Tests and diagnosis

By Mayo Clinic Staff

Nearly all colon cancers develop from polyps, but the polyps grow slowly, usually over a period of years. Screening tests play a key role in detecting polyps before they become cancerous. These tests can also help find colorectal cancer in its early stages, when you have a good chance of recovery.

Several screening methods exist — each with its own benefits and risks. Be sure to discuss these with your doctor:

  • Colonoscopy. Colonoscopy is performed with a long, slender, flexible tube attached to a video camera and monitor. During colonoscopy, your doctor typically views your entire colon and rectum.

    This procedure is the most sensitive test for colorectal polyps and colorectal cancer. If any polyps are found during the exam, your doctor may remove them immediately or take tissue samples (biopsies) for analysis. In order to prepare for the exam, you follow your doctor's instructions on restricting your diet and taking laxatives to cleanse your bowel. You're likely to receive a mild sedative to make you more comfortable. The risks of diagnostic colonoscopy include hemorrhage and perforation of the colon wall. Complications are more likely to occur when polyps are removed.

  • Computerized tomographic colonography (CTC). Also referred to as virtual colonoscopy, this test involves a computerized tomography scan, a highly sensitive X-ray of your colon. Using computer imaging, your doctor rotates this X-ray in order to view every part of your colon and rectum without actually going inside your body. Before the scan, your large intestine is cleared of any stool, but researchers are looking into whether the scan can be done successfully without the usual bowel preparation.

    This newer technology may make colon screening safer, more comfortable and less invasive. It can be done more quickly and doesn't require sedation. However, it may not be as accurate as regular colonoscopy. Also, this method doesn't allow your doctor to remove polyps or take tissue samples during the procedure. If your doctor finds polyps or wants to sample tissue, you will need a colonoscopy.

  • Flexible sigmoidoscopy. In this test, your doctor uses a slender, lighted tube to examine your rectum and sigmoid — approximately the last two feet (61 centimeters) of your colon. Nearly half of all colon cancers are found in this area. If your doctor finds a polyp during this test, you'll need a colonoscopy so that your doctor can see your entire colon and remove any polyps.

    A sigmoidoscopy looks at only the last third of your colon, and this test doesn't detect polyps elsewhere in the large intestine. A sigmoidoscopy can be somewhat uncomfortable. Also, there's a slight risk of perforating the colon, but the risks are less than they are for colonoscopy.

  • Barium enema. This diagnostic test allows your doctor to evaluate your entire large intestine with an X-ray. A contrast solution containing barium is placed into your bowel in enema form. The barium fills and coats the lining of the bowel, creating a silhouette of your rectum, colon and sometimes a small portion of your small intestine. Air also may be added to provide better contrast on the X-ray.

    The image produced with the barium enema test isn't as detailed as other screening methods and polyps may be missed on this exam. It also doesn't allow your doctor to take a biopsy during the procedure to determine whether a polyp is cancerous. This test can be somewhat uncomfortable because the barium and air stretch your bowel. There's also a slight risk of perforating the colon wall.

  • Fecal blood tests. There are two types of noninvasive tests that check a sample of your stool for blood. One is called fecal occult blood test (FOBT), and the other is known as fecal immunohistochemical test (FIT). Each test can be performed in your doctor's office, but you're usually given a kit that explains how to perform the test at home. It's important to follow the instructions carefully, because your diet and other factors can affect the results. You return the test kit to a lab or your doctor's office to be checked.

    Although relatively easy, these tests are designed to screen for cancer, not for polyps. One problem is that most polyps don't bleed, nor do all cancers. This can result in a negative test result, even though you may have a polyp or cancer. On the other hand, if blood shows up in your stool, it may be the result of hemorrhoids or an intestinal condition other than cancer. For these reasons, many doctors recommend other screening methods instead of, or in addition to, fecal blood tests. Also, even if a test doesn't show blood in your stool, if you have seen blood in the toilet, on toilet paper or in your stool, you will need to go through further testing.

  • Stool DNA testing. This new colon cancer screening approach can detect cancer cells that have shed into your stool. The malignant cells have altered DNA, and this test can detect DNA mutations (markers) for some types of cancerous tumors and precancerous polyps. Research is ongoing to increase the test's accuracy and determine how often it should be done. However, this test is designed primarily to detect colon cancer and not to screen for colon polyps.
  • Genetic testing. If you have a family history of colorectal cancer, you may be a candidate for genetic testing. This blood test may help determine if you're at increased risk of colon or rectal cancer.

    Genetic testing is not without drawbacks. The results can be ambiguous, and the presence of a defective gene doesn't necessarily mean you'll develop cancer. Knowing you have a genetic predisposition can alert you to the need for regular screening.

Jul. 16, 2011

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