The normal pancreas
The pancreas is an organ located behind the stomach. It is shaped a little bit like a fish with a wide head, a tapering body, and a narrow, pointed tail. It is about 6 inches long but less than 2 inches wide and extends horizontally across the abdomen. The head of the pancreas is on the right side of the abdomen (belly), behind the place where the stomach meets the duodenum (the first part of the small intestine). The body of the pancreas is located behind the stomach and the tail of the pancreas is on the left side of the abdomen next to the spleen.
The pancreas contains 2 different types of glands: exocrine and endocrine.
The exocrine glands make pancreatic “juice,” which is released into the intestines. This juice contains enzymes that help you digest fats, proteins, and carbohydrates in the food you eat. Without these, some of the food you eat would just pass through your intestines without being absorbed. The enzymes are released into tiny tubes called ducts. These tiny ducts merge to form larger ducts that empty into the pancreatic duct. The pancreatic duct merges with the common bile duct (the duct that carries bile from the liver), and empties the pancreatic juice into the duodenum (the first part of the small intestine) at the ampulla of Vater. More than 95% of the cells in the pancreas are exocrine glands and ducts.
A small percentage of the cells in the pancreas are endocrine cells. These cells are arranged in small clusters called islets (or islets of Langerhans). The islets release important hormones, such as insulin and glucagon, directly into the blood. Insulin reduces the amount of sugar in the blood, while glucagon increases it. Diabetes results from a defect in insulin production.
Types of pancreatic tumors
The exocrine cells and endocrine cells of the pancreas form completely different types of tumors.
Exocrine tumors are by far the most common type of pancreas cancer. When someone says that they have pancreatic cancer, they usually mean an exocrine pancreatic cancer. Benign (non-cancerous) cysts and benign tumors called cystadenomas can occur, but most pancreatic exocrine tumors are malignant (cancerous).
An adenocarcinoma is a cancer that starts in gland cells. About 95% of cancers of the exocrine pancreas are adenocarcinomas. These cancers usually begin in the ducts of the pancreas, but they sometimes develop from the cells that make the pancreatic enzymes (acinar cell carcinomas).
Less common types of cancers of the exocrine pancreas include adenosquamous carcinomas, squamous cell carcinomas, signet ring cell carcinomas, undifferentiated carcinomas, undifferentiated carcinomas with giant cells, and solid pseudopapillary neoplasms of the pancreas. These types are distinguished from one another based on how they look under the microscope.
The treatment of an exocrine pancreatic cancer is mostly based on the stage of the cancer, not its exact type. The stage of the cancer describes how large the tumor is and how far it has spread. Pancreatic cancer staging is described later in this document.
A special type of cancer, called ampullary cancer (or carcinoma of the ampulla of Vater) is a cancer that starts in the place where the bile duct and pancreatic duct come together and empty into the duodenum (the ampulla of Vater). These cancers often block the bile duct while they are still small and have not spread far. This blockage causes bile to build up in the body, which leads to yellowing of the skin and eyes (jaundice) and can turn urine dark. This is an easily recognized sign that something is wrong. This is why ampullary cancers are usually found at an earlier stage than most pancreatic cancers, which means they usually have a better prognosis (outlook) than typical pancreatic cancers.
Ampullary cancers are included with pancreatic cancer in this document because their treatments are very similar.
Tumors of the endocrine pancreas are uncommon. As a group, they are known as pancreatic neuroendocrine tumors (NETs), or sometimes as islet cell tumors. There are several subtypes of islet cell tumors. Each is named according to the type of hormone-making cell it starts in:
- 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).
- PPomas come from cells that make pancreatic polypeptide.
Carcinoid tumors are another type of pancreatic NET. These tumors often make serotonin (also called 5-HT) or its precursor, 5-HTP.
About half of pancreatic NETs make hormones that are released into the blood and so are called functioning tumors. Tumors that do not make hormones are called non-functioning.
Islet cell tumors can be benign or malignant. Benign tumors are called pancreatic neuroendocrine tumors, while malignant tumors are called pancreatic neuroendocrine cancers or carcinomas. Malignant and benign tumors can look very similar under the microscope, so it isn’t always clear at the time of diagnosis whether or not a NET is cancer. Sometimes the diagnosis only becomes clear when the tumor has spread outside of the pancreas.
Pancreatic neuroendocrine cancers make up less than 4% of all pancreatic cancers diagnosed. Treatment and prognosis depend on the specific tumor type and the stage (extent) of the tumor but the prognosis is generally better than that of pancreatic exocrine cancers. The most common types of pancreatic endocrine tumors are gastrinomas and insulinomas. The other types occur very rarely.
It is very important to distinguish between exocrine and endocrine cancers of the pancreas. They have distinct risk factors and causes, have different signs and symptoms, are diagnosed using different tests, are treated in different ways, and have different prognoses. In this document, the term pancreatic neuroendocrine tumor is used for both benign and malignant endocrine pancreatic tumors.