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What is Colon Cancer?

Colon cancer, or colorectal cancer, is cancer of the colon or rectum. These structures are located at the end of the digestive system. The colon is where stool is formed before it goes through the rectum and out of the body.

Colon cancer often starts as a growth called a polyp. Polyps grow on the wall of the colon or rectum. Some polyps eventually become cancer. The best way to prevent colon cancer is find and remove polyps early.

Colon cancer is a common cancer, but the number of colon cancers diagnosed each year is going down. This is likely because more people are getting a colonoscopy and other tests for colon cancer. With a colonoscopy, the doctor can see inside the colon and find and remove any polyps.


Colon cancer may not have symptoms at first, so it is important to follow the recommendations for getting tested. Symptoms of colon cancer include:

  • A change in your bowel movements, like constipation or diarrhea, that lasts more than a few days
  • Bright red blood from the rectum
  • Blood in the stool (can make the stool look dark or black)
  • Pressure that feels like you need to have a bowel movement, even if you just had one
  • Weight loss that is not planned

When to See a Doctor

See a doctor if you notice any bleeding from the rectum or blood in your stool. You should also see a doctor if you have any new bowel problems or pain that does not go away within a few days.


Like other cancers, colon cancer is caused by changes, or mutations, to the DNA that makes up your genes. In most cases of colon cancer, the mutation is not inherited. It develops during a person’s life.

It’s not always clear what causes these mutations. But there are some factors that increase a person’s risk of colon cancer:

  • Older age
  • Drinking 3 or more alcoholic beverages a day
  • Family history of colon cancer
  • Smoking cigarettes
  • Polyps in the colon, which can take 10 to 15 years to develop into colon cancer
  • Previous history of colon cancer, ovarian cancer, or inflammatory bowl diseases like Crohn’s disease or ulcerative colitis
  • Previous history of polyps that looked like they could become cancer
  • Obesity, which increases the both the risk of getting the disease and the risk of dying from it

Some mutations that cause colon cancer can be inherited. Colon cancers caused by inherited gene mutations are a very small percentage of all colon cancer cases. These mutations tend to cause colon cancer in younger people.

  • Familial adenomatous polyposis (FAP). There are several types of FAP. In the most common type, a person develops hundreds of polyps in their colon and rectum. One or more of these eventually turn into cancer. FAP makes up about 1% of all colon cancers.
  • Lynch syndrome (hereditary non-polyposis colon cancer, or HNPCC). In this colon cancer, the genes that usually repair damaged DNA don’t work properly. Normally these genes will stop a cancerous tumor from growing, but not in people with Lynch syndrome. Lynch syndrome makes up about 2% to 4% of all colon cancers.

Diagnosis and Tests

Many colon cancers are preventable if pre-cancerous polyps are removed. Polyps often take from 10 to 15 years to become cancerous. Usually these growths start before you have any symptoms. It is important to get screened for colon cancer to find and remove them.

People who have an average risk of colon cancer should start getting screened at age 50. If you have any of the risk factors for colon cancer, talk to your doctor about when to start getting screened.

These tests are used to screen for colon cancer:

  • Colonoscopy. A colonoscopy allows the doctor to see inside the rectum and colon. A very thin tube (colonoscope) with a light and lens is inserted into the rectum and colon. The doctor may remove a polyp during the colonoscopy or take a tissue sample to look at in a lab.
  • Virtual colonoscopy. This test uses CT scans (X-rays) to get images of the colon. Researchers are still testing to see if this works as well as a regular colonoscopy.
  • Sigmoidoscopy. This test is like a colonoscopy except the instrument (sigmoidoscope) only goes to the lower part of the colon (the sigmoid colon).
  • Fecal occult blood test. This test checks the stool for blood that can’t be seen by the naked eye. The lab tests a small sample of stool.
  • DNA stool test. This test checks the stool for DNA mutations that could lead to colon cancer.


Treatment for colon cancer depends on what kind of colon cancer it is and its stage, or how far along the cancer is. Treatment may include:

  • Surgery to remove a tumor or part of the colon, or both
  • Ablation, which destroys the tumor without removing it
  • Embolization, which cuts off blood supply to the tumor so it can’t grow
  • Chemotherapy, either locally (near the tumor) or systemically (throughout the body)
  • Immunotherapy, which helps the immune system get rid of cancer cells, may be used for advanced colon cancer


You can lower your risk of colon cancer by doing the following:

  • Limit alcohol to no more than 2 alcoholic beverages a day.
  • Maintain a healthy weight.
  • Don’t smoke.
  • Stay physically active.
  • Get screened for colon cancer. Screening tests can find a polyp before it becomes cancer so it can be removed. You should start getting screened for colon cancer at age 50 if you are at average risk. Ask your doctor about a screening schedule if you have any risk factors for colon cancer.