Also Known As
Colorectal Cancer
This article was last reviewed on
This article waslast modified on
December 12, 2017.
What is colon cancer?

Colon cancer is the uncontrolled growth of abnormal cells within the layers of tissue that line the colon. The colon is part of the digestive tract that processes nutrients, such as vitamins, minerals, carbohydrates and fat from food. It is five feet long and makes up the majority of the large intestine, also called the large bowel. In the path that food takes through the body (alimentary canal), the colon follows the small intestine and comes before the rectum. The colon absorbs water and salts, forms stools, and rids the body of waste.

Colon and rectum cancers are sometimes referred to together as "colorectal cancer." In this article, they will be referred to as "colon cancer." Together, they are the third most common non-skin cancer in adults and the second leading cause of cancer deaths in men and women in the United States after lung cancer.

Glands in the colon produce mucus and lubricate the lining of the colon and rectum. Most colon cancers are adenocarcinomas; they start in the cells that form these glands. Most cases of colon cancer begin with the development of benign polyps, finger-like growths that protrude into the intestinal cavity. These polyps are relatively common in people over age 50 and most remain benign. Some, however, can become cancerous with the ability to invade colon tissues and to spread to other parts of the body (metastasize). The tumors they form can create blockages in the intestine, preventing elimination.

The American Cancer Society (ACS) estimates the number of new colon cancer cases in the U.S. to be nearly 133,000 annually. The lifetime risk of developing colon cancer is about 1 in 20 (or 5%), according to the ACS. Within the last 20 years, however, the rate of colon cancer has dropped significantly. This is likely due to a few different factors. Improved screening has led to removal of more pre-cancerous polyps, preventing the development of cancer. Likewise, better screening has detected more cancers in the earlier stages, when they are most treatable; this has led to a decline in colon cancer deaths as well.

Accordion Title
About Colon Cancer
  • Risk Factors

    The exact causes of colon cancer are not known, but risk increases with age, being overweight or obese, and with the occurrence of cancers in other parts of the body. Risk also appears to be associated with factors, such as:

    • Genetics—having family members with colon cancer or colorectal polyps
    • Diet—high fat and meat diets are a risk factor, especially combined with not eating enough fruits, vegetables, and/or high-fiber foods
    • Lifestyle—these risk factors include cigarette smoking and lack of regular exercise


    In particular, people with a personal or family history of colon cancer or polyps are at a higher risk as are those with ulcerative colitis, a form of inflammatory bowel disease, and immunodeficiency disorders. A rare inherited disease called familial adenomatous polyposis (FAP) causes benign polyps to develop early in life and causes cancer in almost all affected persons unless the colon is removed. People with another genetic syndrome called hereditary non-polyposis colorectal cancer (HNPCC, Lynch syndrome) also have a high risk of developing colon cancer. (For more, see Genetics Home Reference articles on FAP and Lynch syndrome.)

  • Signs and Symptoms

    Colon cancer frequently develops without producing early signs or symptoms. Symptoms that can occur include:

    • Diarrhea, constipation, or other changes in bowel habits lasting 10 days or more
    • Blood in the stool (either bright red or dark in color)
    • Unexplained anemia
    • Abdominal pain and tenderness in the lower abdomen
    • Abdominal discomfort (frequent gas pains, bloating, fullness, and cramps)
    • Intestinal obstruction
    • Weight loss with no known reason
    • Stools narrower than usual
    • Constant tiredness


    These signs and symptoms can be caused by cancer or by a number of other conditions. It is important to talk to a healthcare provider if any of these signs and symptoms are present and to screen for colon cancer even when they are not. If the polyps that lead to cancer are detected and removed, colon cancer can often be prevented. If colon cancer is detected early, it is curable in up to 90% of cases. Currently, the American Cancer Society reports that there are more than one million colorectal cancer survivors in the U.S.

  • Tests

    In March 2008, screening guidelines for the early detection of pre-cancerous polyps and colon cancer were released jointly by the American Cancer Society, the US Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology. The US Preventive Services Task Force released similar recommendations in 2008 and they are currently under review. The guidelines divide screening options into two categories:

    • Full or partial structural exams that inspect the colon itself and can detect both cancer and precancerous polyps
    • Laboratory tests on stool samples that detect blood (which may be caused by existing cancer) or that detect cancerous cells shed in the stool


    Another difference between these screening options is that direct examinations such as sigmoidoscopy and colonoscopy allow for removal of polyps at the time the test is done. All other tests must be followed by another procedure to remove any suspected growths.

    High risk:
    People with a family history of colon cancer or polyps or a personal history of inflammatory bowel disease or certain inherited conditions may be advised to start screening before age 50. A colonoscopy is usually recommended because it is the most accurate and thorough. Also, the recommended screening interval for high-risk individuals is shorter than for people with average risk, such as every 1-2 years compared to every 10 years.

    Average risk:
    It is recommended that people at average risk for colon cancer begin regular screening when they turn 50 years old. The following tables summarize the screening tests that are options for people with average risk.

