• Also Known As:
  • Colorectal Cancer
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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. 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, 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.

Cancers in the colon and rectum 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 third leading cause of cancer deaths in men and women in the United States.

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 certain types of benign polyp called adenomas, finger-like growths that protrude into the intestinal cavity. These polyps are relatively common in people over age 50 and most remain benign.

Adenomas of the colon have varying degrees of risk of becoming cancerous. The type, size, and number of adenomas a person has determines the type and extent of follow-up testing and/or treatment required. Once a cancer develops in a polyp, it can invade colon tissues and spread to other parts of the body (metastasize). The tumors they form can sometimes create blockages in the intestine, preventing elimination of stool.

The American Cancer Society (ACS) estimates the number of new colon cancer cases in the U.S. to be nearly 140,000 annually. The lifetime risk of developing colon cancer is about 1 in 21 (or 4.7%) for men and 1 in 23 (4.4%) for women, according to the ACS. Over the last several years, however, the number of deaths from colon cancer has dropped significantly. Improved screening techniques and protocols have 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.

While the incidence of colon cancer has decreased over the last twenty years in people age 55 and older because of improved screening, there has been a 51 percent increase in colon cancer and deaths from the cancer among people younger than age 50 since 1994, according to the ACS. In 2018, the ACS lowered its recommended starting age for colon cancer screening from age 50 to age 45. Experts on the ACS Guideline Development Committee determined that a beginning screening age of 45 for adults of average risk will result in more lives saved from colon cancer.


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.

Examples of other risk factors include:

  • Genetics—having family members with colon cancer or multiple 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, drinking excessive amounts of alcohol, and lack of regular exercise
  • Having ulcerative colitis, a form of inflammatory bowel disease
  • Having type 2 diabetes
  • Racial or ethnic background—African Americans and Ashkenazi Jews have higher risk and rates of colon cancer compared to others.
  • Having a personal history of colon cancer and/or high risk precancerous polyps (see Increased and High Risk under Tests).
  • Having a rare inherited disease called familial adenomatous polyposis (FAP)—this causes benign polyps to develop early in life and causes cancer in almost all affected persons unless the colon is removed.
  • Having a genetic syndrome called Lynch syndrome (hereditary non-polyposis colon cancer or HNPCC) (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. As the disease progresses, signs and symptoms can occur, such as:

  • Diarrhea, constipation, stools narrower than usual, or other changes in bowel habits lasting 10 days or more
  • Rectal bleeding, blood in the stool (either bright red or dark in color)
  • Sensing the need to have a bowel movement that does not go away after having one
  • Unexplained iron-deficiency anemia
  • Abdominal pain and tenderness in the lower abdomen
  • Abdominal discomfort (frequent gas pains, bloating, fullness, and cramps)
  • Weight loss with no known reason
  • 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 in the absence of any signs or symptoms. 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.

Screening Tests

In 2017, screening guidelines for the early detection of pre-cancerous polyps and colon cancer were released by the U.S. Multi-Society Task Force (MSTF) on Colorectal Cancer. The American Cancer Society (ACS) updated their guidelines in 2018 and the U.S. Preventive Services Task Force (USPSTF) released updated recommendations in 2021. While the MSTF recommends that people with average risk for colon cancer begin screening at age 50, the ACS and USPSTF recommend that colon cancer screening begin at age 45. Various options should be discussed with your healthcare practitioner.

The screening options fall 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.

The MSTF groups screening tests into three tiers based upon their effectiveness:

  • Tier 1 tests—these are the tests of choice.
    • Colonoscopy every 10 years should be the primary test offered to patients.
    • A yearly fecal immunochemical test (FIT) is offered if colonoscopy is declined.
  • Tier 2 tests—these have some disadvantages compared to tier 1 tests.
    • Computed tomographic colonography (CTC) every 5 years
    • Combination FIT-DNA stool test every 3 years
    • Flexible sigmoidoscopy every 5-10 years
  • Tier 3 test—capsule colonoscopy every 5 years; a vitamin-sized capsule that is swallowed and contains a wireless camera that transmits pictures as it travels through the digestive tract; not enough evidence is available on this test yet and it is not widely available, so MSTF placed it in a lower category.

The American Cancer Society does not prioritize a particular test, but recommends the same timing of screening listed above, depending on which test a patient chooses.

In 2016, the Canadian Task Force on Preventive Health Care (CTFPHC) issued colon cancer screening recommendations that differ in part from U.S. groups.

  • CTFPHC recommends that adults aged 50 to 74 years be screened with guaiac-based fecal occult blood test (gFOBT) or FIT every 2 years or flexible sigmoidoscopy every 10 years.
  • CTFPHC recommends against using colonoscopy for primary screening. However, like the U.S. Preventive Services Task Force, CTFPHC recommends against screening adults 75 years and older.

Increased and High Risk:

People with increased or high risk of colon cancer may be advised to start screening well 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).

Additionally, people who have been screened and found to have colon cancer or high risk pre-cancerous polyps also need more frequent re-testing, usually at least every 3 years. (This is called surveillance.) For example, the MSTF guidelines advise enhanced surveillance for people with 3-10 small tubular adenomas as well as those with 1 or more high-risk polyps (i.e., villous features, larger than 10 mm diameter tubular adenoma or serrated sessile polyp, or any polyp that has very atypical features, called high grade dysplasia).

