Colon and rectal cancers
Description
An in-depth report on the causes, diagnosis, treatment, and prevention of colorectal cancer.
Alternative Names
Colorectal cancer
Introduction
Cancers of the colon and rectum, often referred to collectively as
colorectal cancer
, are life-threatening tumors that develop in the large intestine.
More than 80% of colorectal tumors evolve from
adenomatous polyp
s. These gland-like growths develop on the mucous membrane that lines the large intestine. They are usually one of the following types:
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Tubular polyps, which protrude mushroom-like
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Villous adenomas, which are flat and spreading and are more apt to become malignant (cancerous)
It should be noted that these polyps are very common and almost always benign. Their numbers increase with age. Polyps are found in about 25% of people by age 50 and 50% of people by age 75. Fewer than 1% of polyps under 1 centimeter (slightly less than half an inch) become cancerous. About 10% of larger polyps become cancerous within 10 years, and about 25% of these larger polyps become cancerous after 20 years. Certain inherited polyps can become cancerous more rapidly.
The Gastrointestinal Tract
Digestion takes place in the gastrointestinal (GI) tract, essentially a long tube that extends from the mouth to the anus. It is a complex organ system that first carries food from the mouth down the esophagus to the stomach. Food then travels through the small and large intestines before being excreted through the rectum and out the anus.
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The esophagus, stomach, large and small intestine -- aided by the liver, gallbladder, and pancreas -- convert the nutritive components of food into energy and break down the non-nutritive components into waste to be excreted.
Esophagus
The
esophagus
is a narrow muscular tube, about nine and a half inches long that begins below the tongue and ends at the stomach.
Stomach
In the
stomach
, acids and stomach motion break food down into particles small enough so that the small intestine can absorb nutrients.
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Small Intestine
The small intestine, despite its name, is the longest part of the gastrointestinal tract, extending for about 20 feet. Food passes from the stomach through its three parts: first the
duodenum
, then the
jejunum
, and finally the
ileum
. Most of the digestive process occurs in the small intestine.
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Large Intestine
Undigested material, such as plant fiber, is passed next to the
large intestine
, mostly in liquid form. The large intestine is wider than the small intestine but only about 6 feet long. It is the final portion of the digestive tract and includes the
cecum
, the
appendix
, the
colon
, and the
rectum
, which extends to the
anus
.
Cecum and Appendix.
The
cecum
and the
appendix
are located in the lower-right quadrant of the abdomen.
Colon.
The colon absorbs excess water and salts into the blood. The remaining waste matter is converted to feces through bacterial action. The colon is divided into four major sections.
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The first section, the
ascending colon
, extends upward from the cecum on the right side of the abdomen.
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The second section, the
transverse colon
, crosses the upper abdomen to the left side.
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The third section extends downward on the left side of the abdomen toward the pelvis and is called the
descending colon
.
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The final section is the
sigmoid colon
.
Rectum and Anus.
Feces are stored in the descending and sigmoid colon until they are passed through the
rectum
and
anus
. The rectum extends through the pelvis from the end of the sigmoid colon to the anus.
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Review Date: 3/6/2007
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Reviewed By: A.D.A.M. Editorial Team: Greg Juhn, M.T.P.W., David R. Eltz, Kelli A. Stacy. Previously reviewed by Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital (9/1/2006).
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