    Imaging Tests

    These tests visualize the colon and can detect both pre-cancerous polyps and existing cancers.
    Test Description Recommended Screening Interval Starting at age 50 for people at average risk Pros Cons
    Sigmoidoscopy Examination of the rectum and lower colon with a rigid or flexible lighted instrument Every 5 years Minimal preparation ahead of time

    Does not usually need sedation

    Fairly quick and safe
    Only examines about 30% of colon
    Can’t remove all polyps

    Small risk of bleeding, infection or bowel tear

    May need to have colonoscopy if abnormal result found
    Colonoscopy Examination of the rectum and entire colon with a lighted instrument Every 10 years Can examine the entire colon

    Can remove polyps and take biopsies for pathological testing
    Extensive full bowel preparation ahead of time

    Sedation needed to perform

    Takes at least one day for prep and recovery

    Risk of bleeding, infection or bowel tears
    Double contrast barium enema Series of x-rays of the colon and rectum; patient is given an enema with a white, chalky solution that outlines the colon and rectum on the x-rays; tube inserted in rectum, bowel is inflated with air Every 5 years Does not require sedation

    Can view entire colon

    Relatively safe; minimal risk of tear to colon
    Same full bowel preparation needed as for colonoscopy

    Cannot remove polyps

    May need to have colonoscopy if suspicious results found
    Virtual colonoscopy (CTC, or Computed Tomographic Colonography) Examination of the rectum and entire colon to the small intestine using x-rays and computers; tube inserted in rectum and bowel is inflated with air Every 5 years No sedation required

    Can view entire colon

    Relatively safe; minimal risk of tear to colon

    Full bowel preparation required

    Cannot remove polyps

    May need colonoscopy if abnormal results

    Effectiveness as a screening tool is not fully accepted

    Laboratory Tests

    These tests are performed on stool samples and detect mainly existing cancers.
    Stool Tests Description Recommended Screening Interval Starting at Age 50 for People at Average Risk Pros Cons
    Guaiac-based fecal occult blood test (gFOBT) Test to detect hidden blood in stool sample Annually No bowel preparation

    No direct risk to bowel

    Sample can be collected at home
    Dietary restriction before testing

    Cannot detect precancerous changes

    Detects any blood in colon, not just from cancers but from food or dental procedures
    Fecal Immuno-chemical test (FIT or iFOBT) Test to detect hidden blood in stool sample; different collection technique than gFOBT Annually No dietary or drug restrictions

    No bowel preparation

    No direct risk to bowel

    Sample can be collected at home
    Cannot detect precancerous changes

    May miss some cancers; one time testing not effective
    DNA test Detects mutations in specific genes associated with colon cancer in DNA isolated from a stool sample Every three years, according to the American Cancer Society No bowel preparation or dietary restrictions

    Sample can be collected at home

    No risk of bowel tear
    Cannot detect precancerous changes

    Adequate stool sample must be obtained

    Special handling needed

    In addition to these tests, a health practitioner may perform a digital rectal examination (DRE) to feel for a rectal mass with a gloved finger. Most colon cancers, however, are beyond the reach of a finger and have no symptoms.

    If a test other than colonoscopy gives a result suggestive of polyps or cancer, a full colonoscopy is often done to examine the full colon and collect tissue samples (biopsies) of polyps or potentially cancerous areas.

    Tests for Diagnosis, Staging, and Prognosis
    Biopsy:
    When a suspected cancer is found during a colonoscopy, a biopsy is taken, removing some tissue from the suspicious site for examination under a microscope by a pathologist. (For in-depth information, see the article on Anatomic Pathology.)

    If the tissue is cancerous, the next step is to determine the stage (or extent) of disease. Treatment will depend in part on the "stage" of the colon or rectal cancer; it is categorized by how far it has spread from its original site. Staging systems for colon cancer vary in different parts of the world, and some use letters instead of numbers. One common system used to describe colon cancer stages is:

    • Stage 0: Very early cancer on the innermost layer of the colon or rectum (carcinoma in situ)
    • Stage I: Tumor in the inner layers of the colon but has not grown through the wall of the colon
    • Stage II: Tumor in the outer layers of the colon and/or nearby tissue but has not spread to lymph nodes
    • Stage III: Tumor that has spread to the lymph nodes but not to distant organs of the body
    • Stage IV: Tumor that has spread to distant organs, such as the lungs, bone, or liver (metastatic)


    Other laboratory tests:

    • A laboratory test for carcinoembryonic antigen (CEA) may be ordered to help in staging. This protein is increased in many people with colon cancer, and blood levels generally correlate with the stage of the disease. CEA testing also may be used to evaluate the success of surgery or other treatments.
    • Genetic tests to detect KRAS, BRAF and/or NRAS gene mutations in tumor tissue may be used to guide cancer treatment and to evaluate prognosis in people with metastatic colon cancer. The presence of certain mutations indicates that anti-EGFR drug therapy, such as cetuximab and panitumumab, will not be effective in treating the cancer and a likely poorer prognosis.
    • Microsatellite instability (MSI) testing—this is another type of genetic test used to help guide treatment. About 15% of colon cancers have microsatellite instability. Studies have shown that tumors with MSI have better prognosis than tumors with no MSI, but MSI tumors may not respond to certain chemotherapies.
  • Treatment

    All stages of colon cancer are usually treated by surgically removing the cancer and possibly some of the surrounding tissue. Other treatment may include chemotherapy and/or radiation therapy, which may be added to help kill any remaining cancer cells.

    Targeted therapy is a relatively new approach to treatment that uses drugs to target specific proteins that control cell growth and maturation. Malfunctioning proteins may contribute to a cancer's uncontrolled growth. For example, drugs such as cetuximab and panitumumab attack the epidermal growth factor receptor (EGFR). Targeted therapies such as these often have less severe side effects than standard chemotherapy.

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