Carcinoid

Carcinoid (also carcinoid tumour or carcinoid tumor) is slow-growing but often malignant type of neuroendocrine tumour, originating in the cells of the neuroendocrine system. Carcinoid tumours are apudomas that arise from the enterochromaffin cells throughout the gut. They are most commonly found in the appendix, where they rarely metastasise. The next most common affected area is the small intestine especially the ileum. In cases of metastases it can lead to carcinoid syndrome. This is due to the production of 5-hydroxytryptamine, which is released into the systemic circulation, which leads to symptoms of cutaneous flushing, diarrhea, bronchoconstriction and right-sided cardiac valve disease.

 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 tumour despite having a malignant appearance microscopically. The recognition of their endocrine-related properties were later described by Gosset and Masson in 1914, and these tumours are now known to arise from the enterochromaffin (EC) and enterochromaffin-like (ECL) cells of the gut.

 Symptoms 

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 percent of carcinoids secrete excessive levels of a range of hormones, most notably serotonin (5-HT), causing:

    * Flushing

    * Diarrhea

    * Wheezing

    * Abdominal cramping

    * Peripheral edema

 This constellation of symptoms is called carcinoid syndrome or (if acute) carcinoid crisis. Occasionally, haemorrhage or the effects of tumour 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, and ovarian origin, though reported, is rare.

Treatment

 Surgery, if feasible, is the only curative therapy. If the tumour has metastasized (most commonly, to the liver), the tumour may be ineligible for surgery (unresectable), though there are many promising treatment modalities, such as radiolabeled octreotide, that are very effective in arresting the growth of the tumours and prolonging survival in patients with liver metastases. 

Chemotherapy is of little benefit and is generally not indicated. Octreotide (a somatostatin analogue) may decrease the secretory activity of the carcinoid.

 Carcinoid tumours are the most common malignant tumour of the appendix, but they are most commonly associated with the small intestine, but they are also found in the rectum and stomach. They are known to grow in the liver, but this finding is usually a manifestation of metastatic disease from a primary carcinoid occurring elsewhere in the body. They have a very slow growth rate compared to most malignant tumours.

 Goblet Cell Carcinoid 

This is considered to be a hybrid between an exocrine and endocrine tumour derived from crypt cells of the appendix. Histologically, it forms clusters of goblet cells containing mucin with a minor admixture of Paneth cells and endocrine cells. The growth pattern is distinctive: typically producing a concentric band of tumour nests interspersed among the muscle and stroma of the appendiceal wall extending up the shaft of the appendix. This makes the lesion difficult to suspect grossly and difficult to measure. Small tumour nests may be camouflaged amongst the muscle or in periappendiceal fat; cytokeratin preparations best demonstrate the tumour cells; mucin stains are also helpful in identifying them. They behave in a more aggressive manner than do classical appendiceal carcinoids. Spread is usually to regional lymph nodes, peritoneum, and particularly the ovary. They do not produce sufficient hormonal substances to cause the carcinoid or other endocrine syndromes. In fact, they more closely resemble exocrine than endocrine tumors. The term 'crypt cell carcinoma' has been used for them, and though perhaps more accurate than considering them carcinoids, has not been a successful competitor.