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Preventing colon cancer from ever starting is the focus of new colorectal cancer screening guidelines issued Wednesday by the American Cancer Society.

The new recommendations add 2 new screening methods, stool DNA testing and CT colonography (also referred to as "virtual colonoscopy"), to the list of options. And the new guidelines are more specific than in the past about the differences between the various types of tests. The guidelines say some tests, which are typically more invasive, offer the best chance of preventing cancer because they can find colon growths (polyps) that can be removed before they become cancerous. Other tests are less likely to find polyps, but can still detect most cancers.

The guidelines say doctors should encourage patients to get the tests that find both polyps and cancer, provided the tests are readily available and people are willing to have them done. It is the first time the screening guidelines have expressed such a preference for one specific type of test over another, says Durado Brooks, MD, ACS Director of Prostate and Colorectal Cancer.

"In the past we've created a list [of options] and left it up to providers and patients to decide what would work best for them," he says. "But the science has made it clear that there are certain tests that have a higher potential to prevent cancer -- and for the first time our guidelines state that colorectal cancer prevention should be the primary goal of screening."

Colorectal cancer, commonly called colon cancer, is the third most common cancer diagnosed among both men and women in the US, and the second leading cause of cancer death. Although rates of this disease have been going down in recent years, they could be even lower if all people who should be screened actually got screened, the guidelines say.

Not only can some types of screening find polyps before they become cancerous, all types have the potential to find cancer at its earliest stage, when it is most treatable. About 90% of people whose colon cancer is caught before it has spread to lymph nodes or nearby organs survive 5 years after diagnosis. But only 10% of patients whose cancer has spread to distant parts of the body survive 5 years.

Tests that Find Both Polyps and Cancer

The new guidelines list 4 testing options that are likely to find both polyps and cancer:

• Flexible sigmoidoscopy every 5 years 
• Colonoscopy every 10 years 
• Double contrast barium enema every 5 years 
• CT colonography (virtual colonoscopy) every 5 years

Sigmoidoscopy and colonoscopy both involve inserting a flexible tube with a camera on the end into the colon to look for cancer or polyps. If polyps are found, they can be removed at that time. Sigmoidoscopy examines only the lower one-third of the colon, while colonoscopy views the entire colon, so if anything is found during sigmoidoscopy, a colonoscopy is recommended as follow-up to be sure there aren't problems in areas that weren't originally examined.

For double contrast barium enema, a catheter is used to put barium (a chalk-like liquid) and air into the colon, then x-rays are taken to look for polyps or cancer. If either is found, colonoscopy is recommended.

CT colonography widens the colon with air, then takes CT (computed tomography) images to look for polyps or cancer. If any are found, regular colonoscopy is needed as follow-up.

Tests that Primarily Find Cancer

The guideline options also include 3 types of stool tests that primarily find cancer, rather than polyps:

• Guaiac-based fecal occult blood testing (gFOBT) every year 
• Fecal immunochemical test (FIT) every year 
• Stool DNA test (unclear how often this is needed)

The gFOBT and FIT tests look for blood in the stool, a possible sign of cancer or perhaps a large bleeding polyp.

The stool DNA test is a relatively new screening method. It tests the stool for certain changes in DNA known to be related to colon cancer. Because it is so new, more research is needed to determine how often the test needs to be done to have the best chance of finding cancer. The test may also not be able to find every DNA change that may indicate cancer.

If any of these tests return a positive result, follow-up with colonoscopy is needed.

Some premalignant polyps may be found by these stool tests; however they are much less likely to find polyps than are the endoscopic and x-ray tests discussed above.

New Emphasis on Quality

The new screening guidelines, which were developed by ACS experts in collaboration with the US Multi Society Task Force on Colorectal Cancer and the American College of Radiology, aim to help doctors and patients make more informed decisions about colon cancer screening. They apply to people 50 and over with an average risk of developing colon cancer. People at increased or high risk of colon cancer may need to follow a more intensive screening schedule.

Each of the different screening options has benefits and limitations, which are discussed in detail in the guidelines. Only tests that have been shown in the medical literature to detect at least half of all cancers present at the time of testing are being recommended.

In another first, the guidelines emphasize the quality of the various types of testing.

"There's a fair amount of testing going on in the marketplace that is of such poor quality that in many ways you're doing patients a disservice," Brooks explains.

For instance, if an FOBT test returns a suspicious result, patients should be sent to get a colonoscopy -- but that doesn't always happen.

"That's a mistake and cancers are going to be missed," Brooks says, "so we go to great lengths to spell out the quality issues that need to be in place for any type of test."

Although the new guidelines do state a preference for certain types of tests over others, Brooks emphasizes that getting tested -- with any of the approved methods -- is still what counts.

"The best test is the test the patients can get and will take, but patients should be aware that there is a greater potential for certain types of tests to prevent cancer," he says.

The guidelines are published in the May-June 2008 issue of the American Cancer Society journal CA: A Cancer Journal for Clinicians.