Bowel Carcinoid
A bowel carcinoid is a type of neuroendocrine tumor (NET) that arises from specialized hormone-producing cells in the wall of the small or large intestine. Bowel carcinoids most commonly occur in the small bowel (particularly the ileum) and the appendix, and less commonly in the colon or rectum. They are generally slow-growing tumors, though their behavior varies widely from indolent lesions found incidentally to more aggressive tumors that can spread to the liver or other sites. Some carcinoids produce hormones (such as serotonin) that, when the tumor has spread to the liver, can cause carcinoid syndrome—a distinctive combination of flushing, diarrhea, and other symptoms. Evaluation combines imaging (including specialized nuclear medicine studies), laboratory testing, endoscopy, and, when needed, biopsy or surgical evaluation.
What is it?
Throughout the wall of the digestive tract are specialized cells called neuroendocrine cells that have features of both nerve cells and hormone-producing cells. These cells produce various hormones and signaling molecules that help regulate digestion, blood flow, and other functions. Neuroendocrine tumors (NETs) arise from these cells. The term “carcinoid” is an older term that has traditionally been used for a subset of neuroendocrine tumors—particularly those arising in the gastrointestinal tract and lungs. Modern classification systems (such as those from the World Health Organization) use the broader term “neuroendocrine neoplasm” and further divide these tumors based on grade (from well-differentiated, low-grade tumors to poorly differentiated neuroendocrine carcinomas) and other characteristics. However, “carcinoid” remains in common use, particularly for well-differentiated gastrointestinal neuroendocrine tumors.
Bowel carcinoids can occur throughout the small and large intestines but have specific patterns of distribution.
The small bowel is one of the most common sites for gastrointestinal neuroendocrine tumors, and small bowel carcinoids (also called midgut NETs when arising from the jejunum, ileum, or proximal colon) are the most common tumor type in the small bowel overall. They arise most commonly from the ileum (particularly the terminal ileum) and are frequently multifocal (multiple tumors in the same patient), an important consideration during evaluation and surgery.
The appendix is also a common site for carcinoids, and small appendiceal carcinoids are relatively common incidental findings in specimens from appendectomies performed for appendicitis or other reasons. Most small appendiceal carcinoids (particularly those less than 1 cm) are considered cured by appendectomy alone, though larger or more concerning tumors may require additional surgery.
Colon and rectum carcinoids are less common than small bowel carcinoids but do occur. Rectal carcinoids are often small and detected during colonoscopy performed for other reasons; those that are small and confined to the inner layers of the rectum are often cured by endoscopic resection.
Duodenal carcinoids are less common. They may be associated with specific conditions, such as gastrinoma in patients with Zollinger-Ellison syndrome, or MEN1 (multiple endocrine neoplasia type 1) syndrome.
Bowel carcinoids can be classified by grade based on how quickly the tumor cells are dividing (using markers such as Ki-67). Grade 1 tumors have a very low proliferation rate (Ki-67 less than 3%) and are considered well-differentiated and low-grade. Grade 2 tumors have a somewhat higher proliferation rate (Ki-67 3–20%) and intermediate features. Grade 3 well-differentiated tumors have higher proliferation rates but retain the well-differentiated morphology. Neuroendocrine carcinomas (small cell or large cell) are poorly differentiated and behave much more aggressively, more like other high-grade cancers, and are typically treated with different regimens than well-differentiated tumors.
Some carcinoids produce hormones that can cause characteristic symptoms. Serotonin is one of the most commonly produced hormones by small bowel carcinoids. When the tumor is confined to the bowel, hormones released into the portal vein are typically metabolized by the liver before reaching the general circulation, and symptoms are uncommon. When the tumor has spread to the liver (liver metastases) or, less commonly, is producing hormones that bypass first-pass liver metabolism, hormones can enter the general circulation and cause carcinoid syndrome.
