Carcinoid Tumor

Carcinoid tumors are slow-growing cancers that arise from specialized hormone-producing cells scattered throughout the body, most commonly in the digestive tract and lungs. Today they are classified under the broader umbrella of neuroendocrine tumors (NETs) — a term reflecting their dual nature as cells that behave like both nerve tissue and hormone-secreting glands. Most carcinoid tumors grow so slowly that people live with them for years without symptoms. However, when they spread to the liver, a dramatic set of symptoms called carcinoid syndrome can emerge: episodes of flushing, watery diarrhea, and, over time, damage to the right side of the heart. Modern imaging using radioactive tracers that lock onto somatostatin receptors has transformed how these tumors are found and staged, and a new class of treatments — including targeted radiation delivered directly to tumor cells — has significantly improved outcomes for patients with advanced disease.

Table of Contents

1. What Carcinoid Tumors Are
2. Where They Arise — Primary Sites
3. WHO Grading and Classification
4. Carcinoid Syndrome
5. Diagnosis — Biochemical Markers
6. Diagnosis — Imaging and Staging
7. Treatment — Surgery and Locoregional Approaches
8. Treatment — Systemic Therapy
9. Research Papers
10. Connections
11. Featured Videos

What Carcinoid Tumors Are

The term "carcinoid" — meaning "cancer-like" — was coined in 1907 by the German pathologist Siegfried Oberndorfer, who noticed that certain intestinal tumors behaved far more indolently than typical carcinomas. The name stuck for over a century, but it has gradually given way to the more precise term neuroendocrine tumor (NET), which acknowledges that these growths originate from enterochromaffin cells and other diffuse neuroendocrine cells dispersed throughout the gut, lung, and pancreas.

Enterochromaffin cells are the most abundant endocrine cells in the body. They line the mucosal layer of the gastrointestinal tract and serve as sensors that respond to food, stretch, and chemical signals by releasing serotonin and other signaling molecules. When one of these cells accumulates the right combination of genetic changes, it can begin dividing uncontrollably while retaining its ability to manufacture and secrete hormones — the defining feature that gives NETs their clinical character. Because these tumors often grow slowly and retain normal-looking tissue architecture for years, they can reach significant size before causing trouble, and many are discovered incidentally during imaging or surgery for unrelated conditions.

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Where They Arise — Primary Sites

NETs can arise anywhere along the diffuse neuroendocrine system, but the gastrointestinal tract is by far the most common location. A landmark analysis of nearly 35,000 U.S. cases found the following distribution:

• Small intestine (ileum): The most frequent site, accounting for roughly 40% of GI NETs. Ileal tumors are often multiple and tend to be functional — meaning they secrete serotonin. They are a leading cause of carcinoid syndrome when they metastasize.
• Appendix: Classically associated with the term "carcinoid," appendiceal NETs are usually found incidentally during appendectomy for appendicitis. The vast majority are less than 2 cm, have not spread, and are cured by appendectomy alone.
• Rectum: Rectal NETs have increased in incidence, likely due to more widespread colonoscopy screening. Most are small, non-functional submucosal nodules detected during screening and cured by endoscopic resection.
• Lung (bronchial): Pulmonary carcinoids represent about 25% of all NETs. They are divided into typical carcinoid (lower grade, better prognosis) and atypical carcinoid (intermediate grade). Many present with cough or hemoptysis, or are found incidentally on chest imaging.
• Pancreas: Pancreatic NETs (PNETs) include functional tumors such as insulinoma (causes hypoglycemia), gastrinoma (Zollinger-Ellison syndrome), and glucagonoma, as well as non-functional tumors that present with mass effect or incidentally.
• Stomach: Gastric NETs are classified into three types depending on whether they arise in the setting of hypergastrinemia (Types I and II, generally low-risk) or sporadically (Type III, higher risk).

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WHO Grading and Classification

The World Health Organization's 2022 classification system grades NETs based on two measures of how rapidly the tumor cells are dividing: the Ki-67 proliferation index (the percentage of tumor cells actively cycling, stained by a monoclonal antibody) and the mitotic count (the number of cell divisions visible per 10 high-power microscope fields). These numbers divide NETs into three grades:

• G1 (low grade): Ki-67 <3%, mitotic count <2 per 10 HPF. Slow-growing; median survival measured in many years even with metastases.
• G2 (intermediate grade): Ki-67 3–20%, mitotic count 2–20 per 10 HPF. Intermediate behavior; still typically well-differentiated.
• G3 (high grade): Ki-67 >20%. This grade now encompasses two very different entities:
  • Well-differentiated G3 NET: Still retains neuroendocrine architecture; responds better to SSAs and targeted agents.
  • Poorly differentiated neuroendocrine carcinoma (NEC): Small-cell or large-cell type; highly aggressive; treated more like small-cell lung cancer with platinum-based chemotherapy.

