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The majority of neuroendocrine tumors can be divided into two classes: carcinoid and pancreatic neuroendocrine tumors. Carcinoid commonly refers to neuroendocrine tumors that originate in the gastrointestinal (GI) tract, lungs, appendix and thymus, although they can also occur in the lymph nodes, brain, bone, gonads (ovaries and testes) and skin. Carcinoid tumors are usually indolent (slow-growing) by nature and develop over the course of many years. However, aggressive, fast-growing forms of carcinoid cancer also exist.
Carcinoid tumors can secrete a variety of functional hormones and chemicals although not all carcinoid tumors do (Reidy, Tang, & Saltz, 2008). A few of the substances that are commonly secreted by carcinoid tumors are: serotonin, chromogranin A, histamine, pancreatic polypeptide and gastrin. Carcinoid tumors are referred to as functioning if they secrete hormones that cause a clinical syndrome (Reidy, Tang, & Saltz, 2008).
Functioning carcinoid tumors that occur in the digestive tract and pancreas release the substances they produce directly into the hepatic portal vein (a blood vessel in the abdominal cavity) which carries them directly to the liver where they are metabolized (broken down). Since the liver metabolizes these substances, their message is not sent to the rest of the body. Consequently, tumors of the digestive tract and pancreas are not usually detected until they have metastasized to the liver or cause obstructive symptoms (Kulke, 2007).
When carcinoid metastasizes to the liver, the liver is not always able to metabolize all of the hormones secreted. This excess of hormone called hypersecretion can cause an array of symptoms called carcinoid syndrome.
Carcinoids that occur in areas outside of the intestine and pancreas such as the lung and stomach do not release their hormones into the hepatic portal vein but release them directly into the bloodstream, bypassing the liver. Consequently, individuals with carcinoid in these locations can sometimes develop carcinoid syndrome without liver metastases as well as other symptoms and syndromes.
Common symptoms associated with Carcinoid Syndrome include diarrhea, flushing of the skin, abdominal cramping, asthma, arthritis, niacin deficiency, swelling of the feet, and wheezing. Carcinoid Syndrome occurs in approximately 10% of all individuals diagnosed with carcinoid (Vinik, Feliberti, Perry, & Nakave, 2008) and can lead to right-sided heart failure (Moller, Connolly, Rubin, Steward, Modesto, & Pellikka, 2003). Carcinoid Syndrome is most common in individuals with liver metastases from ileal carcinoids (Kulke, 2007).
Individuals with Carcinoid Syndrome can also experience carcinoid crisis which can occur spontaneously or be stress induced. A Carcinoid Crisis can be a life-threatening event that requires careful monitoring. Symptoms of a Carcinoid Crisis may include severe hypotension or hypertension, irregular and/or rapid heartbeat, wheezing, prolonged flushing, severe dyspnea (shortness of breath), and peripheral cyanosis (lack of oxygenated blood) (Oberg, Reubi, Kwekkeboom, & Krenning, 2010).
Carcinoid is classified as a rare cancer. Recent studies have determined that 4 to 5 out of every 100,000 people are diagnosed yearly with a neuroendocrine tumor and that there are over 100,000 people currently living with neuroendocrine tumors within the United States (Yao et al., 2009). For reasons not well understood, the incidence of carcinoid is rising. Since most individuals with carcinoid are asymptomatic until the tumors metastasize, the average time between tumor development and diagnosis is between 5 to 10 years (Vinik, 2008; Vinik et al., 2009). Survival rates for individuals with carcinoid vary and depend on tumor type, location, extent of metastases, and many other factors. Currently, surgery is the only cure for localized tumors (those which have not spread) and there is no cure for metastatic carcinoid (Yao, 2007).
What is a Tumor?
Cells are the building blocks of all life. All cells have highly specific functions, but not all cells have the same function. When cells that have similar functions are grouped together they form a tissue. Tissues when grouped together to perform a specific function are called organs. All cells of the human body have the same DNA (genetic language). Cell growth and replication is highly controlled and is encoded in each cell’s DNA. However if there are enough mutations (changes) within a cell’s DNA, a cell can grow and replicate uncontrollably.
