Rectal Neuroendocrine Tumor (Carcinoid)

What is a rectal neuroendocrine tumor?

A rectal neuroendocrine tumor (NET), historically called a carcinoid tumor, is a type of neoplasm that arises from the neuroendocrine cells in the lining of the rectum. These cells have features of both nerve cells and hormone-producing endocrine cells. Most rectal NETs are small, slow-growing (indolent), and are discovered incidentally during a routine colonoscopy. While the majority have a low potential to spread (metastasize), larger tumors or those that invade deeper into the rectal wall have a higher risk of metastasis and require more aggressive treatment.

Do rectal NETs cause symptoms?

The vast majority of rectal NETs are small and asymptomatic. They are typically found by chance during a colonoscopy performed for screening or other reasons. When symptoms do occur, they are usually related to the tumor's size and location and can include:

• Rectal bleeding
• A change in bowel habits or constipation
• Rectal pain or a feeling of fullness

It is extremely rare for rectal NETs to cause carcinoid syndrome (flushing, diarrhea, wheezing), as the hormones they might produce are typically broken down by the liver. This syndrome usually only occurs if the cancer has spread extensively to the liver.

What causes rectal NETs?

The exact cause of most rectal neuroendocrine tumors is unknown. They occur when neuroendocrine cells in the rectum undergo genetic changes that cause them to grow uncontrollably. Unlike many other cancers, there are no well-established lifestyle or environmental risk factors. They are typically sporadic, although very rare inherited syndromes, like multiple endocrine neoplasia type 1 (MEN1), can increase the risk of developing NETs.

How are rectal NETs diagnosed and staged?

The diagnosis is confirmed by a biopsy taken during a colonoscopy. The pathologist will examine the tissue to confirm it is a neuroendocrine tumor and will determine its grade, which reflects how quickly the cells are dividing (e.g., G1, G2, or G3). This is a very important factor in determining the tumor's potential to metastasize.

Once a rectal NET is diagnosed, staging is performed to determine its size, depth of invasion, and whether it has spread. This is crucial for deciding on the appropriate treatment.

• Endoscopic Ultrasound (EUS) or Pelvic MRI: These are the most important tests for small rectal NETs. They provide detailed images to accurately measure the tumor's size and determine if it has invaded into the muscular wall of the rectum (the muscularis propria) or spread to nearby lymph nodes.
• CT Scan and/or Somatostatin Receptor Scintigraphy (Octreoscan or Ga-68 DOTATATE PET/CT): For larger or more aggressive tumors, these imaging studies are used to look for spread to lymph nodes or distant organs like the liver.

How are rectal NETs treated?

Treatment depends on the size of the tumor, its depth of invasion, and its grade.

• For small, low-grade tumors (<1 cm): These have a very low risk of metastasis and can usually be completely removed endoscopically during a colonoscopy, a procedure called endoscopic resection. No further treatment is usually needed.
• For intermediate tumors (1-1.9 cm): Treatment for this group is more controversial. If the tumor is confined to the superficial layers and has no high-risk features, complete removal via transanal local excision (using techniques like TAMIS) is often sufficient. If the tumor invades the muscularis propria or has other high-risk features, a more radical surgery may be recommended.
• For large or high-risk tumors (≥2 cm or with deep invasion): These tumors have a higher risk of spreading to lymph nodes. The recommended treatment is a radical resection, which is the same type of surgery performed for rectal adenocarcinoma. This involves removing the section of the rectum containing the tumor along with its surrounding lymph nodes (a low anterior resection or abdominoperineal resection).

Is a carcinoid tumor a type of cancer?
Yes, all neuroendocrine tumors are considered malignant because they have the potential to spread to other parts of the body. However, most small, low-grade rectal NETs are very slow-growing and have an extremely low risk of metastasis, and are often cured with simple removal.

If my tumor is removed completely, do I need follow-up?
For small (<1 cm), low-grade tumors that are completely removed endoscopically, no further specific follow-up beyond routine colon cancer screening is usually necessary. For larger or more aggressive tumors, regular follow-up with imaging and blood tests (like chromogranin A) may be recommended for several years to monitor for any recurrence.

• Alavi K, Boutros M. Gastrointestinal Stromal Tumors, Neuroendocrine Tumors, and Lymphoma. In: Steele SR, Hull TL, Hyman N, Maykel JA, Read TE, Whitlow CB, eds. The ASCRS Textbook of Colon and Rectal Surgery. 4th ed. Springer; 2022:590-597.
• National Comprehensive Cancer Network. Neuroendocrine and Adrenal Tumors (Version 1.2024). Accessed October 12, 2025. https://www.nccn.org/professionals/physician_gls/pdf/neuroendocrine.pdf
• Mayo Clinic. Neuroendocrine tumors. Accessed October 12, 2025. https://www.mayoclinic.org/diseases-conditions/neuroendocrine-tumors/symptoms-causes/syc-20354285