This distinction between well-differentiated G3 NET and poorly differentiated NEC is clinically critical — they are biologically distinct tumors that require different treatment approaches, even though both carry a Ki-67 above 20%. Pathologists use additional markers (DAXX/ATRX mutations for PNETs, Rb loss for NEC) to make this distinction when Ki-67 alone is ambiguous.

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Carcinoid Syndrome

Carcinoid syndrome is the dramatic collection of symptoms that occurs when a functioning NET — typically a midgut tumor — has spread to the liver and those metastases secrete serotonin and other vasoactive substances directly into the systemic circulation. The key word here is liver metastases: primary gut tumors drain into the portal vein, and the liver efficiently clears serotonin on first pass. Once the liver is seeded with metastatic deposits, that clearance mechanism is overwhelmed, and excess serotonin and tachykinins reach the rest of the body.

The classic features of carcinoid syndrome are:

• Flushing: Episodic reddening or purplish discoloration of the face and upper chest, often triggered by alcohol, certain foods, stress, or anesthesia. Episodes typically last minutes but can be prolonged in severe cases.
• Watery diarrhea: Secretory diarrhea driven by serotonin's direct stimulation of intestinal motility. Can be severe enough to cause significant dehydration and electrolyte imbalance.
• Bronchospasm: Wheezing and breathlessness from bronchoconstriction, present in a minority of patients but important to recognize before surgery or procedures.
• Carcinoid heart disease (Hedinger syndrome): Fibrous plaques deposit on the right-sided heart valves — particularly causing tricuspid regurgitation and pulmonary stenosis. Left-sided valves are largely spared because the lungs metabolize serotonin. Carcinoid heart disease occurs in up to 50% of patients with long-standing carcinoid syndrome and is a major cause of morbidity and death. It is assessed by echocardiography and monitored with NT-proBNP levels.

A feared acute complication is carcinoid crisis — life-threatening hypotension, extreme flushing, and bronchospasm, most often triggered by tumor manipulation during surgery or interventional procedures. Prevention requires perioperative somatostatin analogue infusion.

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Diagnosis — Biochemical Markers

Two laboratory tests form the cornerstone of biochemical diagnosis for carcinoid tumors:

• 24-hour urine 5-HIAA (5-hydroxyindoleacetic acid): Serotonin is metabolized to 5-HIAA, which is excreted in urine. A 24-hour collection captures integrated secretion over a day, avoiding the fluctuations seen in spot samples. Sensitivity for midgut NETs is approximately 70–75%. The test requires dietary restriction of serotonin-rich foods (avocado, banana, walnut, pineapple) for 48 hours before and during collection to avoid false positives.
• Serum chromogranin A (CgA): A protein co-released with peptide hormones from neuroendocrine granules. CgA is elevated in up to 80–90% of NETs, including non-functional ones that do not cause clinical syndrome. It is useful for monitoring treatment response and detecting recurrence. However, CgA is subject to important false positives: proton pump inhibitor (PPI) use is the most common cause of spuriously elevated CgA (PPIs drive hypergastrinemia, which stimulates enterochromaffin-like cell proliferation and CgA release). Renal failure also elevates CgA. Always check PPI use before interpreting a mildly elevated result.

Additional markers are used for specific tumor types: gastrin for gastrinoma, insulin and C-peptide for insulinoma, glucagon for glucagonoma, and vasoactive intestinal peptide (VIP) for VIPoma. Pancreastatin (a CgA fragment) may be more specific than intact CgA in some settings.

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Diagnosis — Imaging and Staging

Because most NETs overexpress somatostatin receptors (particularly SSTR2), functional imaging with somatostatin receptor-targeted tracers is central to staging and treatment planning.

