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Tests and diagnosis
By Mayo Clinic staffNearly 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. When early-stage cancers are found and removed during routine screening, the five-year survival rate may be as high as 90 percent.
Several screening methods exist — each with its own benefits and risks. Be sure to discuss these with your doctor:
Fecal occult (hidden) blood test
This noninvasive test checks a sample of your stool for blood. It can be performed in your doctor's office, but you're usually given a kit that explains how to take 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.
- Risks and benefits. Although this test is relatively easy, it's 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 occult blood tests. Also, even if the test is negative, if you have seen blood in the toilet, toilet paper or stool, you will need to go through further testing.
Flexible sigmoidoscopy
In this test, your doctor uses a slender, lighted tube to examine your rectum and sigmoid — approximately the last 2 feet (61 centimeters) of your colon. Nearly half of all colon cancers are found in this area.
- Risks and benefits. A sigmoidoscopy looks at only the last third of your colon, and this test doesn't detect polyps elsewhere in the large intestine. It's often combined with a barium enema to better visualize the entire colon, or your doctor may recommend performing a colonoscopy instead. 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. Barium, a contrast dye, is placed into your bowel in enema form. The barium fills and coats the lining of the bowel, creating a clear silhouette of your rectum, colon and sometimes a small portion of your small intestine. Air may also be added to provide better contrast on the X-ray.
- Risks and benefits. Because barium enema has a higher miss rate for colon polyps, it's not nearly as reliable as some other screening tests. 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 distend your bowel. There's also a slight risk of perforating the colon wall.
Colonoscopy
Colonoscopy is similar to flexible sigmoidoscopy. However, the instrument used — a colonoscope, which is a long, slender tube attached to a video camera and monitor — allows your doctor to view your entire colon and rectum.
- Risks and benefits. This procedure is the most sensitive test for colorectal polyps and colorectal cancer. It's better at detecting polyps than is a barium enema X-ray alone. 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.
- Risks and benefits. 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. Some studies have shown it is effective, but there are not enough data to make a good comparison between CTC and 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.
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 underway to increase the test's accuracy and determine how often it should be done.
- Risks and benefits. This test is designed primarily to detect colon cancer and not to screen for colon polyps. It is available but has not been certified by the Food and Drug Administration.
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.
- Risks and benefits. 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.
- Ahnen DJ, et al. Approach to the patient with colonic polyps. http://www.uptodate.com/home/index.html. Accessed June 15, 2009.
- Ahnen DJ, et al. Epidemiology and risk factors for colorectal cancer. http://www.uptodate.com/home/index.html. Accessed June 15, 2009.
- What I need to know about colon polyps. National Digestive Diseases Information Clearinghouse. http://digestive.niddk.nih.gov/ddiseases/pubs/colonpolyps_ez/. Accessed June 14, 2009.
- Torpy JM, et al. Colon polyps. Journal of the American Medical Association. 2008;300:1480.
- Fletcher RH. Screening for colorectal cancer: Strategies in patients at average risk. http://www.uptodate.com/home/index.html. Accessed June 14, 2009.
- Levin B, et al. Screening and surveillance for the early detection of colorectal cancer and adenomatous polyps, 2008: A joint guideline from the American Cancer Society, the U.S. Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology. CA, A Cancer Journal for Clinicians. 2008;58:130.
- Colorectal cancer screening. National Cancer Institute. http://www.cancer.gov/cancertopics/factsheet/Detection/colorectal-screening. Accessed June 15, 2009.
- How is colorectal cancer found? American Cancer Society. http://www.cancer.org/docroot/CRI/CRI_2_1x.asp?rnav=criov&dt=10. Accessed June 15, 2009.
- Seitz U, et al. Endoscopic removal of large colonic polyps. http://www.uptodate.com/home/index.html. Accessed June 15, 2009.
- How is colorectal cancer treated? American Cancer Society. http://www.cancer.org/docroot/CRI/content/CRI_2_2_4X_How_Is_Colorectal_Cancer_Treated.asp?sitearea=. Accessed June 15, 2009.
- Can colorectal cancer be prevented? American Cancer Society. Accessed June 15, 2009.
- Picco MF (expert opinion). Mayo Clinic, Jacksonville, Fla. June 19, 2009.
- MYH-associated polyposis. Cancer Net, American Society of Clinical Oncology. http://www.cancer.net/patient/Cancer+Types/MYH-Associated+Polyposis. Accessed June 19, 2009.
