Colon cancer starts with a growth in the inner lining of the colon. These growths are called polyps. Some types of polyps can change into cancer over time but it depends on the type of polyp it is. Polyps are of two types:

Adenomatous polyps:

These polyps are called adenomas and can sometimes turn to be cancerous.

Hyperplastic polyps and inflammatory polyps:

These polyps are not precancerous.

The factors that can make a polyp cancerous include if the polyp is larger than 1 cm, if two or more polyps are found and if dysplasia is seen after the removal of a polyp. Dysplasia is a precancerous condition which means that there is an area in the polyp that contains cells that look abnormal.

What are the types of colon cancer?

Adenocarcinomas make up about 96% of the colon cancer cases. These cancers mainly start in the cells that line the inner surface of the colon and these cells are also responsible for lubricating the colon. When doctors speak of colon cancer they are often referring to adenocarcinomas. There are also rarer tumor types which include:

Carcinoid tumors

These tumors start in the hormone-producing cells of the intestine and are most common type neuroendocrine tumors. Most of these kinds of tumors are generally present in the stomach, intestines, colon, liver, and rectum. Carcinoid tumors can take several years to grow and for the same reason, they are classified as low-grade tumors. The most common methods of treatment used for these tumors are surgery, chemotherapy and, hormone therapy.

Gastrointestinal stromal tumors

Gastrointestinal stromal tumors start in cells that are in the wall of the digestive tract. These tumors start in special cells known as interstitial cells of the Cajal (ICCs). These cells are a part of the autonomic nervous system and send messages to muscles in the gastrointestinal tract to contract to move food and liquid along.
Gastrointestinal stromal tumors can start anywhere along the gastrointestinal tract but a very small number grow outside of the gastrointestinal tract. Gastrointestinal stromal tumors can grow and spread to other parts of the body too and doctors look into a number of factors to tell if the tumor is likely to grow and spread.

Lymphomas

Lymphomas are cancer cells that are found in the lymph nodes and can grow and start anywhere including areas like the colon or other organs.

Sarcomas

These cancers can start in the blood vessels, connective tissues and muscle layers in the walls of the colon. However, sarcomas of the colon are rare.


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