• 68Ga-DOTATATE PET/CT: The current standard of care. A gallium-68 labeled somatostatin analogue (DOTATATE) is injected and imaged with PET. Sensitivity exceeds 90% for well-differentiated NETs — far superior to conventional CT or the older Octreoscan. It can detect lesions as small as 5–7 mm and is essential for identifying sites of disease before deciding on PRRT eligibility (SSTR expression must be confirmed). The scan also provides prognostic information: high uptake generally correlates with well-differentiated, slower-growing disease.
• Octreoscan (111In-pentetreotide scintigraphy): The earlier-generation somatostatin receptor imaging test. Now largely replaced by 68Ga-DOTATATE PET where available, due to lower resolution and longer acquisition time. Still used where PET is unavailable.
• CT and MRI: Cross-sectional imaging defines anatomy, measures tumor burden, and assesses vascular involvement. Triphasic CT of abdomen/pelvis captures the arterial enhancement typical of NETs. MRI with hepatocyte-specific contrast (gadoxetate) is highly sensitive for liver metastases.
• Endoscopy and EUS: Upper endoscopy, colonoscopy, and capsule endoscopy are used to locate primary small-bowel or gastric lesions. Endoscopic ultrasound (EUS) is the most accurate technique for staging pancreatic NETs and for biopsy.

Staging follows the European Neuroendocrine Tumor Society (ENETS) and the American Joint Committee on Cancer (AJCC) TNM systems, which differ slightly by primary site. Stage IV (distant metastases) is present at diagnosis in roughly 20–25% of patients but carries a far better prognosis than stage IV of most other cancers — 5-year survival for stage IV midgut NET exceeds 50% in contemporary series.

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Treatment — Surgery and Locoregional Approaches

Surgery remains the only potentially curative treatment for NETs and is the preferred approach whenever resection is feasible.

• Resection of the primary tumor: Even in the setting of metastatic disease, resection of the primary midgut tumor is recommended to prevent mesenteric ischemia, obstruction, and ongoing hormonal secretion. Extensive mesenteric lymph node dissection is performed given the tendency for nodal spread.
• Liver-directed surgery: Hepatic resection for liver metastases can improve symptoms, reduce hormonal burden, and extend survival in selected patients. R0 resection (clear margins) is the goal. Patients with bilobar or unresectable hepatic disease may still benefit from cytoreductive (debulking) surgery if >90% of liver disease volume can be removed — symptom control can be dramatic.
• Radiofrequency ablation (RFA) and microwave ablation: Thermal ablation is used for small (<3–4 cm) liver metastases not amenable to resection, often combined with surgical debulking of larger lesions.
• Hepatic artery embolization and TACE: Because liver metastases from NETs are hypervascular and derive most of their blood supply from the hepatic artery (unlike normal liver parenchyma, which is portal-dominant), bland embolization or transarterial chemoembolization (TACE) can cause significant tumor necrosis. Somatostatin analogue cover is mandatory perioperatively to prevent carcinoid crisis from tumor lysis.
• Appendiceal tumors: Simple appendectomy is curative for tumors <2 cm. Right hemicolectomy is recommended for tumors >2 cm or those with adverse histologic features (lymphovascular invasion, positive margins), given the higher risk of nodal spread.

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Treatment — Systemic Therapy

For patients with unresectable or metastatic NETs, several systemic options have demonstrated benefit in large randomized trials:

Somatostatin Analogues (SSAs)

Octreotide and lanreotide are long-acting SSA formulations given by monthly injection. They were established first as antisecretory agents that control carcinoid syndrome symptoms. Their antiproliferative activity was demonstrated in two pivotal trials:

• PROMID trial (2009): Octreotide LAR significantly lengthened time to tumor progression (14.3 vs 6 months) in patients with metastatic midgut NETs.
• CLARINET trial (2014): Lanreotide autogel (120 mg every 28 days) significantly improved progression-free survival vs placebo (not reached vs 18 months) in patients with G1/G2 non-functional enteropancreatic NETs, including those with stable disease at baseline.

SSAs are generally well tolerated; the main side effects are GI (loose stools, fat malabsorption) and, over time, gallstone formation from reduced gallbladder motility.

Everolimus (mTOR Inhibitor)

Everolimus (Afinitor) inhibits the mTOR pathway, which is frequently activated in NETs. Two phase III trials established its role:

• RADIANT-3 (2011): Everolimus improved median PFS from 4.6 to 11 months in advanced pancreatic NETs.
• RADIANT-4 (2016): In non-functional lung and GI NETs (excluding pancreas), everolimus reduced the risk of progression or death by 52% vs placebo.

Common side effects include stomatitis, rash, diarrhea, fatigue, and non-infectious pneumonitis. Hyperglycemia can be significant in patients with diabetes or pre-diabetes.

