A carcinoid tumor is a slow-growing cancer that forms in neuroendocrine cells, which are specialized cells that receive signals from the nervous system and respond by releasing hormones. These tumors most commonly develop in the digestive tract or lungs, and many grow so slowly that people can live with them for years without symptoms. When they do cause problems, it’s often because they release excess hormones into the bloodstream.
Where Carcinoid Tumors Develop
About 60 to 70% of carcinoid tumors form somewhere in the gastrointestinal tract. The second most common site is the airways of the lungs. Within the GI tract, the distribution is uneven. A large U.S. analysis of over 35,000 neuroendocrine tumors found the rectum was the single most common location (34%), followed by the small bowel (26%), stomach (12%), colon (8%), duodenum (8%), cecum (6%), and appendix (6%).
These tumors originate from primitive stem cells in the gut wall or from similar neuroendocrine cells in the lining of the airways. Because neuroendocrine cells exist throughout the body, carcinoid tumors can technically appear in other organs too, though this is far less common.
Why Most Carcinoid Tumors Cause No Symptoms
Many carcinoid tumors are small, slow-growing, and never release enough hormones to cause noticeable problems. They’re frequently discovered by accident during surgery or imaging done for an unrelated reason. Lung carcinoids, for example, grow more slowly than other types of lung cancer. GI carcinoid tumors can sit quietly in the intestinal wall for years before being detected.
When symptoms do appear, they tend to be vague. A small bowel carcinoid might cause intermittent abdominal pain or a partial blockage. A lung carcinoid might trigger a persistent cough or recurrent pneumonia. These nonspecific signs often lead to a delayed diagnosis.
Carcinoid Syndrome
The more distinctive set of symptoms, called carcinoid syndrome, happens when a tumor secretes serotonin or other chemicals directly into the bloodstream in large enough quantities. This typically occurs only after the cancer has spread to the liver, because the liver normally filters out those hormones before they can circulate. Once liver metastases bypass that filter, the hormones reach the rest of the body.
The hallmark symptom is skin flushing, where the face and upper chest feel hot and turn pink to purple. Episodes can last anywhere from a few minutes to several hours. Other common symptoms include frequent watery diarrhea (sometimes with abdominal cramps), wheezing or shortness of breath similar to asthma, and episodes of rapid heartbeat. Over time, some people develop purplish spider-vein patterns on the nose and upper lip.
Left uncontrolled, the excess serotonin from carcinoid syndrome can damage heart valves, a complication known as carcinoid heart disease. This is one reason early detection and hormone control matter.
How Carcinoid Tumors Are Diagnosed
The classic diagnostic test measures a serotonin byproduct called 5-HIAA in a 24-hour urine collection. When serotonin is broken down, 5-HIAA is what’s left over, so elevated levels point toward a hormone-producing carcinoid. Newer blood tests measuring the same byproduct in serum are increasingly used because they’re more convenient for patients. In one study, a serum 5-HIAA test detected carcinoid syndrome with 96% sensitivity, meaning it caught nearly all true cases.
Imaging plays a major role too. CT scans, MRI, and specialized nuclear medicine scans that target receptors on neuroendocrine cells can locate the primary tumor and identify any spread. A biopsy confirms the diagnosis and determines the tumor’s grade.
Grading: What It Means for Your Outlook
Carcinoid tumors are graded based on how quickly their cells are dividing, measured by something called the Ki-67 index (essentially the percentage of tumor cells actively reproducing). Grade 1 tumors have a Ki-67 of 2% or less, meaning very few cells are dividing at any given time. Grade 2 tumors fall between 3% and 20%. Grade 3 tumors exceed 20% and behave more aggressively. Most carcinoid tumors fall into grade 1 or 2.
The survival outlook for low- and intermediate-grade GI carcinoid tumors is notably good compared to many other cancers. Based on data from 2015 to 2021, the five-year relative survival rate for localized tumors is 97%. Even when the cancer has reached nearby lymph nodes (regional stage), five-year survival remains at 96%. When it has spread to distant organs, the rate drops to 68%, which is still considerably higher than most metastatic cancers. Across all stages combined, the five-year survival rate is 94%.
Typical vs. Atypical Lung Carcinoids
Lung carcinoids come in two distinct types with very different behavior. Typical carcinoids are well-differentiated, meaning the cells still look and act relatively normal. They’re associated with good survival, and most patients who undergo surgery remain disease-free afterward.
Atypical carcinoids are more aggressive. In one study of patients whose cancer had already reached nearby lymph nodes, those with typical carcinoids mostly remained disease-free at follow-up. Among patients with atypical carcinoids and lymph node involvement, however, seven out of eleven developed distant metastases within a median of 17 months, and most of those patients died within about two years. The difference in outcomes between the two types was statistically significant, which is why atypical lung carcinoids often require more aggressive treatment beyond surgery alone.
Risk Factors
The exact cause of carcinoid tumors remains unknown, though genetic abnormalities, including chromosomal losses and numerical imbalances, are suspected. Several factors increase your risk. Older adults are diagnosed far more often than younger people or children. Women develop carcinoid tumors more frequently than men.
The most well-established genetic link is a family history of multiple endocrine neoplasia type 1 (MEN1), an inherited condition that causes tumors to form in multiple hormone-producing glands. If MEN1 runs in your family, your risk of developing carcinoid tumors is elevated.
How Carcinoid Tumors Are Treated
Surgery is the primary treatment when the tumor can be removed. For small, localized carcinoids, particularly in the appendix or rectum, complete removal is often curative. Lung carcinoids are also treated primarily with surgery, and outcomes for typical carcinoids are generally excellent.
For tumors that can’t be fully removed, or for those causing carcinoid syndrome, medications that mimic a natural hormone-blocking substance in the body are a cornerstone of treatment. These drugs reduce the amount of hormones the tumor secretes, which controls flushing, diarrhea, and other symptoms. They also slow tumor growth. Long-acting formulations allow for injections every two to four weeks rather than multiple daily doses, making ongoing treatment more manageable.
Additional options for advanced disease include targeted radiation delivered directly to tumor cells via specialized injections, liver-directed therapies when metastases are concentrated there, and in some cases chemotherapy, though carcinoid tumors tend to respond less dramatically to chemotherapy than faster-growing cancers do. Treatment plans are highly individualized based on tumor location, grade, extent of spread, and whether carcinoid syndrome is present.