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Pancreatic neuroendocrine tumors (NETs), or islet cell tumors, are a type of cancer that starts in the pancreas. (Cancer starts when cells in the body begin to grow out of control. To learn more about how cancers start and spread, see What Is Cancer?)

Pancreatic NETs are a less common type of pancreatic cancer. They make up less than 2% of pancreatic cancers, but tend to have a better outlook (prognosis) than the more common type.

Where pancreatic neuroendocrine tumors start

Pancreatic neuroendocrine tumors start in neuroendocrine cells, a special kind of cell found in the pancreas. Neuroendocrine cells are also found in other areas of the body, but only cancers that form from neuroendocrine cells in the pancreas are called pancreatic neuroendocrine tumors.

The neuroendocrine system

Neuroendocrine cells are like nerve cells in some ways and like hormone-making endocrine cells in other ways.  Cells in this system don't form actual organs. Instead, they are scattered throughout other organs like the esophagus, stomach, pancreas, intestines, and lungs.

Neuroendocrine cells (sometimes just called endocrine cells) in the pancreas are found in small clusters called islets (or islets of Langerhans). These islets make important hormones like insulin and glucagon (which help control blood sugar levels), and release them directly into the blood.

The pancreas

The pancreas is an organ that sits behind the stomach. It's shaped a bit like a fish with a wide head, a tapering body, and a narrow, pointed tail. In adults it's about 6 inches (15 centimeters) long but less than 2 inches (5 centimeters) wide.

• The head of the pancreas is on the right side of the abdomen (belly), behind where the stomach meets the duodenum (the first part of the small intestine).
• The body of the pancreas is behind the stomach.
• The tail of the pancreas is on the left side of the abdomen next to the spleen.

Neuroendocrine tumors start in the endocrine cells of the pancreas. But most of the pancreas is actually made up of another type of cell called exocrine cells. These cells form the exocrine glands and ducts. The exocrine glands make pancreatic enzymes that are released into the intestines to help you digest foods (especially fats). The most common type of pancreatic cancer, adenocarcinoma of the pancreas, starts from exocrine cells. See Pancreatic Cancer for more about this type.

If you are diagnosed with pancreatic cancer, it’s very important to know if it's an exocrine cancer (see Pancreatic Cancer) or endocrine cancer (discussed here). They have distinct risk factors and causes, have different signs and symptoms, are diagnosed with different tests, are treated in different ways, and have different outlooks.

Types of pancreatic neuroendocrine tumors

Tumor grade

Pancreatic neuroendocrine tumors (NETs) are classified by tumor grade, which describes how quickly the cancer is likely to grow and spread. 

• Grade 1 (also called low-grade or well-differentiated) neuroendocrine tumors have cells that look more like normal cells and are not multiplying quickly.
• Grade 2 (also called intermediate-grade or moderately differentiated) tumors have features in between those of low- and high-grade (see below) tumors.
• Grade 3 (also called high-grade or poorly differentiated) neuroendocrine tumors have cells that look very abnormal and are multiplying faster.

Cancers that are grade 1 or 2 are called pancreatic neuroendocrine tumors. These cancers tend to grow slowly and can possibly spread to other parts of the body.

Cancers that are grade 3 are called pancreatic neuroendocrine carcinomas (NECs). These cancers tend to grow and spread quickly and can spread to other parts of the body.

Another important part of grading is measuring how many of the cells are in the process of dividing into new cells. This is described in more detail in Pancreatic Neuroendocrine Tumor Stages. 

Tumor function

Pancreatic NETs are also named based on whether they are functioning (making hormones that cause symptoms) or non-functioning (not making hormones).

Functioning NETs: About half of pancreatic NETs make hormones that are released into the blood and cause symptoms. These are called functioning NETs. Each one is named for the type of hormone the tumor cells make.

• Insulinomas come from cells that make insulin.
• Glucagonomas come from cells that make glucagon.
• Gastrinomas come from cells that make gastrin. 
• Somatostatinomas come from cells that make somatostatin.
• VIPomas come from cells that make vasoactive intestinal peptide (VIP).
• ACTH-secreting tumors come from cells that make adrenocorticotropic hormone (ACTH).

Most (up to 70%) functioning NETs are insulinomas. The other types are much less common.

Non-functioning NETs: These tumors don’t make enough excess hormones to cause symptoms. Because they don’t make excess hormones that cause symptoms, they can often grow quite large before they're found. Symptoms that may occur when they grow to a large size include abdominal (belly) pain, lack of appetite, and weight loss.

Carcinoid tumors: These NETs are much more common in other parts of the digestive system, although rarely they can start in the pancreas. These tumors often make serotonin.

The treatment and outlook for pancreatic NETs depend on the specific tumor type and the stage (extent) of the tumor, but the outlook is generally better than for pancreatic exocrine cancers.