Best way to screen for colon cancer?
Posted by medconsumers on May 7, 2010
Whenever I write about screening for colon cancer, I mention an inconvenient truth shown in three randomized trials worldwide. Screening symptomless older people reduced their chance of dying from colorectal cancer by about 30% …but this benefit was canceled by an equivalent increase in deaths from cardiovascular disease. I learned this little known medical fact at an international gathering of colorectal cancer researchers, held in San Francisco and sponsored by the U.S. National Cancer Institute. It was the year 2000, and the speaker was Dr. Wendy Atkin, a colon cancer researcher at Imperial College London, U.K.
Fast forward to the present. The very same Dr. Atkin recently made international news (though less prominently in this country). She led a large study that showed screening can in fact reduce deaths from colorectal cancer without increasing the risk of death from other causes. And it was accomplished with a five-minute sigmoidoscopy examination given just one time between the age of 55 and 64 years. . “Patient acceptance was surprisingly good,” said Dr. Atkin at a press briefing just before the study was published in The Lancet online.
“This is the largest and longest study to date and the first to show direct evidence that removing polyps prevents cancer. All we had before was lots of indirect evidence,” said Dr. Atkin. “A once-only sigmoidoscopy gives a big effect, reducing the risk of cancer by one-half in the portion of the colon examined and one-third including the part not examined. It’s best to undergo the sigmoidoscopy in your late 50s because you’ve already got your polyps by then.” Most people who develop left-sided (distal) colon cancer have polyps called adenomas by age 60, according to the British researchers. “The use of sigmoidoscopy to detect and remove adenomas affords long-term protection against distal colon cancer.”
The British study randomly assigned 40,621 people to a one-time sigmoidoscopy examination and 116,478 people to remain unscreened (the control group). After 11 years of follow-up, there were 40% fewer colorectal cancer deaths and a one-third reduction in colorectal cancer incidence in the screened group. In response to my e-mailed question about whether her study collected data on deaths from all causes, Dr. Atkin called my attention to the slight reduction in deaths from all causes shown in the screened group. The study was funded by Medical Research Council, National Health Service R&D, Cancer Research UK, KeyMed.
This study is very British because it took cost into consideration along with the best evidence in order to get the biggest bang for the bucks. But there is already speculation that it will not change practice here in the U.S. where the more expensive colonoscopy has long been the colorectal cancer screening tool of choice. With no supporting evidence, by the way. (The flexible sigmoidoscopy is falling out of favor in the U.S. because it reaches only the lower portion of the colon; whereas colonoscopy allows inspection of the upper and lower portion of the colon.) A lucrative industry has already built around the procedure that isn’t likely to shrink in the near future.
The lack of proof that colonoscopy is superior to sigmoidoscopy was noted by Dr. David Ransohoff, University of North Carolina at Chapel Hill, in an editorial that accompanied the new British study. The inflated claims for large reductions in colorectal cancer deaths often quoted for colonoscopy, wrote Dr. Ransohoff, are drawn from nonrandomized data. And colonoscopy’s purported superiority, based on its ability to reach the entire colon, has been undermined, he said, by a 2009 Canadian study that found that colonoscopy often misses polyps in the right, or ascending, colon where many colorectal cancers develop. