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The term "carcinoid" tumor was originally used to describe a group of tumors that were thought to be benign (non-malignant). It is now clear that while carcinoid tumors are slow growing, depending on certain factors, they do carry a significant metastatic potential. Morphologically (by cellular appearance) there are approximately five different types of carcinoid tumor cells ranging from very well defined types, which carry the best prognosis, to undifferentiated types, which are associated with a poorer prognosis. There is also increased vascularity (blood supply) within the tumor.

Histologically, carcinoid tumors are characterized by monotonous sheets of cells containing small compact nuclei that are uniform in appearance and contain electron dense granules that contain biologically active amines and/or neuropeptides. These granules contain chromogranins (A and C), neuron specific enolase and synaptophysins in varying concentrations that can be measured to assess the biological function of the tumors.

Carcinoid tumors are unique from other cancer cell in that they produce numerous hormone-like substances including:

• Amines

  • serotonin
  • noradrenalin

• Polypeptides

  • ACTH (adrenocorticotropic hormones)
  • gastrin
  • insulin
  • growth hormone

• Prostaglandins

Each of these hormone-like substances can cause varying types of symptoms.

In many cases, the location of the primary tumor can be identified based on the bioactive hormone-like substances that it produces. These include:

• Foregut tumors - produce low amounts of serotonin but higher levels of other substances such as gastrin, 5-HTP, and corticotropins.

• Midgut tumors - tend to produce serotonin and tachykinins which are responsible for the symptoms of carcinoid syndrome when the tumor spreads to the liver

• Hindgut tumors - do not produce serotonin but may produce other hormones (e.g., low levels of gastrin)

Studies have shown that serum (blood) levels of a particular hormone have little predictive value for assessing the malignant potential of carcinoid tumors. Furthermore, the size of a particular tumor does not necessarily correlate with the level of hormone production or the severity of the clinical syndrome.