Common Colon Cancer Test Often Misses Mark
The most commonly used test to detect colorectal cancer identifies only about a quarter of precancerous growths and is not an effective tool unless used yearly, Department of Veterans Affairs researchers report today.
Colorectal cancer is the second most deadly cancer in Americans, trailing lung cancer, but it is treatable if detected early. An estimated 135,400 cases will be found in the United States this year, with 56,700 deaths.
The American Cancer Society and other groups recommend that people older than 50 have a yearly screening with the fecal occult-blood test, which looks for signs of blood in the patient’s stool, and, every five years, a sigmoidoscopy, a more complex and expensive test.
“But what happens in real life is that patients often get only one test and no follow-up,” said Dr. David A. Lieberman of the Portland Veterans Affairs Medical Center, the lead author of the new study.
“This study tells us that physicians can’t use that single negative test to reassure patients and that people need to return for repeat testing in order for screening programs to be effective,” he added.
The study found that fecal occult-blood screening finds only 24% of polyps and other growths in a onetime test; sigmoidoscopy detects 70%. The two together identify 76% of the growths, the team reports in today’s New England Journal of Medicine.
Annual fecal occult-blood tests can reduce mortality from colorectal cancer by at least 30% if they are repeated over several years, noted Dr. Robert Smith, director of cancer screening for the American Cancer Society.
While sigmoidoscopy and colonoscopy, a more extensive test, are the best ways to screen for colorectal cancer, they are not widely used because they are more uncomfortable and are substantially more expensive. The low cost of fecal occult-blood testing, about $10, makes it a good alternative if it is used regularly, Smith said.
“The best test for colorectal screening,” he added, “is the one you get.”
The most effective screening tool for colorectal cancers is colonoscopy, in which a tiny television camera mounted on the end of a thin flexible tube is threaded all the way through the colon to search for small tumors, or polyps, which are often precursors of tumors. The process takes about 20 minutes--or longer if polyps are discovered and must be removed--and can cost more than $1,000. Researchers estimate that it identifies at least 95% of all lesions.
Second best is sigmoidoscopy, a similar process that covers only the lower one-third or so of the colon. It costs from $100 to $300.
The fecal occult-blood test uses a test strip that changes color if small amounts of blood--too small to be seen visually--are present in the stool.
Lieberman and his colleagues studied nearly 2,300 apparently healthy subjects, ages 50 to 75, at 13 VA medical centers around the country. All submitted stool samples for testing and each underwent colonoscopy.
The colonoscopy was conducted in two stages. The first stage mimicked sigmoidoscopy by examining only the lower third of the colon. The second stage involved a full-scale colonoscopy.
Colonoscopy detected cancer in 24 of the patients and troublesome precancerous growths in 282 others. The sigmoidoscopy identified only 70% of those lesions, the team found. The onetime fecal occult-blood testing found 24%. The latter two combined identified 76%.
“That’s a predictable result, but it provides a scientific foundation for what had previously been felt to be the case,” said Dr. Anil K. Rustgi of the University of Pennsylvania Cancer Center, who is a medical advisor to the National Colorectal Cancer Research Alliance.
Lieberman’s group noted that the sensitivity of both the fecal occult-blood test and sigmoidoscopy decline as the patient gets older. In elderly people, a higher percentage of tumors are found in the upper two-thirds of the colon, which are not reached by a sigmoidoscopy.
Lieberman cautioned that virtually all of the subjects were men. The research conclusions are probably the same for women, he noted, but they generally develop colon tumors five to seven years later than men.
“The next issue is how to implement the findings,” Rustgi added. Screening has been increasing in the last couple of years, especially after the high-profile colon cancer deaths of cartoonist Charles M. Schulz and Katie Couric’s 42-year-old husband. But the overall rate is still low, less than 30%, Smith said.
To be effective, colon cancer screening will have to become as common as mammography for breast cancer and Pap smears for cervical cancer, they added.
For further information about colon cancer screening: Colon Cancer Alliance at https://www.ccalliance.org; National Colorectal Cancer Research Alliance at https://www.nccra.org; National Cancer Institute at https://cancernet.nci.nih.gov/Cancer_Types/ Colon_And_Rectal_Cancer.shtml.
