Colorectal cancer is cancer that occurs in the colon (a muscular tube about five to six feet long that is part of the large intestine) and the rectum (the lower six inches of the large intestine). The colon absorbs water and nutrients from food, and the rectum serves as a holding place for stool, which then passes out of the body through the anus.
The colon is divided into four sections: the ascending colon, transverse colon, descending colon, and sigmoid colon. Most colorectal cancers arise in the sigmoid colon — the portion just above the rectum. They usually start in the innermost layer and can grow through some or all of the several layers of tissue that make up the colon and rectum. The extent to which a cancer penetrates the various tissue layers determines the stage of the disease.
Most colorectal cancers grow slowly over a period of several years, often beginning as small benign growths called polyps. A small percentage of colorectal cancers — usually hereditary forms of the disease — can cause large numbers of polyps to appear, but these types of colorectal cancer are rare.
Colorectal cancers are usually contained within the colon, but when they become advanced the cancer can metastasize, or spread, to other organs. When colorectal cancer spreads, it tends to move to the liver and lungs.
Types of Colorectal Cancer
More than 95 percent of colorectal cancers are adenocarcinomas — cancers of the cells that line the interior of the colon and rectum. Rarer types of tumors include carcinoid tumors, gastrointestinal stromal tumors, and lymphomas. These rare types are reviewed elsewhere.
Because colon cancer and rectal cancer are similar diseases, they are usually grouped together under the label “colorectal cancer.” For the purpose of diagnosis, the main attribute that distinguishes one type from the other is simply the location where the tumor develops.
However, there are important differences in how colon cancer and rectal cancer are treated. Surgery for rectal cancer is often more complex than surgery for colon cancer, due to the narrow confines of the bony pelvis. This area contains the nerves that control sexual and bladder function, so care must be taken to avoid damaging them as much as possible. Rectal cancer is also much more likely to recur after surgery.
Genetics and Colorectal Cancers
About 5 to 10 percent of colorectal cancer cases arise because of specific genetic mutations passed from parents to children. Physicians today recognize two well-established forms of inherited colon cancer that account for some, but not all, of those cases: familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC), also known as Lynch syndrome. Researchers are also learning about a number of other types of hereditary colorectal cancer.
Learn more about hereditary forms of colon cancer.
Colorectal cancers often arise without any symptoms. By the time symptoms are present, a colorectal cancer is likely to be more advanced. For this reason, screening tests such as colonoscopy and tests for blood in the stool are recommended to detect the cancer early, when it is more curable than advanced disease. More important, as discussed in more detail below, colorectal cancer is largely preventable with routine screening, because colorectal cancers arise in benign growths called polyps, and a screening colonoscopy can identify and remove most polyps, and thereby greatly reduce the risk of ever getting colorectal cancer.
When symptoms do occur, however, they may include the following:
- rectal bleeding or blood in the stool
- a change in bowel habits (such as diarrhea, constipation, or narrowing of the stool) that lasts for more than a few days
- abdominal pain
- a continuous feeling that you need to have a bowel movement, which does not resolve after passing stool
Some of these symptoms may be caused by other conditions, but you should see your doctor if they persist. Any incidence of rectal bleeding or blood in the stool should be brought to your doctor's attention as soon as possible.