Carcinoid (also carcinoid tumor) is a slow-growing type of neuroendocrine tumor, originating in the cells of the neuroendocrine system.
In 2000, the World Health Organization redefined "carcinoid", but this new definition has not been accepted by all practitioners. This has led to some complexity in distinguishing between carcinoid and other neuroendocrine tumors in the literature. According to the American Cancer Society, the 2000 WHO definition states:
The WHO now divides these growths into neuroendocrine tumors and neuroendocrine cancers. Neuroendocrine tumors are growths that look benign but that might possibly be able to spread to other parts of the body. Neuroendocrine cancers are abnormal growths of neuroendocrine cells which can spread to other parts of the body.
Carcinoid tumors are those neuroendocrine tumors that look more benign, in distinction to the neuroendocrine cancers.
They are most commonly found in the midgut at the level of the ileum (75–87% cases). The next most common affected area is the respiratory tract with 28% cases of all cases — per PAN-SEER data (1973–1999).
Carcinoid tumors are apudomas that arise from the enterochromaffin cells throughout the gut. Over two-thirds of carcinoid tumors are found in the gastrointestinal tract. Some sources list the appendix as the most common site.
Carcinoid metastasis can lead to carcinoid syndrome. This is due to the over-production of many substances, including serotonin, which is released into the systemic circulation, and which can lead to symptoms of cutaneous flushing, diarrhea, bronchoconstriction and right-sided cardiac valve disease. It is estimated that less than 10% of carcinoid patients will develop carcinoid syndrome.
Carcinoid tumors are also found in the lungs.
They were first characterized in 1907 by Siegfried Oberndorfer, a German pathologist at the University of Munich, who coined the term karzinoide, or "carcinoma-like", to describe the unique feature of behaving like a benign tumor despite having a malignant appearance microscopically. The recognition of their endocrine-related properties were later described by Gosset and Masson in 1914, and these tumors are now known to arise from the enterochromaffin (EC) and enterochromaffin-like (ECL) cells of the gut.
Some sources credit Otto Lubarsch with the discovery.
Most carcinoids are asymptomatic through the natural lifetime and are discovered only upon surgery for unrelated reasons; these are called coincidental carcinoids. But all carcinoids are considered to have malignant potential.
About 10% of carcinoids secrete excessive levels of a range of hormones, most notably serotonin (5-HT), causing:
The outflow of serotonin can cause a depletion of tryptophan leading to niacin deficiency. Niacin deficiency, aka pellagra, is associated with dermatitis, dementia, and diarrhea.
This constellation of symptoms is called carcinoid syndrome or (if acute) carcinoid crisis. Occasionally, haemorrhage or the effects of tumor bulk are the presenting symptoms. The most common originating sites of carcinoid is the small bowel, particularly the ileum; carcinoid tumors are the most common malignancy of the appendix. Carcinoid tumors may rarely arise from the ovary or thymus.The Carcinoid Cancer Foundation
Encouraging, conducting, and supporting carcinoid and related neuroendocrine tumor (NETs) research, awareness, and education, with the ultimate goal of finding a cure.Caring for Carcinoid Foundation
The Caring for Carcinoid Foundation (CFCF) is the leading 501(c)(3) nonprofit dedicated to discovering a cure for carcinoid cancer.General Practice Notebook
A concise synopsis of the entire field of clinical medicine focussed on the needs of the General Practitioner.MedLine Plus
Carcinoid tumors are rare, slow-growing cancers that usually start in the lining of the digestive tract or in the lungs. Because they grow slowly and don't produce symptoms in the early stages, the average age of people diagnosed with digestive or lung carcinoids is about 60.Wikipedia
Wikipedia's article on carcinoid tumors.