PNETs arise from neuroendocrine cells within the pancreas. Most (up to 90 % in some series) do not secrete hormones (nonfunctioning) and may be an incidental finding on imaging. The rest are described as functioning. These exhibit hypersecretion of various hormones and are generally defined by their secretory products. They may produce one or more hormones, which subsequently give rise to a specific clinical picture (Table 8.1).

PNETs are rare, occurring with an incidence of less than 1:100,000. However, postmortem findings suggest that these tumors are more common than previously thought, up to 10 % in some studies. Many small tumors that are nonfunctioning, and therefore asymptomatic, remain undiagnosed. Patients with functioning tumors are more likely to be symptomatic and so tend to be diagnosed at a younger age. Overall survival is better in those with functional lesions although it is important to bear in mind that lesions found incidentally on screening may otherwise have been one of those that patients would otherwise have never known about. Aggressive treatment of such lesions may suggest that whatever treatment is used is effective, leading to publication bias in favor of surgical or other treatment.