Pancreatic neuroendocrine tumors (pancreatic NETs or PNETs) account for about 6% of all pancreatic tumors. They may be benign or malignant and they tend to grow slower than exocrine tumors. They develop from the abnormal growth of endocrine (hormone-producing) cells in the pancreas called islet cells. This is why these tumors are sometimes referred to as "islet cell tumors."

Some of the hormones islet cells produce include insulin, glucagon and somatostatin. Insulin and glucagon are the two main pancreatic hormones. Insulin lowers blood sugar levels, while glucagon raises blood sugar levels. Together, these two main hormones work to maintain the proper level of sugar in the blood. Somatostatin regulates the levels of a variety of other hormones in the blood.

Pancreatic neuroendocrine tumors are either functional (produce hormones) or nonfunctional (produce no hormones).

Functional neuroendocrine tumors cause the pancreas to overproduce hormones consequently causing hormone-related symptoms. The majority of PNETs are nonfunctional tumors. Nonfunctional tumors do not produce any hormones so they do not cause any hormone-related symptoms. As a result, these tumors are typically diagnosed once the tumor is advanced and is causing symptoms such as pain or jaundice.