Carcinoid syndrome is a distinctive combination of symptoms including facial flushing (episodes of reddening of the face, neck, and upper chest), diarrhea (which can be substantial), and, less commonly, wheezing or shortness of breath (from bronchoconstriction). Long-standing severe carcinoid syndrome can lead to carcinoid heart disease, characterized by fibrotic changes on the right side of the heart (particularly affecting the tricuspid and pulmonary valves) that can cause heart failure. Carcinoid crisis is a severe form that can be triggered by anesthesia, surgery, or other stressors and involves severe symptoms and hemodynamic instability; preventive measures with somatostatin analogues are important during procedures.
Most bowel carcinoids are diagnosed after they cause symptoms, though many are also discovered incidentally—during evaluation for other conditions, during imaging performed for unrelated reasons, or at surgery for suspected other conditions (such as appendicitis). Symptoms of bowel carcinoids can be subtle and nonspecific, contributing to often long delays between symptom onset and diagnosis. Small bowel carcinoids often cause intermittent vague abdominal pain, sometimes with symptoms of partial bowel obstruction, and characteristic mesenteric fibrosis (scarring in the tissue supporting the bowel) that can further contribute to pain and obstruction. Gastrointestinal bleeding can occur but is less common.
Diagnosis typically involves several components.
Imaging is central. CT enterography and MR enterography (dedicated protocols for small bowel evaluation) are particularly useful for identifying small bowel carcinoids and evaluating associated findings such as mesenteric fibrosis and lymph node involvement. Standard CT of the chest, abdomen, and pelvis is used for staging and evaluation for liver metastases. MRI can provide detailed evaluation, particularly of liver metastases. Ga-68 DOTATATE PET/CT is a specialized nuclear medicine study that images somatostatin receptors, which are expressed by most well-differentiated neuroendocrine tumors. It is highly sensitive for identifying primary tumors, lymph node involvement, and metastases, and has largely replaced older imaging studies (such as the octreoscan) that also targeted somatostatin receptors but with lower resolution.
Laboratory testing may include chromogranin A (a general marker of neuroendocrine tumors, though not specific), 5-HIAA (5-hydroxyindoleacetic acid, a breakdown product of serotonin measured in 24-hour urine or blood, useful for tumors that produce serotonin), and, in selected cases, other specific hormone measurements based on symptoms.
Endoscopy is used for evaluation of carcinoids in accessible locations (such as the duodenum, colon, and rectum) and allows biopsy. Deep enteroscopy or capsule endoscopy can help evaluate the small bowel, though the mid and distal small bowel remain challenging to visualize with standard techniques.
Biopsy provides definitive diagnosis and grade determination. This may be obtained through endoscopy for accessible tumors or through image-guided or surgical approaches for others.
For patients with suspected hereditary syndromes (such as MEN1), evaluation for other associated tumors and genetic counseling and testing may be appropriate.
Important to Know
Management of bowel carcinoids is highly individualized based on the specific location, size, grade, extent of disease, hormone production, and patient factors. Care is best delivered by multidisciplinary teams at centers with experience in neuroendocrine tumors and typically includes medical oncologists (particularly those specializing in NETs), surgeons (colorectal, hepatobiliary, or surgical oncology), endocrinologists (particularly for functional tumors), gastroenterologists, radiologists, nuclear medicine specialists, pathologists, and, when needed, other specialists.
For localized small bowel carcinoids, surgery is the mainstay of curative treatment. Surgical resection typically involves removal of the affected segment of small bowel, along with a wide resection of the mesentery (the tissue containing the blood supply and lymph nodes to that segment) to remove associated lymph node metastases, which are common even with small primary tumors. Multifocal disease should be carefully searched for during surgery, as multiple tumors in different segments are common. Careful examination of the entire small bowel, either at open or laparoscopic surgery, is important.
For small appendiceal carcinoids (typically less than 1 cm), appendectomy alone is generally considered adequate treatment and is often curative. For larger appendiceal carcinoids or those with certain features (such as involvement of the base of the appendix, deep invasion, or high grade), a more extensive procedure such as right hemicolectomy may be recommended.
For rectal carcinoids, small tumors confined to the inner layers of the rectum can often be treated with endoscopic resection, while larger tumors or those with invasion beyond the inner layers may require surgical resection with different approaches depending on the specific circumstances.
