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Screening Tests

Screening Tests for those at Average Risk

Most experts agree that colorectal cancer is an illness uniquely suited to the use of screening tests. There are several reasons:

  • The illness is common.
  • It is lethal if left untreated.
  • Early detection dramatically improves survival.
  • Abnormal growths (polyps) may be present in the colon and rectum for many years without spreading, providing a lengthy opportunity for early detection.
  • Polyps and early colorectal cancer, which have not spread beyond the lining of the bowel wall, are very treatable, with a cure rate close to 100 percent.
  • Simple screening tests are available.

There is a still debate, however, about the most efficient means of screening the large group of people who are at average risk for colorectal cancer. Screening tests include:

Fecal Occult Blood Test
A fecal occult blood (FOB) test detects blood in the stool even if the amounts are too small to be visible to the naked eye. The drawbacks of FOB tests are that they do not always detect colorectal cancers and most people who have positive FOB tests do not have colorectal cancer, as there are many causes of gastrointestinal bleeding. For example, polyps, ulcers and hemorrhoids may also cause positive FOB tests, as will blood in the meat that a person has eaten. An FOB test can be useful, however, because it can provide an early warning of colorectal cancer.

If you are having an FOB test, you must avoid eating meat for several days before the test. You should also avoid medications like Aspirin, which can cause a small amount of blood loss in the stool. For this test, you will be asked to collect three stool samples at home, and take them to the laboratory for testing.

Flexible Sigmoidoscopy
This procedure examines the rectum and the lower part of the colon, with a flexible tube carrying a tiny video camera. A biopsy can be performed, if needed. The tube does not extend the entire length of the colon and will, therefore, not detect polyps or cancer farther up the colon. If a polyp is found, a colonoscopy should be done to examine the rest of the colon.

Your doctor will tell you how to prepare for a sigmoidoscopy. Preparation usually takes the form of a liquid diet for 12-24 hours before the procedure, a laxative and an enema. This will clean out your lower colon so your doctor can see it clearly during the sigmoidoscopy. The test can be done in a clinic or in a doctor's office, and will take about 15 minutes. You may experience some cramping during the exam, but will be able to return to your regular activities later in the day.

Many provinces are either considering or have already established programs to screen for colorectal cancer.

The Canadian Cancer Society recommends that all men and women who are 50 years and older and at average risk for colorectal cancer have a fecal occult blood test every two years. If abnormalities are found, followup testing should include a colonoscopy, a double contrast barium enema or a sigmoidoscopy.

Screening for Those at Increased Risk of Colorectal Cancer

If you are at high risk of colorectal cancer, or if other screening tests (FOB test or flexible sigmoidoscopy) indicate an abnormality, your health care provider will recommend a colonoscopy.

Colonoscopy is considered the gold standard when it comes to diagnosing colorectal cancer. A colonoscope consists of a long flexible tube with a tiny video camera on the end. It is similar to a sigmoidoscope except it is longer and allows your doctor to view the entire colon. With this instrument, a physician can inspect the entire colon and rectum, and remove polyps or biopsy any abnormalities, which can then be examined in the lab.

If you are having a colonoscopy, you will be given a preparation the day before your procedure, to clear your colon. A colonoscopy is usually done as a day surgery procedure. You will generally be given a mild sedative intravenously. The procedure takes about 30 minutes. It may cause mild bloating and discomfort for a few hours following the procedure.

Colonoscopy is recommended when abnormalities are found using FOB test or flexible sigmoidoscopy. Regular colonoscopies are also recommended for those with family or personal histories of colorectal cancer, polyps or inflammatory bowel disease, as outlined in the previous section. This may be at one- to five-year intervals, depending on your situation.

Barium Enema
Another procedure sometimes used to examine the colon is a barium enema. This can identify abnormalities in the lining of the bowel. An enema is used to partially fill the colon with a chalky substance. Air is then added to expand the colon and x-rays are taken.

Your doctor will tell you how to prepare for the procedure. Test preparations include a diet of clear liquids, a laxative and an enema, the day before your procedure, to clear your colon and rectum.

Barium enemas are done in the radiology department of a hospital or clinic. The procedure usually takes 45 minutes to an hour and may cause some bloating and discomfort.

It is not possible to remove polyps or to take biopsies using a barium enema. Generally, if an abnormality is found, a colonoscopy will be recommended so that the abnormality can be biopsied.

Imaging Tests for Colorectal Cancer

If cancer is found, some imaging tests may be used to identify how far the cancer has spread.

Rectal ultrasound is an examination in which a device is inserted into the rectum. This device produces sound waves that are reflected by tissues of the rectum and nearby organs. The pattern of sound wave echoes is detected and analyzed by a computer to create an image of the rectum. Ultrasound may be used to see how deeply a rectal cancer has spread.

Computed tomography (CT) scan is an examination in which an x-ray beam is rotated around the body, to produce a series of two-dimensional images. A computer combines these images to produce a detailed, cross-sectional image map. This allows the physician to find out if a colorectal cancer has spread outside the bowel to other organs, such as the liver.

Magnetic resonance imaging (MRI) is an examination, which uses radio waves and strong magnets to scan tissues. The patterns are then taken by a computer and translated into a detailed image. MRIs can provide more detailed images than CT scans, and can also be used to detect the spread of colorectal cancer to the liver and other organs in the abdomen.

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  • A publication of:
  • Women's College Hospital