A carcinoid tumor of the small intestine is a slow-growing cancer that develops from hormone-producing cells in the intestinal lining. These tumors have become the most common type of small bowel cancer, with incidence rising significantly since 2000. Most are diagnosed in people between ages 60 and 64, and they affect men and women at roughly equal rates.
Where These Tumors Come From
The small intestine is lined with specialized cells called enterochromaffin cells, which produce hormones that help regulate digestion. Carcinoid tumors arise when these cells begin growing out of control. The hallmark hormone they produce is serotonin, a chemical better known for its role in mood but also heavily involved in gut function. When overproduced, serotonin drives many of the symptoms these tumors cause.
About 70% of small intestine carcinoid tumors develop in the ileum, the final and longest section of the small bowel. The jejunum (the middle section) accounts for most of the rest, while the duodenum (the first section, nearest the stomach) is least commonly affected. Together, the ileum and jejunum account for roughly 28% of all neuroendocrine tumors found anywhere in the body.
Why Symptoms Often Appear Late
Small intestine carcinoid tumors are notoriously quiet in their early stages. The tumors grow slowly, and the small bowel has enough room for a mass to enlarge without causing obvious blockages. Many are discovered incidentally during surgery or imaging for an unrelated issue. When symptoms do appear, they’re often vague: intermittent abdominal pain, bloating, or changes in bowel habits that are easy to attribute to something else.
The more dramatic symptoms come from what’s known as carcinoid syndrome, which typically only develops after the tumor has spread to the liver. The reason is straightforward: when serotonin and other hormones are released into the blood from the gut, they flow first to the liver, which breaks them down before they can circulate through the body. Once tumor cells are growing in the liver itself, those hormones bypass this filtering step and enter the bloodstream at full strength.
Carcinoid Syndrome
Carcinoid syndrome affects a subset of patients, specifically those whose tumors have metastasized to the liver. The classic symptoms include skin flushing, diarrhea, and wheezing or shortness of breath. Flushing episodes typically affect the face and upper chest, turning the skin anywhere from pink to purple, and can last from a few minutes to several hours. Diarrhea is often frequent and watery, sometimes accompanied by cramping. Over time, small purplish spider-like veins may appear on the nose and upper lip.
Treatment with somatostatin analogs, a class of drugs that block hormone release from the tumor, can dramatically reduce these symptoms. Pooled data from more than a dozen clinical trials show that about 71% of patients experience improvement in both diarrhea and flushing. Flushing episodes drop by roughly 84% on average, from about 4.5 episodes per day to fewer than one. Diarrhea frequency falls by about 42%. These drugs also reduce circulating serotonin levels by up to 50% and can help stabilize tumor growth.
Heart Damage From Serotonin
One of the most serious complications of carcinoid syndrome is damage to the heart valves, known as carcinoid heart disease or Hedinger syndrome. This affects at least 20% of patients with carcinoid syndrome. Sustained high levels of serotonin in the bloodstream stimulate the growth of fibrous tissue on the inner lining of the heart, particularly on the right side. The right heart valves are hit hardest because blood returning from the liver passes through them before reaching the lungs, where serotonin is partially inactivated.
The tricuspid valve (between the right upper and lower chambers) is affected in up to 97% of patients with carcinoid heart disease, with the vast majority developing moderate to severe leaking. The pulmonary valve is involved in about 88% of cases. Left-sided valve damage is rare, occurring in only about 7% of patients, because the lungs filter out most of the serotonin before blood reaches the left heart. Over time, this valve damage can lead to right-sided heart failure.
How These Tumors Are Graded
When a biopsy or surgical sample is taken, pathologists grade the tumor based on how quickly its cells are dividing. This is measured two ways: by counting cells actively splitting under a microscope (mitotic rate) and by a lab marker called the Ki-67 index, which reflects the percentage of cells in an active growth phase.
- Grade 1 (low): Fewer than 2 dividing cells per field and a Ki-67 index below 3%. These are the slowest growing.
- Grade 2 (intermediate): 2 to 20 dividing cells per field and a Ki-67 index of 3 to 20%.
- Grade 3 (high): More than 20 dividing cells per field and a Ki-67 index above 20%. These behave more aggressively.
Most small intestine carcinoid tumors are grade 1 or 2, which partly explains their relatively favorable survival rates compared to other cancers. A separate category exists for poorly differentiated neuroendocrine carcinomas, which look and behave very differently from typical carcinoid tumors and carry a worse prognosis.
Spread and Survival
Despite their slow growth, small bowel carcinoid tumors have a surprisingly high rate of spread even at small sizes. Tumors under 1 centimeter already have lymph node or liver metastases 20 to 30% of the time. For tumors between 1 and 2 centimeters, about two-thirds have spread to lymph nodes. And tumors larger than 2 centimeters show lymph node involvement in over 80% of cases and liver metastases in 40 to 50%.
Even so, survival rates are relatively good for grade 1 and 2 tumors. Based on data from people diagnosed between 2015 and 2021, the five-year relative survival rate is 97% for localized disease (confined to the intestinal wall), 96% for regional disease (spread to nearby lymph nodes), and 68% for distant disease (spread to the liver or other organs). Younger patients, those under 50 at diagnosis, and those with tumors under 2 centimeters tend to do better. Complete surgical removal is strongly associated with improved outcomes.
Diagnosis
Diagnosing a small intestine carcinoid tumor often involves a combination of imaging, blood tests, and urine tests. The two main biomarkers are chromogranin A, a protein released by neuroendocrine cells, and 5-HIAA, the main breakdown product of serotonin measured in a 24-hour urine collection. Neither test is perfect on its own. Chromogranin A has a sensitivity of about 64%, meaning it catches roughly two-thirds of cases, with a specificity of 86%. The 5-HIAA urine test is more specific at 93% but only picks up about 36% of tumors, so a normal result doesn’t rule out the diagnosis.
Imaging plays a critical role. CT and MRI scans can reveal the primary tumor and any liver metastases. A specialized scan that targets somatostatin receptors on the surface of neuroendocrine tumor cells is particularly useful for mapping the extent of disease throughout the body. Capsule endoscopy, where you swallow a tiny camera, can sometimes detect tumors in parts of the small bowel that are difficult to reach with standard scopes.
Surgical Treatment
Surgery is the primary treatment for small bowel carcinoid tumors and the only option that offers a potential cure. Because even small tumors can spread to lymph nodes, the standard approach involves removing a wide segment of the intestine along with the surrounding lymph nodes, not just the tumor itself. This is recommended regardless of tumor size.
Surgery is also recommended for patients whose cancer has already spread to the liver. Removing the primary tumor can reduce the hormonal load driving carcinoid syndrome, relieve or prevent intestinal obstruction, and potentially improve the effectiveness of other treatments targeting liver metastases. For liver disease specifically, options range from surgical removal of individual metastases to procedures that cut off blood supply to tumor deposits in the liver.
Patients with duodenal tumors and those who achieve complete surgical removal tend to have the best long-term outcomes. For advanced disease that can’t be fully removed, the combination of somatostatin analogs to control symptoms and targeted therapies to slow growth forms the backbone of ongoing management.