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. One is where they started. The other is whether they release hormones. Both matter for knowing what to expect.
Grouped by location
- Gastrointestinal (GI) NETs start in the digestive tract. The small intestine, rectum, stomach, and appendix are the usual sites. These are sometimes called carcinoid tumors.
- Pancreatic NETs start in hormone-making cells of the pancreas. They are also called islet cell tumors.
- Lung NETs start in the lungs. They are also called lung carcinoid tumors. About 9 in 10 are low-grade ("typical carcinoid") and grow slowly. The rest are intermediate-grade ("atypical carcinoid") and grow faster.
NETs can start elsewhere in the body too. Neuroendocrine cells sit in many organs.
Grouped by hormone activity
- Functioning tumors release hormones that cause symptoms. In the pancreas, each has a name based on that hormone. An insulinoma releases insulin, which lowers blood sugar. A gastrinoma releases gastrin, which causes stomach ulcers and reflux. A glucagonoma releases glucagon, which raises blood sugar and brings a skin rash. There are others.
- Non-functioning tumors do not release hormones that cause obvious symptoms. They are often found later, or by chance.
Why grade matters more than stage
Grade describes two things. First, how closely the tumor cells resemble normal cells, which is called differentiation. Second, 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. Their outlook is often more favorable, even if they have spread.
- Poorly differentiated, high-grade tumors are sometimes called neuroendocrine carcinomas. They grow and spread faster, and they are treated more urgently.
So two people with a NET at a similar stage can have very different experiences. Grade sits alongside stage and location. It is one of the most important facts for planning treatment and knowing what to expect.
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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: 2026-08-11Next 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 checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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