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Neuroendocrine tumors (NET) arise from endocrine (hormone producing) cells in various organs in the body, including the gastrointestinal tract, lungs, adrenal glands, and thyroid. These endocrine cells bear microscopic similarities to neurons (nerve cells) and interact with the nervous system: hence the term ‘neuroendocrine’.
NETs range in behavior from very slow growing to highly aggressive. Microscopic features on a tumor specimen (biopsy or surgical tissue) can help to categorize these tumors. Well-differentiated tumors resemble normal endocrine tissues and are less aggressive than poorly differentiated tumors, in which the normal tissue architecture is lost. In fact, poorly differentiated tumors are called neuroendocrine carcinomas (NEC) rather than neuroendocrine tumors (NET). Tumor grade refers to the number of dividing cells in the tumor specimen and can be measured by counting the number of cells undergoing mitosis (mitotic rate) or the ki-67 proliferative index. For gastrointestinal NETs, low grade (G1) is defined as ki-67% of 0-2%, intermediate grade (G2) as 3-20%, and high grade (G3) as >20%. A small minority of well-differentiated NETs are high grade, but all poorly differentiated NECs are high grade.
NETs arising in the gastrointestinal tract and lungs used to be known as ‘carcinoid tumors’, and the term persists in the medical literature.
A common question is whether low-grade NETs can be benign. All tumors that have spread to lymph nodes or other organs are, by definition, malignant. However, some low-grade tumors are small and non-invasive. Depending on where they originate, these tumors can have an extremely low chance of spread (in some cases essentially zero). Nevertheless, a pathologist cannot, strictly speaking, label a NET as ‘benign’ because there are no pathologic features that can definitively characterize a NET as non-malignant.
Other hormonal syndromes are associated particularly with pancreatic NETs. Insulinomas are pancreatic NETs that produce insulin, a hormone that causes hypoglycemia (low blood sugar). Insulinomas are typically detected early and cured surgically. Gastrinomas usually originate in the pancreas or duodenum, and produce gastrin, a hormone that causes excess acid secretion in the stomach. This leads to ulcers, heartburn, and diarrhea. Other very rare pancreatic NETs include VIPomas which produce vasoactive intestinal peptide, a hormone that leads to extreme watery diarrhea, and glucagonomas which produce glucagon, a hormone that causes weight loss, diabetes, and an unusual rash.
NETs of the adrenal medulla are called pheochromocytomas. They often produce hormones like adrenalin (epinephrine) which cause severe hypertension, palpitations, headaches, and panic attack like symptoms. Pheochromocytomas rarely metastasize. Similar tumors arising in nervous plexuses are called paragangliomas. Most pheochromocytomas and paragangliomas have very low malignant potential and can be cured surgically.
NETs are known for their capacity to produce (secrete) hormones. Hormone-producing tumors are called ‘functional’. These hormones can result in various clinical syndromes. The most common is carcinoid syndrome, which typically occurs in patients with metastatic small bowel NETs, particularly when tumors have spread to the liver. Carcinoid syndrome is caused by production of serotonin and other vasoactive substances. Serotonin is often measured in the urine as 5-hydroxyindoleacetic acid (5-HIAA), a metabolite of serotonin. Carcinoid syndrome is characterized by flushing (redness/warmth in the face and neck) and diarrhea. Over time, carcinoid syndrome can result in damage to heart valves, a condition known as ‘carcinoid heart disease’.
Other hormonal syndromes are associated particularly with pancreatic NETs. Insulinomas are pancreatic NETs that produce insulin, a hormone that causes hypoglycemia (low blood sugar). Insulinomas are typically detected early and cured surgically. Gastrinomas usually originate in the pancreas or duodenum, and produce gastrin, a hormone that causes excess acid secretion in the stomach. This leads to ulcers, heartburn, and diarrhea. Other very rare pancreatic NETs include VIPomas which produce vasoactive intestinal peptide, a hormone that leads to extreme watery diarrhea, and glucagonomas which produce glucagon, a hormone that causes weight loss, diabetes, and an unusual rash.
NETs of the adrenal medulla are called pheochromocytomas. They often produce hormones like adrenalin (epinephrine) which cause severe hypertension, palpitations, headaches, and panic attack like symptoms. Pheochromocytomas rarely metastasize. Similar tumors arising in nervous plexuses are called paragangliomas. Most pheochromocytomas and paragangliomas have very low malignant potential and can be cured surgically.