On the other hand, those with 1-2 small (less than 10 mm) tubular adenomas in the colon can be re-screened at normal intervals (i.e., every 10 years). Another common polyp, termed a hyperplastic polyp, is not felt to increase risk of colon cancer.

For more information on these types of polyps, read the American Cancer Society’s webpage Understanding Your Pathology Report: Colon Polyps

Average Risk:
The MSTF recommends that people at average risk for colon cancer begin regular screening when they turn 50 years old, and that African-Americans begin at age 45. ACS and USPSTF recommend that all average-risk people begin screening at age 45.

The following tables summarize the screening tests that are options for people with average risk.

Screening Tests

Test Description Screening Interval for people at average risk Pros Cons
Tier 1 tests
Colonoscopy Examination of the rectum and entire colon with a lighted instrument Every 10 years Can examine the entire colon

Detects pre-cancerous polyps and cancer

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

Fecal Immuno-chemical test (FIT) stool test Test to detect hidden blood in stool samples Annually No dietary or drug restrictions

No bowel preparation

No direct risk to bowel

Samples can be collected at home

Cannot detect precancerous changes

May miss some cancers

May need to have colonoscopy if positive result

Tier 2 tests
Flexible sigmoidoscopy Examination of the rectum and lower colon with a rigid or flexible lighted instrument Every 5-10 years Minimal preparation ahead of time

Detects pre-cancerous polyps and cancer

Does not usually need sedation

Fairly quick and safe

Only examines about 30% of colon

Small risk of bleeding, infection or bowel tear

May need to have colonoscopy if abnormal result 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

Detects pre-cancerous polyps and cancer

Relatively safe; minimal risk of tear to colon

Full bowel preparation required

May need standard colonoscopy if abnormal results

Effectiveness as a screening tool is not fully accepted

Fecal Immunochemical test (FIT)-DNA Detects blood and mutations in specific genes associated with colon cancer in DNA isolated from a stool sample Every three years, according to the American Cancer Society and MSTF No bowel preparation or dietary restrictions

Sample can be collected at home

No risk of bowel tear

Cannot detect precancerous changes

Not as effective as annual FIT

Adequate stool sample must be obtained

Special handling needed

May need colonoscopy if abnormal result found

Capsule colonoscopy Examination of the colon performed by swallowing an indigestible pill with embedded video cameras Every 5 years per MSTF Detects pre-cancerous polyps and cancer

No sedation required

Relatively safe

May need standard colonoscopy if abnormal results

Not approved by the FDA for screening people at average risk

No Tier recommendation
Guaiac-based fecal occult blood test (gFOBT) stool test Test to detect hidden blood in stool sample Annually No bowel preparation

No direct risk to bowel

Sample can be collected at home

Dietary restrictions before testing

Cannot detect precancerous changes

Detects any blood, not just from cancers but from food or dental procedures

May need colonoscopy if positive result

In addition to screening tests, a healthcare 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 detection range of a DRE.

If a test other than colonoscopy gives a result suggestive of polyps or cancer, a colonoscopy is often done to examine the full colon and remove polyps or potentially cancerous areas.

Tests for Diagnosis, Staging, and Prognosis


Typically, if a polyp or polyps are found during a colonoscopy, they are removed. This prevents any possible precancerous polyps from becoming cancerous. Any samples taken (biopsies) are usually examined under a microscope by a pathologist. Different types of polyps look different under the microscope. During the examination, the pathologist determines factors such as the type of polyp and whether it precancerous or cancerous. (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 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. This blood test is not used to diagnose colon cancer.
  • 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. (For more on this, read the article on KRAS Mutation.)
  • Mismatch repair (MMR) and microsatellite instability (MSI) testing—genetic testing can be done on tumor tissue to determine whether cancer cells have changes in mismatch repair genes and a high level of genetic changes. Testing may be used to help guide treatment. About 15% of all people with colon cancer and 90% of people with colon cancer associated with Lynch syndrome 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.

Common question: I have heard about a septin 9 gene test for colon cancer. What is it?

The septin 9 gene in a patient’s blood is being studied as potential marker of colorectal cancer (CRC). Studies have shown that there is a higher risk for CRC when a person has higher levels of “hypermethylated” septin 9 gene in their blood versus normally methylated septin 9. (Methylated means that compounds called methyl groups are added to the gene). Septin 9 testing is not recommended for routine use in CRC screening. However, septin 9 testing could be offered to patients who do not agree to first tier or second tier screening tests for CRC. First-tier tests are colonoscopy every 10 years or an annual fecal immunochemical test (FIT). The second-tier tests include CT colonography every 5 years, the FIT-DNA test every 3 years, or flexible sigmoidoscopy every 5 to 10 years.


All stages of colon cancer are usually treated by surgically removing the cancer, possibly some of the surrounding tissue, and in some instances, lymph nodes. 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.

Immunotherapy is also relatively new and may be used to treat some patients with advanced colon cancer. Immunotherapy works by helping a patient’s own immune system recognize and kill cancer cells more effectively.

For more on treatment, see the links in the Related Content section.

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