Tuesday, January 10, 2012


Several tests are available for colorectal cancer screening, including tests that can detect cancers at an early treatable stage (eg, stool tests), and tests that also detect pre-cancerous polyps (adenomas) and can lead to cancer prevention.

Guidelines from expert groups recommend that you and your healthcare provider discuss the available options and choose a testing strategy that makes sense for you. Tests that detect pre-cancerous polyps are preferable; these include colonoscopy, CT colonography, and flexible sigmoidoscopy. Stool tests that detect blood or abnormal DNA are another option [1]. Being screened with any test is more important than which test is used.

Colonoscopy — Colonoscopy allows a physician to see the lining of the rectum and the entire colon.

Procedure — Colonoscopy requires that you prepare by cleaning out your entire colon and rectum. This usually involves consuming a liquid medication that causes temporary diarrhea. You are given a mild sedative drug before the procedure. During colonoscopy, a thin, lighted tube is used to directly view the lining of the rectum and the entire colon. Polyps and some cancers can be removed during this procedure.

Effectiveness — Colonoscopy detects most small polyps and almost all large polyps and cancers.

Risks and disadvantages — The risks of colonoscopy are greater than those of other screening tests. Colonoscopy may lead to serious bleeding or a tear of the intestinal wall in some individuals (about 1 in 1,000). Because the procedure usually requires sedation, you must be accompanied home after the procedure and you should not return to work or other activities on the same day.

CT colonography ("virtual colonoscopy") — Computed tomography colonography (CTC, sometimes called "virtual colonoscopy") is a test that uses a CT scanner to take images of the entire bowel. These images are in two- and three-dimensions, and are reconstructed to allow a radiologist to determine if polyps or cancers are present. The major advantages of CTC are that it does not require sedation, it is non-invasive, the entire bowel can be examined, and abnormal areas (adenomas) can be detected about as well as with traditional (optical) colonoscopy.

There are several disadvantages of CTC. Like traditional colonoscopy, CTC requires a bowel prep to clean out the colon. If an abnormal area is found with CTC, a traditional colonoscopy will be needed at a later time to see the area and take a tissue sample (biopsy). CTC may detect abnormalities other than polyps and colorectal cancer. Many of these incidental findings will require further testing. This test may not be covered by health insurance plans in the United States. CTC, like many other imaging tests, exposes patients to radiation which may have long-term risks.

Sigmoidoscopy — Sigmoidoscopy allows a physician to directly view the lining of the rectum and the lower part of the colon (the descending colon). This area accounts for about one-half of the total area of the rectum and colon.

Procedure — Sigmoidoscopy requires that you prepare by cleaning out the lower bowel. This usually involves consuming a clear liquid diet and using an enema shortly before the examination. Most people do not need sedative drugs and are able to return to work or other activities the same day. During the procedure, a thin, lighted tube is advanced into the rectum and through the left side of the colon to check for polyps and cancer; the procedure may cause mild cramping. Biopsies (small samples of tissue) can be taken during sigmoidoscopy. Sigmoidoscopy may be performed in a doctor's office.

Effectiveness — Sigmoidoscopy can identify polyps and cancers in the descending colon and rectum with a high degree of accuracy. Studies have shown that sigmoidoscopy reduces the incidence of colorectal cancer and overall mortality.

Risks and disadvantages — The risks of sigmoidoscopy are small. The procedure creates a small tear in the intestinal wall in about 2 per every 10,000 people; death from this complication is rare. A major disadvantage of sigmoidoscopy is that it cannot detect polyps or cancers that are located in the right side of the colon.

Additional testing — Finding polyps or cancers in the lower colon increase the likelihood that there are polyps or cancer in the remaining part of the colon. Thus, if sigmoidoscopy reveals polyps or cancer, colonoscopy is recommended to view the entire length of the colon.

Double contrast barium enema — A barium enema test provides a detailed x-ray picture of the rectum and the entire colon. This test has largely been replaced by other options. Polyps or cancers cannot be removed during a barium enema, and CT colonography is more accurate for detecting abnormalities.

Stool tests — Colorectal cancers often release microscopic amounts of blood and abnormal DNA into the stool. Stool tests can detect blood or abnormal DNA makers.

Two types of tests, called guaiac tests (typically Hemoccult) and immunochemical tests, evaluate the stool for blood, which may be present if there is bleeding from a colon cancer (or other source of blood).

With guaiac testing, you collect two samples of stool from three consecutive bowel movements, which you apply to home collection cards. You mail the cards back to the healthcare provider.

With immunochemical testing, you use a long handled tool to brush the surface of your stool in the toilet. You apply the brush to a card, and then mail the card to a laboratory. You do not have to change your diet or stop any medications with this test.

With stool DNA testing, you collect an entire bowel movement and mail it to a laboratory with an ice pack. You do not have to change your diet or stop any medications with this test. The stool DNA test evaluates stool for several DNA markers, which can be associated with colon cancers.

Effectiveness — Guaiac testing, when performed once per year, reduces the risk of dying from colorectal cancer by as much as one-third.

Risks and disadvantages — Because polyps seldom bleed, guaiac testing is less likely to detect polyps than other screening tests. In addition, only 2 to 5 percent of people with a positive stool test actually have colorectal cancer.

Additional testing — If the stool test is positive, your entire colon should be examined with colonoscopy.

Fecal occult blood test and sigmoidoscopy — Combined screening with a fecal occult blood test (guaiac) and sigmoidoscopy is a possible screening strategy and may be more effective than either test done alone.

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