Sometimes, NETs can be discovered after patients present with hormonal symptoms, such as those described above. Most NETs are not hormone producing (nonfunctional) and are discovered either from symptoms of tumor growth, or incidentally (by accident). Tumor-related symptoms depend on location of tumors. For example, NETs of the small intestine can cause chronic intermittent abdominal pain, or even bowel obstruction. Pancreatic NETs can cause jaundice from blockage of bile ducts, or upper abdominal pain and weight loss. Lung NETs can cause cough, sometimes with bloody sputum. Most metastatic NETs spread to the liver, and can cause pain in the right upper abdomen, fatigue and weight loss.
NETs that are found incidentally (during colonoscopy, for example, or scans done for other purposes) are often small and localized.
Most NETs are sporadic (non-hereditary), but a small minority are inherited. Multiple endocrine neoplasia type 1 (MEN-1) leads to tumors in the pituitary gland, parathyroid glands, and NETs in the pancreas and duodenum. Pheochromocytomas are often hereditary, and some are associated with multiple endocrine neoplasia type 2 (MEN-2) which is caused by mutation to the RET gene. MEN2 causes pheochromocytomas, medullary thyroid cancers, and parathyroid tumors.
Appendiceal NETs are almost always found incidentally (by accident) during appendectomy for appendicitis or other reasons. If they are under 2cm in size, they almost never spread to other organs.
Poorly differentiated NEC are highly aggressive cancers that can originate in many different organs, including the lung, gastrointestinal tract, prostate and cervix. NECs can be composed of small cells (small cell carcinoma) or large cells. Small cell lung cancer is an aggressive neuroendocrine cancer that is usually managed by lung cancer specialists.
NECs are usually metastatic at diagnosis, but even if they are found at a local stage, they have a high potential to metastasize. The treatment of localized NECs can consist of a combination of surgery, radiation, and chemotherapy: typically, at least two modalities of treatment.
Definitive diagnosis of NET requires evaluation of tumor tissue, either through a biopsy or through surgical resection. Biopsies can be performed endoscopically, or through the skin using image guidance (CT or ultrasound).
PET scans use a radioactive isotope to detect tumors. Conventional PET scans use FDG, a radioactive glucose isotope, to detect metabolically active tumors. While this can be useful for some NETs (particularly poorly differentiated and/or high grade), most well-differentiated NETs will not be well-visualized with a standard PET. Instead, special PET scans that image somatostatin receptors are used. These include 68-Gallium-Dotatate PET (Netspot), and 64-Copper-Dotatate PET (Detectnet).
Other diagnostic blood or urine tests are performed to evaluate hormone production and are ordered based on the presence of a clinical syndrome. For example, in patients with suspected carcinoid syndrome (flushing, diarrhea), a blood serotonin or a blood or 24-hour urine 5-HIAA test should be performed. These tests are usually more accurate with avoidance of certain foods before and during the test.
There are certain tumor makers that can be used to evaluate NETs and can correlate with progression or response of disease. The most common one is chromogranin A (CgA). Others include pancreastatin, pancreatic polypeptide or neuron specific enolase (NSE). The benefit of these tests has been increasingly questioned in recent years.
Tumors that have spread to other organs through the blood or lymphatic system are called metastatic (stage IV).
Well-differentiated metastatic NETs can sometimes be managed surgically, even if all tumors cannot be removed by the surgeon. Such non-curative surgery is called ‘cytoreductive’ or ‘debulking’ surgery.
Neuroendocrine tumors often express somatostatin receptors on their cell surface. Drugs called somatostatin analogs attach to these receptors and can both inhibit tumor growth as well as decrease hormone production in patients with hormonally active (functional) tumors. The two somatostatin analogs commonly used are octreotide (sandostatin) and lanreotide (somatuline). These drugs are typically injected once every 4 weeks in the buttocks. While generally well-tolerated, they can cause side effects such as malabsorption of fatty foods and gallstones.
Since NETs often metastasize to the liver, various liver-directed therapies can be used. Small numbers of tumors can be ablated using radiofrequency or microwave ablation via a probe inserted through the skin or treated with stereotactic radiation. Larger number of tumors can be treated with hepatic arterial embolization using microparticles that are infused into the liver arteries to block the blood supply to the tumors. Hepatic arteries are usually accessed through a catheter inserted in the groin. Embolization can be performed with microparticles alone (bland embolization), mixed with chemotherapy (chemoembolization) or radioactive particles using Yttrium-90 (radioembolization). Embolization often causes pain in the right upper abdomen, nausea, fever, and fatigue. Most symptoms tend to resolve within a week; serious complications such as abscess or liver dysfunction are rare.
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