Colon cancer is a disease in which malignant (cancerous) cells appear in the tissues of the colon.
The colon is part of the body’s digestive system. The digestive system absorbs and processes nutrients (vitamins and minerals, carbohydrates, fats, proteins, and water) from food and facilitates the elimination of waste products from the body. The digestive system consists of the esophagus, stomach, large intestine and small intestine. The colon (large intestine) is the first part of the large intestine and is about 1.5 meters long. The last part of the large intestine consists of the rectum and the anal canal, which is about 15-20 cm long. (The anal canal ends at the anus, which is the opening of the large intestine to the outside of the body.)
Anatomy of the lower digestive system; shows the colon and other organs.
Interstitial tumors of the gastrointestinal tract can occur in the colon. For more information, see the PDQ summary for more information on the treatment of gastrointestinal mesenchymal tumors (adults).
To learn more about colorectal cancer in children, see the PDQ summary on treatment options for rare cancers in children.
Health history can affect the risk of developing colon cancer.
Any factor that increases the chance of getting the disease is called a risk factor. Having a risk factor does not mean you will get cancer; not having a risk factor does not mean you will not get cancer. If you think you are at risk for colorectal cancer, talk to your doctor.
Risk factors for colorectal cancer include the following:
A family history of colon or rectal cancer in a first-degree relative (parent, sibling, or child).
Have a personal history of colon, rectal, or ovarian cancer.
Personal history of high-risk adenomas (colorectal polyps 1 cm or larger or with cells that look abnormal under the microscope).
Genetic changes in certain genes that increase the risk of familial adenomatous polyposis (FAP) or Lynch syndrome (hereditary nonpolyp colorectal cancer).
History of chronic ulcerative colitis or Crohn’s disease for more than 8 years.
Drinking three or more glasses of alcohol per day.
Smoking.
Black.
Obese.
Older age is a major risk factor for most cancers. As you get older, the chance of developing cancer increases.
Colonic polyps. Some polyps have a tip and some do not. The inset shows a picture of a polyp with a tip.
Signs of colon cancer include blood in the stool or a change in bowel habits.
These signs and other symptoms may be caused by colon cancer or other diseases. If you have any of the following symptoms, consult your doctor.
Change in bowel habits.
Blood in the stool (bright red or dark red).
Diarrhea, constipation, or a feeling that the bowel is not emptying.
Failure is finer than usual.
Frequent painful flatulence, bloating, feeling of fullness, stomach cramps.
Uncaused weight loss.
Feeling very tired.
Vomiting.
Colon and rectal exams are used to detect (find) and diagnose colon cancer.
The following tests and procedures may be used:
Physical exam and medical history: The physical exam is used to check for general health, including checking for signs of disease, such as a lump or any other abnormal signs. A history of the patient’s health habits and history of previous illnesses and treatments will also be taken.
rectal examination: A rectal exam. The doctor or nurse inserts a lubricated, gloved finger into the rectum to check for a mass or any other abnormality.
Fecal occult blood test (FOBT): A test that examines stool (solid waste) through a microscope for the presence of blood. A small stool sample is placed on a special card or in a special container and given to a doctor or laboratory for testing. Blood in the stool may be a sign of polyps, cancer, or other diseases.
There are two types of fecal occult blood tests (FOBTs):
Guaiac method fecal occult blood test: A stool sample is placed on a special card and tested using a chemical. If there is blood in the stool, this special card will change color. 
guaiacin method fecal occult blood test (FOBT) checks for potential (hidden) blood in the stool. A small stool sample is placed on a special card and given to a doctor or laboratory for testing.
Immunochemical fecal occult blood test (FOBT):A liquid is added to the stool sample. The mixture is injected into a machine that contains antibodies that detect blood in the stool. If there is blood in the stool, a line will appear in the window of the machine. The test is also known as the fecal immunochemical test or FIT.
The fecal immunochemical test (FIT) checks for potential (hidden) blood stains in stool. A small stool sample is placed in a special collection tube or on a special card and given to a doctor or laboratory for testing.
Barium enema: A series of x-rays of the lower gastrointestinal tract. A liquid containing barium (a silvery-white metallic compound) is instilled into the rectum. The barium covers the lower GI tract, and then radiographs are taken. This procedure is also known as a series of lower GI exams. 
Barium enema procedure. The patient is lying on the x-ray table. The barium enters the rectum and flows through the colon. The abnormal area is viewed with x-ray.
Sigmoidoscopy: This procedure is used to look at polyps (small pieces of raised tissue), other abnormal sections, or cancer in the rectum and sigmoid (lower) colon. The sigmoidoscope is inserted through the rectum into the sigmoid colon. A sigmoidoscope is a thin, tube-like instrument with a light and lens for observation. It may also be equipped with tools for removing polyps or tissue samples that are used to find signs of cancer under the microscope. 
Sigmoidoscopy. A bright, thin tube is inserted through the anus and rectum into the lower part of the colon to view abnormal areas.
Colonoscopy: This procedure is used to look at polyps, abnormal areas, or cancer in the rectum and colon. A colonoscope is a thin, tube-like instrument with a light and lens for viewing. It may also come with tools for removing polyps or tissue samples that are used to spot signs of cancer under the microscope. 
Colonoscopy. A bright, thin tube is inserted through the anus and rectum into the colon to look at abnormal areas.
Virtual colonoscopy: This procedure uses a series of x-rays called computed tomography to take a series of images of the colon. The computer combines these images into detailed pictures that may show polyps on the inner surface of the colon and any other abnormalities. This test is also known as colon imaging or CT colon imaging.
Biopsy: Removal of cells or tissue so that the pathologist can examine the signs of cancer under a microscope.
Some factors that affect the prognosis (chance of recovery) and treatment options.
Prognosis (chance of recovery) and treatment depends on:
The stage of the cancer (whether the cancer is confined to or has spread through the colon wall, or has spread to the lymph nodes or other parts of the body).
Whether the intestinal tumor has blocked the bowel lumen or perforated.
Whether there are cancer cells remaining after surgery.
Whether the cancer has recurred.
The patient’s general health.
The prognosis also depends on the concentration of carcinoembryonic antigen (CEA) in the blood before treatment begins. cEA is a substance in the blood that may increase when there is a tumor.