For colon carcinoids, treatment principles broadly follow those for colorectal cancer, with surgical resection appropriate for localized disease.
For patients with liver metastases, several treatment options exist and are often used in combination.
Somatostatin analogues (such as octreotide and lanreotide) are important treatments that both control symptoms of carcinoid syndrome (by blocking hormone release from the tumor cells) and can slow tumor growth (based on evidence from clinical trials such as PROMID and CLARINET). They are typically given as long-acting injections every few weeks and are often continued for extended periods.
Liver-directed therapies can help control liver metastases. Options include surgical resection of liver metastases in selected patients, thermal ablation (radiofrequency or microwave ablation), and transarterial therapies such as bland embolization, chemoembolization, or radioembolization (with yttrium-90).
Peptide receptor radionuclide therapy (PRRT) with lutetium-177 DOTATATE (Lutathera) is a targeted therapy that delivers radiation specifically to tumor cells expressing somatostatin receptors. It is approved for progressive well-differentiated NETs and has been shown to improve outcomes in appropriate patients based on the NETTER-1 trial and other studies.
Targeted therapies such as everolimus (an mTOR inhibitor) are used for progressive well-differentiated NETs, based on evidence from the RADIANT trials.
Chemotherapy is typically reserved for higher-grade tumors (Grade 3 well-differentiated and neuroendocrine carcinomas), poorly differentiated neuroendocrine carcinomas (which are treated similarly to small cell lung cancer with platinum-based regimens), or specific circumstances.
Management of carcinoid syndrome involves several components. Somatostatin analogues are the mainstay for symptom control. Telotristat, an oral medication that reduces serotonin synthesis, is available for patients with refractory diarrhea despite somatostatin analogue therapy. For patients undergoing surgery or other procedures, preventive measures with somatostatin analogues are important to reduce the risk of carcinoid crisis. Assessment for carcinoid heart disease with echocardiography and, when needed, evaluation by cardiology is important, particularly for patients with long-standing severe carcinoid syndrome. Surgical or catheter-based valve interventions may be needed for advanced carcinoid heart disease.
Nutritional considerations are important for many patients with bowel carcinoids, particularly those with significant diarrhea, mesenteric involvement, or after significant bowel surgery. Consultation with dietitians or nutritionists can be helpful.
For patients with hereditary syndromes (such as MEN1), coordinated care includes evaluation and management of other associated tumors, genetic counseling, and family screening.
For patients with advanced disease, palliative care is an important component of treatment and can be integrated alongside cancer-directed therapy at any stage.
Long-term follow-up is important because bowel carcinoids can recur years after initial treatment. Standard surveillance includes clinical assessment, imaging (typically CT or MRI, with Ga-68 DOTATATE PET/CT in selected settings), laboratory tests (chromogranin A, 5-HIAA when appropriate), and echocardiography for patients with a history of carcinoid syndrome. Surveillance schedules are individualized based on the specific circumstances and are often continued for many years given the potential for late recurrence.
Care is best coordinated by multidisciplinary teams at centers with experience in neuroendocrine tumors. Imaging, laboratory, pathology, and molecular findings are interpreted alongside the patient’s symptoms, examination, and broader clinical context rather than in isolation.
Patient education plays an essential role. Understanding the diagnosis, the specific characteristics of the tumor including grade and hormone production, the rationale for the recommended treatment plan, the importance of surveillance, the significance of hereditary syndromes when identified, treatment side effects, and warning signs of complications or recurrence all contribute to better outcomes. Patient support groups and organizations focused on neuroendocrine tumors can provide valuable information and community.
Red flag symptoms include severe abdominal pain, symptoms of bowel obstruction (severe abdominal pain, distention, vomiting, inability to pass stool or gas), significant gastrointestinal bleeding, severe symptoms of carcinoid syndrome (severe flushing, severe diarrhea, wheezing, difficulty breathing), symptoms of carcinoid heart disease (progressive shortness of breath, swelling, palpitations), severe dizziness or fainting (particularly during or after procedures, which may suggest carcinoid crisis), high fever with signs of severe infection, sudden severe symptoms, or rapid clinical deterioration. These warrant prompt or urgent medical evaluation.