New noninvasive test for colorectal cancer shows promise
Current screening tests for colorectal cancer have a number of drawbacks. The invasive nature of colonoscopy, sigmoidoscopy, and barium enema leads many people to avoid these tests. In addition, trained experts and expensive equipment are required to perform these procedures. The fecal occult blood test, which can be done at home or in the doctor's office, is not an invasive procedure, but it merely detects blood in feces—a problem that may or may not be due to colorectal cancer.
In a recent article in The New England Journal of Medicine , researchers describe the feasibility and effectiveness of testing stool samples (feces) for a certain gene mutation known to initiate colorectal cancer.
About the study
Researchers at Johns Hopkins University developed a process for testing feces for mutations of the adenomatous polyposis coli (APC) gene, which is known to initiate colorectal cancer. They collaborated with researchers in Texas, Massachusetts, Maryland, and Sweden to test the ability of their APC gene test to detect APC gene mutations in the fecal DNA of people known to have colorectal cancer. All stool samples were collected from patients at the M.D. Anderson Cancer Center in Houston, Texas and the Lahey Clinic in Burlington, Massachusetts between 1997 and 2001.
The researchers used a new method of detecting gene mutations, called digital-protein-truncation assay, to test the feces of 46 patients with colorectal cancer. They also tested the feces of 28 people who underwent colonoscopy but were not found to have colorectal cancer.
The researchers hoped the new test would detect mutated APC genes in patients known to have colorectal cancer but not in patients known to be free of colorectal cancer, who should not have the mutated gene.
The digital-protein-truncation assay detected APC gene mutation in 26 of the 46 patients (57%) known to have colorectal cancer and did not detect APC gene mutation in any of the 28 patients without colorectal cancer. Although the test detected only slightly more than one-half of the colorectal cancers, it is very important that it did not mistakenly identify any cancer-free patients as having cancer.
The colorectal cancer patients in this study had relatively early stage cancers that could reasonably be treated with surgery. The researchers chose earlier stage cancers for this study because, to be effective, a screening test needs to detect cancer early, while it is potentially curable.
There are limitations to this study that should be considered along with the results. It is important to note that this study does not prove the ability of the digital-protein-truncation assay to detect colorectal cancer. What it does show is that it's possible for researchers to detect the APC gene and its mutations in feces. As important, the test was sensitive enough to identify people who do not have this gene mutation. In addition, this study only included people with colorectal cancer and people at high risk for colorectal cancer. Therefore, the validity of this test in people at average risk for colorectal cancer is still unknown.
How does this affect you?
Will your doctor take a stool sample to test for APC gene mutation at your next check-up? No. At this time, the APC gene test has not been proven sensitive or specific enough to replace existing screening tests for colorectal cancer. The authors of this study estimate it will take three to five years before this test is available.
For now, talk to your doctor about your risk of colorectal cancer and which screening options are best for you. In assessing your risk, your health care provider will consider factors such as personal or family history of colorectal cancer or adenomatous polyps, and personal history of chronic inflammatory bowel disease. People at increased risk of colorectal cancer may need to begin screening earlier and get screened more often than people at average or low risk.
The American Cancer Society suggests that starting at age 50, men and women should follow one of the five screening options below:
- Yearly fecal occult blood test (preferably the take-home, multiple sample method)
- Flexible sigmoidoscopy every 5 years
- Yearly fecal occult blood test plus flexible sigmoidoscopy every 5 years*
- Double contrast barium enema every 5 years
- Colonoscopy every 10 years
*The combination of fecal occult blood test and flexible sigmoidoscopy is preferred over either of these two tests alone.
Traverso G, et al. Detection of APC mutations in fecal DNA from patients with colorectal tumors.
New England Journal of Medicine . January 31, 2002;346(5):311-320.
Schwartz RS. A needle in a haystack of genes.
New England Journal of Medicine . January 31, 2002;346(5):302-304.
Last reviewed Jan 30, 2002 by ]]>Richard Glickman-Simon, MD]]>
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