Background: Neuroendocrine tumor/carcinoid tumor features
Where can neuroendocrine tumors be found?
Since neuroendocrine tumors are derived from neuroendocrine tissue that is found in many different organs, neuroendocrine tumors can be found in any part of the body. However, the majority of neuroendocrine tumors occur in several locations in the abdomen.
Carcinoids. The majority of carcinoids occur in the GI tract and are characterized based on the development of the GI tract as either foregut (stomach and duodenum), midgut (small bowel or right colon) or hindgut (left colon and rectum). Approximately half of gastrointestinal carcinoids occur in the appendix with the terminal ileum as the next likely location. Outside of the gastrointestinal tract, the next most likely location for carcinoid tumors is the lung. Rarely, carcinoids can also occur in the pancreas, ovary or thymus.
Figure 1: Upper abdominal anatomy. In the upper left of the picture (actually upper right of the patient) is the liver (brownish), the stomach (pink), and the colon (yellow) in front of the pancreas (salmon colored, pebbled appearance), with the spleen (purple) at the tail of the pancreas, and the small intestine (pink, continuing from the stomach) running around the head of the pancreas and continuing down into the mid-abdomen.
Figure 2: Pancreatic anatomy. Centered is the salmon-colored pancreas, its head cradled with the surrounding C-shaped duodenum (1st part of small intestine), with the superior mesenteric vein (blue) and artery (red) running behind the neck of the pancreas. The gallbladder (green, upper right) connects to the bile duct that runs along the back side of the head of the pancreas, where it joins the pancreatic duct, emptying into the duodenum. The spleen (purple, upper right) is at the tail of the pancreas, with the splenic artery (red) running along its upper edge, and the splenic vein (faintly blue) running in the middle along the back of the body and tail of the pancreas, and joining the superior mesenteric vein behind the pancreas to form the portal vein that delivers blood to the liver. The main pancreatic duct (reddish) runs the length of the pancreas, and collects pancreatic juice through it branches, then empties into the duodenum. The islets that secrete insulin are too small to be shown in this picture.
Figure 3: Microscopic section of pancreas showing normal exocrine cells (secrete bicarbonate and enzymes to digest food) surrounding an islet that contains beta cells (special stain tints the cells browncenter of image) that secrete insulin.
Pancreatic neuroendocrine tumors. Pancreatic neuroendocrine tumors tend to occur in the pancreas and duodenum. In familial cases, multiple tumor nodules occur though out the pancreas and duodenal wall. The pancreas is an organ that lies behind the stomach and in front of the spine, cradled by the first part of the small intestine (duodenum) (Figures 1 and 2), and has 3 principal functions: secrete bicarbonate to neutralize the stomach acid, secrete enzymes that are critical for the digestion of the food as it passes through the intestine, and secrete insulin for regulation of blood glucose (sugar). The cells that secrete bicarbonate and enzymes are called exocrine, and are different than the islet cells (called islets, about a million of which are clustered as small islands throughout the pancreas), of which beta cells (specifically secrete insulin) are the most important (Figure 3). The endocrine nature of insulin and other hormones relates to the fact that they are secreted by the pancreas, but its action takes place elsewhere in the body (liver, muscle, etc.). Most exocrine tumors are ductal adenocarcinomas, which are typically referred to as "pancreas cancer" and frequently have poor prognoses. On the other hand, the "endocrine" pancreas produces hormones that regulate the body's metabolism, including the control of blood sugar levels. "Neuroendocrine" pancreas tumors account for only 1-2% of all pancreas tumors.