Most NETs, like other neoplasms, are clinically silent, producing symptoms only as a consequence of growth. But silence should not be mistaken for a low disease prevalence. In fact, with more than 100,000 cases estimated in the United States, NETs are more prevalent than other stomach and pancreatic cancers combined (see chart).1,2
The majority of NETs occur in the GI tract (67.5%) and the bronchopulmonary system (25.3%). Within the GI tract, most NETs occur in the small intestine (41.8%), rectum (27.4%), and stomach (8.7%) Less than 1% of NETs occur in the pancreas,3 although this number is typically considered to be low because nonmalignant tumors (ie, insulinomas and small, nonfunctioning pancreatic NETs) have not been included in Surveillance, Epidemiology, and End Results (SEER) Program registries.4
Pancreatic NETs also account for only 1% of pancreatic cancers by incidence, while representing 10% of all pancreatic cancers by 28-year limited duration prevalence. Again, however, both of these numbers may be underestimated in SEER databases.2,4
While some studies indicate that roughly half of all pancreatic NETs are nonfunctional, this number may be underrreported. In fact, 85% of pancreatic NETs reported in SEER registries have an unknown functional status. Among pancreatic NETs with known functional status, gastrinomas account for 6%, insulinomas 4%, glucagonomas 2%, and VIPomas 1%.4
Although the bronchopulmonary system is a frequent site of NETs, grade 1 and grade 2 (G1/G2) bronchial NETs — also called typical and atypical lung carcinoids, respectively5 — represent only 1% to 2% of all lung cancers, with an annual age-adjusted incidence of 1.35 per 100,000 individuals. Annual age-adjusted incidence for thymic NETs is 0.02 per 100,000 individuals.1,5
These numbers do not include G3 large cell neuroendocrine carcinomas (LCNEC) and small cell lung carcinomas (SCLC), which are distinct from G1/G2 bronchial NETs in terms of prevalence, biology, and prognosis.6,7
Complex malignancies on the rise
A SEER database analysis shows a dramatic 5-fold increase in diagnosed incidence of NETs from 1973 to 2004 (see chart).1 Diagnosed incidence of NETs is predicted to continue rising at a faster rate than other malignant neoplasms.3
A related observation is that although the annual incidence of malignant pancreatic NETs in the United States is about 0.2 per 100,000 persons,4,8 some studies show that patients with pancreatic NETs tend to have higher incidence of multiple primary malignancies. Gastrinomas and nonfunctional pancreatic NETs, in particular, may be associated with ovarian, breast, endometrial, bladder, prostate, and esophageal tumors.9
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