The short answer
Neuroendocrine tumors are grouped by where they start — most often the digestive tract, pancreas, or lungs — and by whether they release hormones. Grade, based on how fast cells are dividing (often reported with a Ki-67 test), often matters more than stage for predicting how a NET will behave and guiding treatment.
Neuroendocrine tumors are grouped by where they start: gastrointestinal (GI) NETs, pancreatic NETs, and lung NETs are the most common groups.
Within each group, tumors are also described as functioning (hormone-releasing) or non-functioning.
Pancreatic functioning tumors have specific names based on the hormone they release, such as insulinoma, gastrinoma, and glucagonoma.
Grade — how well-differentiated the cells are and how fast they are dividing, often measured with a Ki-67 test — is often more useful than stage for predicting how a NET will behave.
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The full explanation.
The simple version
Neuroendocrine tumors (NETs) are usually described in two ways at once: by where they started, and by whether they release hormones. Both matter for understanding what to expect.
Grouped by location
- Gastrointestinal (GI) NETs — start in the digestive tract, most often the small intestine, rectum, stomach, or appendix. These are sometimes called carcinoid tumors.
- Pancreatic NETs — start in hormone-making cells of the pancreas; also called islet cell tumors.
- Lung NETs — start in the lungs and are also called lung carcinoid tumors. Most are low-grade ("typical carcinoid") and grow slowly; a smaller group are intermediate-grade ("atypical carcinoid") and grow somewhat faster.
NETs can also start elsewhere in the body, since neuroendocrine cells are found in many organs.
Grouped by hormone activity
- Functioning tumors release hormones that cause symptoms. In the pancreas, these have specific names based on the hormone: an insulinoma releases insulin (causing low blood sugar), a gastrinoma releases gastrin (causing stomach ulcers and reflux), and a glucagonoma releases glucagon (causing high blood sugar and a skin rash), among others.
- Non-functioning tumors do not release hormones that cause noticeable symptoms and are often found later or by chance.
Why grade matters more than stage
Grade describes two things: how closely the tumor cells resemble normal cells (differentiation) and how fast they are dividing, often measured with a lab test called a Ki-67 index.
- Well-differentiated, low-grade tumors tend to grow slowly and often have a more favorable outlook, even if they have spread.
- Poorly differentiated, high-grade tumors — sometimes called neuroendocrine carcinomas — grow and spread faster and are treated more urgently.
Because of this, two people with a NET at a similar stage can have very different experiences depending on grade. Grade, alongside stage and location, is one of the most important pieces of information for planning treatment and knowing what to expect.
Sources
- National Cancer Institute, Gastrointestinal Neuroendocrine Tumors Treatment (PDQ) – Health Professional Version, accessed August 3, 2026.
- National Cancer Institute, Pancreatic Neuroendocrine Tumors (Islet Cell Tumors) Treatment (PDQ) – Patient Version, accessed August 3, 2026.
- American Cancer Society, Lung Carcinoid Tumor, accessed August 3, 2026.
Words to know
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Common questions
Where do neuroendocrine tumors usually start?
The digestive tract (including the small intestine, rectum, stomach, and appendix), the pancreas, and the lungs are the most common sites. NETs can also start in other places, since neuroendocrine cells are found throughout the body.
What is the difference between functioning and non-functioning tumors?
A functioning tumor releases hormones that cause noticeable symptoms, such as flushing and diarrhea, low blood sugar, or stomach ulcers. A non-functioning tumor does not release hormones that cause symptoms, so it tends to be found later, often once it is large enough to cause pain or a blockage, or by chance on a scan.
What are examples of pancreatic neuroendocrine tumors?
Pancreatic NETs that release hormones are named for that hormone: insulinoma (insulin, causing low blood sugar), gastrinoma (gastrin, causing stomach ulcers and reflux), and glucagonoma (glucagon, causing high blood sugar and a distinctive skin rash), among others. Non-functioning pancreatic NETs do not have these hormone-based names.
What does grade mean for a NET?
Grade describes how closely the tumor cells resemble normal cells (differentiation) and how quickly they are dividing, often measured with a lab test called a Ki-67 index. Well-differentiated, low-grade tumors tend to grow slowly and have a more favorable outlook. Poorly differentiated, high-grade tumors — sometimes called neuroendocrine carcinomas — grow and spread faster.
Why does grade matter more than stage for NETs?
Two NETs at a similar stage can behave very differently depending on grade. A low-grade tumor that has spread may still grow slowly and be manageable for years, while a high-grade tumor is more aggressive even at an earlier stage. Because of this, grade heavily influences the treatment plan, alongside stage and location.
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Last updated: 2026-08-03Next planned review: 2027-08-03
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source verified — This page was created with AI assistance and checked against the sources listed on it. Source checking is not a medical review.
Human medical review: not completed. Cancer Explained is not clinician-reviewed, and that is a deliberate design choice rather than a gap we are waiting to close. We restate published federal guidance and cite it; the authority belongs to the source, not to us. That is why every page names where its claims come from — so you can verify us instead of trusting us. Use it to understand your situation and to ask better questions of the people treating you.
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