What is colorectal cancer?

Colorectal (large bowel) cancer is a disease in which malignant (cancer) cells form in the inner lining of the colon or rectum. Together, the colon and rectum make up the large bowel or large intestine. The large intestine is the last segment of the digestive system (the esophagus, stomach, and small intestine are the first three sections). The large bowel’s main job is to reabsorb water from the contents of the intestine so that solid waste can be expelled into the toilet. The first several feet of the large intestine is the colon and the last 6 inches is the rectum.

Most colon and rectal cancers originate from benign growths on the inner lining of the colon or rectum called polyps or flat lesions. The difference between polyps and flat lesions is primarily just their shape, with polyps growing more into the lumen than flat lesions. Not all polyps and flat lesions have the potential to transform into cancer. Those that do have the potential are called adenomas and sessile serrated polyps (also called sessile serrated adenomas). It takes more than 10 years in most cases for a precancerous polyp or flat lesion to develop into cancer. This is why some colon cancer prevention tests are effective even if done at 10-year intervals. This 10-year interval is too long, in some cases, such as in persons with ulcerative colitis or Crohn’s colitis, in persons with a strong family history of colorectal cancer or adenomas, in persons who themselves have previously had colorectal cancer, and some persons who have had precancerous polyps or flat lesions.


Who is at risk for colorectal cancer?

  • Everyone age 50 and older

The average age to develop colorectal cancer is 70 years, and 93% of cases occur in persons 50 years of age or older. Current recommendations are to begin screening at age 50 if there are no risk factors other than age for colorectal cancers. A person whose only risk factor is their age is said to be at average risk.

  • Men and women

Men tend to get colorectal cancer at an earlier age than women, but women live longer so they ‘catch up’ with men and thus the total number of cases in men and women is equal.

  • Anyone with a family history of colorectal cancer

If a person has a history of two or more first-degree relatives (parent, sibling, or child) with colorectal cancer, or any first-degree relatives diagnosed under age 60, the overall colorectal cancer risk is three to six times higher than that of the general population. For those with one first-degree relative diagnosed with colorectal cancer at age 60 or older, there is an approximate two times greater risk of colon cancer than that observed in the general population. Special screening programs are used for those with a family history of colorectal cancer. A well-documented family history of adenomas is also an important risk factor.

  • Anyone with a personal history of colorectal cancer or adenomas at any age, or cancer of endometrium (uterus) or ovary diagnosed before age 50

Persons who have had colorectal cancer or adenomas removed are at increased risk of developing additional adenomas or cancers. Women diagnosed with uterine or ovarian cancer before age 50 are at increased risk of colorectal cancer. These groups should be checked by colonoscopy at regular intervals, usually every 3 to 5 years. Woman with a personal history of breast cancer have only a very slight increase in risk of colorectal cancer.


What are the symptoms of colorectal cancer?

Symptoms of colorectal cancer vary depending on the location of the cancer within the colon or rectum, though there may be no symptoms at all. The prognosisis worse on average in symptomatic as compared to asymptomatic individuals (the latter refers to persons with cancer discovered by screening). The most common presenting symptom of colorectal cancer is rectal bleeding. Cancers arising from the left side of the colon generally cause bleeding, or in their late stages may cause constipation, abdominal pain, and obstructive symptoms. On the other hand, right-sided colon cancers may produce vague abdominal aching, but are unlikely to present with obstruction or altered bowel habit. Other symptoms such as weakness, weight loss, or anemia resulting from chronic blood loss may accompany cancer of the right side of the colon. You should promptly see your doctor when you experience any of these symptoms.


Why should you get checked for colorectal cancer if you have no symptoms?

Precancerous polyps and flat lesions can grow for years and transform into cancer without producing any symptoms. By the time symptoms develop, it is often too late to cure the cancer, because it may have spread. Screening identifies cancers earlier and actually results in cancer prevention when it leads to removal of pre-cancerous growths.


What is screening for colorectal cancer?

Screening means looking for cancer or polyps when patients have no symptoms. Finding colorectal cancer before symptoms develop dramatically improves the chance of survival. Identifying and removing polyps before they become cancerous actually prevents the development of colorectal cancer.


What tests are available for screening?

  • Colonoscopy

Your doctor can examine your entire colon and rectum during colonoscopy. The procedure is used to look for early signs of cancer in the colon and rectum where they could not be reached by sigmoidoscopy. Polyps and flat lesions can be removed during colonoscopy. Sedation is usually used for colonoscopy. Colonoscopy is currently the only test recommended for colorectal cancer screening in average-risk persons at 10 year intervals.

  • Fecal occult blood test

One of the presentations of colon cancer is chronic blood loss in the stool. Sometimes, such blood loss is so minimal, it cannot be seen when the stool is inspected in the toilet. Your doctor will ask you to collect a stool sample which is returned to the doctor or lab to test for occult (hidden) blood. There are two types of tests, called the fecal immunochemical test (FIT) and the guaiac test. The fecal immunochemical test (FIT) is the better test. Either test is done annually. If either test is positive, colonoscopy should be done.

  • Fecal DNA Testing

Colorectal cancers contain abnormal DNA which is shed into the stool. In this a sample of stool is checked for abnormal DNA and colonoscopy is performed if any is found. This test should be repeated at 3 years if it’s negative.



Liangpunsakul, S., & Rex, D. K. (2017). Colorectal Cancer Screening. Retrieved from American College of Gastroenterology (ACG): http://patients.gi.org/topics/colorectal-cancer-screening/