The tumors that originate from the neuroendocrine cells and are derived from the primitive stem cells present in the gut walls are known as carcinoid tumors. Carcinoid tumors have a high potential for metastatic growth in organs such as kidneys, thymus, pancreas, liver, lungs, mediastinum, bronchus, prostate, and ovaries.
Understanding the Neuroendocrine System
The neuroendocrine system consists of gland cells and nerve cells, and produces and discharges the hormones into the bloodstream. The neuroendocrine cells are present in the gut, pancreas, food pipe (esophagus), and lungs.
The function of neuroendocrine cells depends on where the cells are actually present in the body. For instance, the blood and airflow in the lungs are controlled by the hormones released by the neuroendocrine cells. The discharge of digestive juices into the gut and the movement of food through the bowel by muscles are controlled by the neuroendocrine cells present in the gut. Alterations in the neuroendocrine cells influence the development and uncontrollable growth of carcinoid tumors.
Neuroendocrine cancers are also termed as neuroendocrine carcinomas. They are classified based on their form or shape, which is identified using a microscope. The features of moderately differentiated cancer lie between the features of poorly differentiated and well-differentiated cancer.
Carcinoid Tumors and Carcinoid Syndrome
Carcinoid tumors grow from enterochromaffin cells that generally possess the capability to generate similar hormones in higher volume. Circulation of these hormones in the blood causes symptoms of carcinoid syndrome.
Carcinoid syndrome is a key feature of this tumor, which differentiates it from other types of tumors in the gastrointestinal (GI) tract. Internal problems in the intestines, such as intestinal obstruction and intestinal bleeding, and abdominal pain are the primary causes of symptoms in many other GI tract tumors such as small bowel lymphomas and colon cancers. These internal symptoms can also result from carcinoid tumors. In addition, carcinoid syndrome can be a result of the growth and discharge of certain components of these tumors. Generally, the symptoms of the carcinoid syndrome are more significant than the local symptoms.
The cells of the dispersed neuroendocrine system are present in places where the carcinoid tumor originates. Cells present in this system are similar to hormone-making endocrine cells and nerve cells in certain aspects. These cells do not form an organ like the thyroid or the adrenal gland, but are dispersed throughout organs such as lungs, esophagus, pancreas, stomach, and intestines. A higher number of neuroendocrine cells are present in the broad digestive system than in other body parts, which explain why carcinoid tumors originate in the digestive system.
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Types of Carcinoid Tumors
Carcinoid tumors can be mainly classified into two categories: lung carcinoid tumors and GI carcinoid tumors.
1. Lung Carcinoid Tumors: Cancer that originates in the lungs is a type of lung cancer known as lung carcinoids or lung carcinoid tumors. Lung carcinoid tumors are rare and tend to develop gradually. They are made of significant cells called neuroendocrine cells. Neuroendocrine lung tumors can be classified into four types. They are:
- Small cell lung cancer (SCLC): one of the fast developing and spreading cancers.
- Large cell neuroendocrine carcinoma (LCNEC): a rare cancer that is a sub-category of non-small cell lung cancer or NSCLC, although it has some characteristics of SCLC, such as the ability to grow quickly.
- Typical lung carcinoid tumor: High chromogranin-A and serotonin levels are produced, which can be measured in blood. High levels of 5-HIAA, which is a product output of serotonin disintegration, are also produced, which can be measured in urine.
- Atypical lung carcinoid tumor: Normal chromogranin-A and serotonin levels in blood and also normal 5-HIAA levels in urine are found in people affected by this tumor and urine can contain high serotonin and amino acid 5-HTP levels as a result of 5-HTP production in atypical lung carcinoid tumors.
2. GI Carcinoid Tumors
Cancer that originates in the lining of the GI tract is due to the presence of GI carcinoid tumors.
They are derived from a certain type of neuroendocrine cell that is similar to hormone-making cells and nerve cells. Most of these cells are found in the GI tract and are dispersed all over the abdomen and the chest. The discharge of digestive juices and the movement of food through the stomach and intestines are controlled by hormones produced by the neuroendocrine cells. GI carcinoid tumors may also produce and discharge hormones into the body.
GI carcinoid tumors are rare and tend to develop gradually in the rectum, appendix, or intestine. It is also possible for more than one tumor to develop. Carcinoid tumors are capable of causing carcinoid syndrome, which is a key feature of this tumor that differentiates it from other types of tumors in the GI tract.