Screening for Colorectal Cancer

The term “screening” means to perform tests for a disease on individuals who do not exhibit any symptoms, as a method of prevention and early detection. Because colorectal cancer almost always develops from a benign (non-cancerous) polyp, the disease can be prevented when these polyps are discovered through screening and removed. In fact, colorectal cancer is over 90 per cent preventable. In addition, when the cancer has occurred, but is caught early, it has an over 90 per cent cure rate.

Common screening tests include:

Fecal Occult Blood Test

Blood vessels at the surface of colorectal cancers and some polyps are often very fragile and can become easily damaged with the passage of feces, releasing an amount of blood too small to be seen with the naked eye. A fecal occult blood test (FOBT) can detect this occult (hidden) blood in stool. The test is done at home by the patient and involves collecting samples from three separate bowel movements and smearing them onto a card treated with a plant-based substance called guaiac. The card is then mailed to a laboratory where it is analyzed for blood. A guaiac-based FOBT requires certain drug and dietary restrictions several days before and during the test, as they can affect its outcome; however a new stool test, called a fecal immunochemical test (FIT), does not require these restrictions. With an FIT, stool samples are obtained much the same way as with an FOBT but are processed differently. An FIT can detect blood specifically emanating from the colon or rectum, whereas an FOBT detects blood from anywhere along the digestive tract (mouth to anus). A positive FOBT or FIT should always be followed up with the internal screening test, colonoscopy (see below), to determine the cause of bleeding, which can be one of many conditions, including ulcers and hemorrhoids.

Double Contrast Barium Enema

A double contrast barium enema (DCBE) is an X-ray test that provides images of the entire colon. A liquid containing the contrast agent barium sulfate is inserted into a patient’s rectum and spreads throughout the colon to define its appearance on the X-rays. Air is also pumped into the colon to expand it before the X-rays are taken. If polyps or other abnormalities show up on the images, a colonoscopy is needed to investigate. With the advent of newer, more sensitive technologies and greater availability of colonoscopy, this test is being used less often.

Flexible Sigmoidoscopy

A flexible sigmoidoscopy is the internal examination of the lower part of the colon called the sigmoid colon, using a thin, bendable tube containing a light and video camera which can be hooked up to a display monitor. The tube is inserted into the anus and slowly guided into the sigmoid colon for inspection. If polyps or other abnormalities are detected, they can be entirely or partially removed by an instrument passed through the end of the tube—this is called a biopsy. The biopsy specimen is sent to a laboratory to determine if cancer cells are present and a colonoscopy is also required to check for growths in the entire colon.

Colonoscopy

A colonoscopy is similar to a sigmoidoscopy except that it examines the entire length of the colon. If polyps or other abnormalities are detected, they can be biopsied and sent to a laboratory to determine if cancer cells are present. Because this procedure is more invasive, sedation is usually administered to the patient. Colonoscopy is considered the most sensitive and thorough of all the tests for colorectal cancer. A newer, less invasive test called a virtual colonoscopy, involves the use of a CT scan (an X-ray procedure) to capture images of the colon and rectum. Through sophisticated computer software, internal, 3-D images of the bowel are generated, allowing doctors to move through the colon with the click of a mouse. If anything suspicious is identified, a conventional colonoscopy is required for biopsy.

Biopsy

Removed tissue examined under a microscope by a pathologist (a doctor specializing in the diagnosis of diseases) is the only definitive way to make a colorectal cancer diagnosis. A biopsy sample can also determine how aggressive a cancer is, and may be able to show the extent it has affected the colon or rectum wall.

Who Should be Screened?

For the average person, age is the main risk factor for colorectal cancer, with more than 90 per cent of cases occurring in those over 50. Therefore, the Colorectal Cancer Association of Canada recommends that all Canadians age 50 and over undergo screening with an FOBT or FIT at least once every two years. If a test is positive for blood, a colonoscopy should be performed to determine the cause of bleeding.

Those at a higher risk of developing the disease should talk to their doctors about earlier and more frequent screening, as well as which test would be appropriate.

People at a higher risk of developing colorectal cancer include those who have:

• a first-degree relative with colorectal cancer
• a personal history of colorectal cancer
• a personal history of benign polyps
• inflammatory bowel disease such as ulcerative colitis or Crohn’s disease
• a family history, or diagnosis, of hereditary syndromes linked to colorectal cancer, such as familial adenomatous polyposis (FAP) or hereditary nonpolyposis colorectal cancer (HNPCC)