Peptide Receptor Radionuclide Therapy (PRRT) — 177Lu-DOTATATE

PRRT exploits the high SSTR2 expression on NETs by delivering a radioactive isotope (lutetium-177) directly to tumor cells via a somatostatin analogue carrier (DOTATATE). The NETTER-1 trial (2017) was the pivotal study: in patients with progressive midgut NETs who had failed or were intolerant of SSAs, 177Lu-DOTATATE (Lutathera) improved median PFS from 8.4 to 28.4 months compared with high-dose octreotide LAR, with a significantly better response rate (18% vs 3%). The treatment is given as four infusions eight weeks apart and is generally well tolerated; the main risks are hematologic toxicity and, rarely, renal toxicity (mitigated by amino acid co-infusion to protect the kidneys). Patients must have sufficient SSTR expression on 68Ga-DOTATATE PET to qualify.

Sunitinib

The tyrosine kinase inhibitor sunitinib is approved for progressive well-differentiated pancreatic NETs, where it improved PFS from 5.5 to 11.4 months in a phase III trial.

Chemotherapy

Cytotoxic chemotherapy has a limited role in well-differentiated NETs but remains the standard for poorly differentiated NECs (cisplatin/carboplatin + etoposide). Streptozocin-based regimens are occasionally used for progressive functional PNETs.

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Research Papers

1. Modlin IM, Lye KD, Kidd M (2003). A 5-decade analysis of 13,715 carcinoid tumors. Cancer, 97(4):934–959. PMID: 12569593. doi: 10.1002/cncr.11105
2. Yao JC, et al (2008). One hundred years after "carcinoid": epidemiology of and prognostic factors for neuroendocrine tumors in 35,825 cases in the United States. J Clin Oncol, 26(18):3063–3072. PMID: 18565894. doi: 10.1200/JCO.2007.15.4377
3. Klimstra DS, et al (2010). The pathologic classification of neuroendocrine tumors: a review of nomenclature, grading, and staging systems. Pancreas, 39(6):707–712. PMID: 20664470. doi: 10.1097/MPA.0b013e3181ec124e
4. Caplin ME, et al; CLARINET Investigators (2014). Lanreotide in metastatic enteropancreatic neuroendocrine tumors. N Engl J Med, 371(3):224–233. PMID: 25014687. doi: 10.1056/NEJMoa1316158
5. Strosberg J, et al; NETTER-1 Trial Investigators (2017). Phase 3 trial of ¹⁷⁷Lu-Dotatate for midgut neuroendocrine tumors. N Engl J Med, 376(2):125–135. PMID: 28076709. doi: 10.1056/NEJMoa1607427
6. Yao JC, et al; RADIANT-3 Study Group (2011). Everolimus for advanced pancreatic neuroendocrine tumors. N Engl J Med, 364(6):514–523. PMID: 21306238. doi: 10.1056/NEJMoa1009290
7. Yao JC, et al; RADIANT-4 Study Group (2016). Everolimus for the treatment of advanced, non-functional neuroendocrine tumors of the lung or gastrointestinal tract (RADIANT-4). Lancet, 387(10022):968–977. PMID: 26703430. doi: 10.1016/S0140-6736(15)00817-X
8. Kulke MH, et al (2010). NANETS treatment guidelines: well-differentiated neuroendocrine tumors of the stomach and pancreas. Pancreas, 39(6):735–752. PMID: 20664473. doi: 10.1097/MPA.0b013e3181ebb168
9. Rinke A, et al; PROMID Study Group (2009). Placebo-controlled, double-blind, prospective, randomized study on the effect of octreotide LAR in the control of tumor growth in patients with metastatic neuroendocrine midgut tumors. J Clin Oncol, 27(28):4656–4663. PMID: 19704057. doi: 10.1200/JCO.2009.22.8510
10. Öberg K, et al (2012). A meta-analysis of the accuracy of somatostatin receptor scintigraphy and chromogranin A in detecting neuroendocrine tumors. Endocr Connect, 1(1):R1–R9. PMID: 23781316. doi: 10.1530/EC-12-0049
11. Bhattacharyya S, et al (2004). Carcinoid heart disease: the cardiac complications of neuroendocrine tumors. Endocrinol Metab Clin North Am, 33(3):555–576. PMID: 15261834. doi: 10.1016/j.ecl.2004.04.001
12. Sundin A, et al (2017). ENETS Consensus Guidelines for the Standards of Care in Neuroendocrine Tumors: radiological, nuclear medicine and hybrid imaging. Neuroendocrinology, 105(3):212–244. PMID: 28355645. doi: 10.1159/000471879

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Connections

• Acromegaly
• Cushing's Syndrome
• Pheochromocytoma
• Endocrinology Conditions
• Gastroenterology Conditions
• Lab Tests
• Diabetes Insipidus

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