Colorectal cancer is the number two cancer killer in the US, but screening for it is a topic many Americans don’t like to discuss, much less do anything about. While none of the screening tests commonly used (fecal occult blood test, barium enema, flexible sigmoidoscopy, or colonoscopy) is perfect, getting people to use any test at all has been a challenge.

Recent studies show that fewer than 40% of the people who should be screened do so on a regular basis. There are many reasons for this, but among the most prominent is that many people are uncomfortable with some of the tests themselves, especially colonoscopy.

But what if newer, less invasive types of tests were found to be just as good? A group of experts from the American Cancer Society Colorectal Cancer Advisory Group recently examined some of these newer technologies in CA: A Cancer Journal for Clinicians (Vol. 53: 44-55).

CT colonography, often referred to as “virtual colonoscopy,” is a promising method for detecting colorectal polyps that might become cancerous. Some doctors and clinics are already offering this test to their patients.

The test involves pumping air into the colon, and then getting a special scan called helical CT or spiral CT. This type of scan takes many thin pictures of the structures in the abdomen, and can usually be completed in a single breathhold. The pictures are then fed into a computer, which constructs two- and three-dimensional images of the inside of the colon. Doctors can then “fly through” the colon on a computer screen, detecting abnormalities that might require a closer look.

The advisory group experts pointed out that virtual colonoscopy has several attractive features, including the fact that it is essentially non-invasive. It does not carry the very small but real risk of bowel perforation associated with colonoscopy. It also seems to detect large polyps about equally as well as colonoscopy. While it may not be as sensitive at detecting smaller lesions, doctors aren’t sure how important it is to identify such lesions.

But virtual colonoscopy has limitations as well. In order to get a good picture, it requires the same type of bowel cleansing regimen before the exam that many patients find to be the worst part of a colonoscopy or barium enema.

And because it only detects shapes (not colors or fine details), doctors can’t actually see the inside of the colon, as they can with colonoscopy. At times this may lead to suspicious findings that turn out not to be polyps. Advances in technology may change this in the future, however.

Also, unlike colonoscopy, polyps can’t be removed during the exam. If something suspicious is found, it will require follow-up colonoscopy anyway. Finally, the procedure is still too new to get an accurate idea of how effective it will be; not enough studies have been done.

The committee members concluded that virtual colonoscopy “is a compelling, emerging technology that shows considerable promise, but it has not yet been studied in a typical screening population.” Therefore, it is not included at this time as one of the recommended colorectal cancer screening methods.

Most people are familiar with the idea of testing a stool sample for blood to screen for colorectal cancer. Fecal occult blood testing (FOBT) is non-invasive, is done in the privacy of the home, and can be a useful method of screening when done correctly on an annual basis.

But conventional (guaiac-based) FOBT has some shortcomings. It is not very sensitive, because it can only detect tumors that are bleeding at the time of sampling. (Tumors generally don’t cause bleeding all the time.) And because of the specific substance the test detects, it can give a false positive result if a person is eating certain meats or vegetables, or is taking certain vitamins or drugs.

Immunochemical tests have been developed that use antibodies to detect only a specific portion of a human blood protein. These tests are done essentially the same way as conventional FOBT, but they are more specific – that is, they reduce the number of false positive results. The tests do not require dietary restrictions before testing, and only require two stool specimens (as opposed to three for conventional FOBT), so people may find them easier to use.

Immunochemical FOBT has some of the same drawbacks as conventional FOBT, such as the fact that it wouldn’t be able to detect a tumor that is not bleeding. But the experts were impressed enough with the technique to recommend adding it to the colorectal cancer screening guidelines, stating, “in comparison with guaiac-based tests for the detection of occult blood, immunochemical tests are more patient-friendly, and are likely to be equal or better in sensitivity and specificity.”

While FOBT can miss tumors because they are not always bleeding, DNA tests do not have this problem. Colon tumors constantly shed cells that contain abnormal DNA, a hallmark of cancer. Tests that can detect DNA mutations specific to cancer may prove to be very useful in screening.

In addition to the ability to detect tumors that are not bleeding, the committee members pointed out other possible advantages of stool DNA testing. The test looks for cancer at the molecular level, so it may be less likely than conventional tests to give false positive or false negative results. It may also be able to detect other kinds of cancers, as DNA from some other parts of the body also passes through the colon.

But there are drawbacks with the test at the present time. It requires an entire stool sample to be sent to a laboratory for testing, something some people may find unacceptable. Because the test is not yet automated, it currently costs more than $400 per test. And, like virtual colonoscopy, it requires further testing to prove it is effective.

These issues must be addressed before DNA stool testing can be recommended as a general screening test, the panel concluded.

These new tests do indeed hold promise for the future, the panel stated, and will continue to be reviewed as new data become available. Until they are better studied, people over 50 at average risk should follow one of five screening options:

·
• A fecal occult blood test (FOBT) every year, or ·
• Flexible sigmoidoscopy every 5 years, or ·
• A fecal occult blood test every year plus flexible sigmoidoscopy every 5 years, or (Of these first three options, the third is preferable: FOBT every year and flexible sigmoidoscopy every 5 years.) ·
• Double-contrast barium enema every 5 years, or ·
• Colonoscopy every 10 years