, the coassociation of elevated PP in PNETs making other hormones has maintained its value in the diagnosis and follow-up of patients with both functional and nonfunctional PNETs. Therefore, PP is a good marker to test in all cases of suspected PNETs, in addition to the hormones suggested by a clinical syndrome, if present. Measurement of CgA and pancreastatin is also useful in monitoring the effects of therapy and for progression.
The vast majority of nonfunctional PNETs are diagnosed as a result of nonspecific abdominal pain or symptoms of obstruction of the pancreatic or bile duct. Because of this, nonfunctional PNETs tend to be larger when detected (5.9 cm), and they have a higher rate of metastases (60%) and a poorer prognosis (5-year survival of 33%).
Current recommendations for biochemical testing in pancreatic neuroendocrine tumors
The NCNN recommends checking PP, CgA, calcitonin, PTH-rP, and GHRH for generic PNETs. If the patient has a recognizable syndrome, they recommend checking specific hormone levels. When insulinoma is suspected, then insulin, proinsulin, and C-peptide should be checked, and consideration should be given to a 72-hour fast. Serum VIP levels should be checked if one suspects VIPoma, serum glucagon for those with glucagonoma symptoms, and basal or stimulated gastrin for suspected gastrinoma patients. ENETs suggests checking CgA in cases of nonfunctional PNETs, with PP being more uncertain, except in MEN1 patients. Further tests are indicated if the patient demonstrates symptoms. The recommendations from NANETs are similar, although given in more detail for functional tumors. For nonfunctional tumors, they recommend CgA and PP.
Biomolecular diagnostics in neuroendocrine tumors
WREN Assay
Modlin and colleagues set out to identify a genetic signature for NETs that could be tested from peripheral blood samples that might be useful for diagnosis, assessment of tumor burden, and response to therapy. They retrieved data from tissue-based microarrays from normal tissues, and from 9 primary GEP NETs and 6 GEP NET metastases. They identified a group of genes that showed elevated expression in GEP NETs and then tested these genes in peripheral blood samples as a training set (67 normal samples, 63 GEP NETs). The validation set included 92 normal samples and 143 GEP NETs. They selected 75 genes for further study by quantitative polymerase chain reaction (qPCR; 21 from tissue-based results, 32 from blood-based, and 22 from the literature), which was then further reduced to a 51-gene panel. In PNETs, 79% of samples were accurately identified using the PCR test, as were 88% of GI NETs; the sensitivity was 90% and specificity was 94% for GI NETs, and 80% and 94%, respectively, for PNETs. In comparison with serum CgA levels from 81 GEP NET patients and 95 controls, the PCR-based test outperformed the biomarker (CgA sensitivity 32%, accuracy 60%), even in patients where CgA was low. The investigators concluded that their panel could identify GEP NETs regardless of primary tumor site or metastasis, which could be useful for screening and potential response to therapy. This group is actively recruiting patients to determine how well it might perform under these circumstances.
Biotheranostics Test
A 92-gene molecular assay (CancerTYPE ID, bioTheranostics, Inc, San Diego, CA, USA) was developed for determining the site of unknown primary tumors using qPCR from paraffin-embedded biopsy specimens. In a trial examining 790 tumors comprising 28 different tumor types and 50 subtypes, it was found to have an 87% sensitivity, greater than 98% specificity, and a positive predictive value of 61% to 100%. This test was later applied specifically to 75 NETs (12 GI, 22 pulmonary, 10 pancreas, 10 pheochromocytoma, 11 medullary thyroid carcinoma, and 10 Merkel cell carcinomas), of which 59% were metastases and 41% were primary tumors. This panel correctly classified the tumors as a NET in 74 of 75 cases. The 4 genes that were most important for making this distinction were ELAVL4, CADPS, RGS17, and KCNJ11 . Fifteen additional genes were used for further subtyping of NETs, which was accurate in 71 of 75 (95%) cases. One shortcoming of this study is the inability of this test to determine the GI NET subtype—the test does not identify the site of the primary tumor (small bowel vs duodenal vs rectal), only that the tumor is a GI NET. Still, the test performed well overall and shows promise in differentiation of lung, pancreatic, and GI NET primaries from tissue samples of metastases, allowing for more tailored therapy for patients.
Gene Expression Classifiers and Immunohistochemistry to Differentiate Small Bowel Neuroendocrine Tumors from Pancreatic Neuroendocrine Tumors
Sherman and colleagues evaluated the expression of a panel of genes by qPCR in primary tumors and metastases from 61 patients with SBNETs and 25 with PNETs. They were able to refine this panel down to 4 genes in the G-protein-coupled receptor pathway ( BRS3, OPRK1, OXTR, and SCTR ), and in 136 metastases accurately predicted the origin from SBNETs in 94 of 97 cases (97%) and 34 of 39 PNETs (87%). The algorithm made primary predictions in 122 cases using qPCR of just BRS3 and OPKR1 , although when one of the genes had undetectable expression (14 cases), the results for OXTR and/or SCTR were used to come up with a prediction. Maxwell and colleagues compared the results of this gene expression classifier (GEC) to an IHC algorithm that used CDX2, PAX6, and Islet1 in first-tier staining, followed by IHC for PR, PDX1, NESP55, and PrAP if the first-step stains were equivocal. The IHC algorithm was correct in determining the site of origin in 23 of 27 (85%) of SBNET metastases, and 10 of 10 (100%) PNET metastases. Comparison of these results revealed improved performance of the GEC for determining SBNET primaries and of IHC for PNET primaries. Although the overall accuracy was 94% for the GEC and 89% for IHC, they concluded that this IHC algorithm should be used first because of its widespread availability, and that GEC be reserved for cases of indeterminate IHC results. The ability to differentiate SBNET from PNET primaries from a biopsy of a metastatic liver tumor could aid in surgical exploration, and selection of therapy for these patients with metastatic disease, such as Everolimus, Sunitinib, or chemotherapy in patients with PNETs.
Related posts:
Management of Thyroid Nodular Disease
Surgical Management of Lymph Node Compartments in Papillary Thyroid Cancer
Current Guidelines for Postoperative Treatment and Follow-Up of Well-Differentiated Thyroid Cancer
Minimally Invasive Parathyroidectomy Versus Bilateral Neck Exploration for Primary Hyperparathyroidism
Pheochromocytoma and Paraganglioma
Minimally Invasive Adrenalectomy
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