A tumor is a mass formed by an abnormal growth of cells within the body. A tumor can be non-cancerous (benign) or cancerous (malignant). A tumor is considered cancerous when it has uncontrolled proliferation (abnormal growth) and can invade and destroy surrounding tissue. Malignant tumors can also have the ability to metastasize (spread to other organs of the body).
What is a Neuroendocrine Tumor?
The neuroendocrine system consists of highly specialized neuroendocrine cells which act as an interface or junction between the nervous system and the endocrine system. The endocrine system is made up of cells whose function is to produce and secrete hormones into the bloodstream. Hormones are biochemical messengers that help to regulate many different processes within the body. The nervous system is composed of specialized cells (neurons) that control the activities of all body parts. A neuroendocrine cell is a cell which receives neuronal input (a signal from a nerve cell) and releases hormones in response to this signal.
A neuroendocrine tumor can develop anywhere there are neuroendocrine cells. The most common sites from which neuroendocrine tumors arise are the lungs, appendix, small intestine, rectum and pancreas (Yao, Hassan, Phan, Dagohoy, Leary, Mares, Abdalla, Fleming, Vauthey, Rashid, & Evans, 2008). Neuroendocrine tumors that arise in the pancreas are called pancreatic neuroendocrine tumors or islet cell tumors. When neuroendocrine tumors originate in other areas, they are often classified as carcinoid tumors.
Since neuroendocrine tumor cells are derived from neuroendocrine cells, many of these tumor cells can behave like cells they originated from and can secrete a variety of hormones. A functioning neuroendocrine tumor is one that secretes biologically active hormones causing a clinical syndrome. Non-functioning neuroendocrine tumors do not cause clinical syndromes.
Carcinoid tumors and pancreatic neuroendocrine tumors share similarities including often indolent behavior, ability to secrete biologically active hormones, and well-differentiated histology (Reidy, Tang & Saltz, 2009).
Kulke, M. H. (2007). Clinical Presentation and Management of Carcinoid Tumors. Hematology/Oncology Clinics of North America, 21, 433-455. Retrieved from: http://www.ncbi.nlm.nih.gov/pubmed/17548033.
Moller, J. E., Connolly, H. M., Rubin, J., Seward, J. B., Modesto, K., Pellikka, P. A. (2003). Factors associated with progression of carcinoid heart disease. New England Journal of Medicine, 13(348), 1005-1015. Retrieved from: http://www.ncbi.nlm.nih.gov/pubmed/12637610.
Oberg, K. E., Reubi, J. C., Kwekkeboom, D. J., Krenning, E. P. (2010) Role of somatostatins in gastroenteropancreatic neuroendocrine tumor development and therapy. Gastroenterology, 139(3), 742-753. Retrieved from: http://www.ncbi.nlm.nih.gov/pubmed/20637207.
Reidy, D. L., Tang, L. H., Saltz, L. B. (2009). Treatment of advanced disease in patients with well-differentiated neuroendocrine tumors. Nature Clinical Practice, Oncology, 6(3), 143-152. Retrieved from: http://www.ncbi.nlm.nih.gov/pubmed/19190591.
Vinik, A. I., Silva, M. P., Woltering, E. A., Go, V., Warner, R., Caplin, M. (2009). Biochemical Testing for Neuroendocrine Tumors. Pancreas, 38(8), 876-899. Retrieved from: http://www.ncbi.nlm.nih.gov/pubmed/19855234.
Yao, J. C. (2007). Neuroendocrine tumors. Molecular targeted therapy for carcinoid and islet-cell carcinoma. Best practice and research, clinical endocrinology and metabolism, 21(1), 163-172. Retrieved from: http://www.ncbi.nlm.nih.gov/pubmed/17382271.
Yao J. C., Hassan, M., Phan, A., Dagohoy, C., Leary, C., Mares, J. E., Abdalla, E. K., Fleming, J. B., Vauthey, J. N., Rashid, A., Evans, D. B. (2008). One hundred years after “carcinoid”: epidemiology of and prognostic factors for neuroendocrine tumors in 35,825 cases in the United States. Journal of Clinical Oncology, 20(26), 3063-3072. Retrieved from: http://www.ncbi.nlm.nih.gov/pubmed/18565894.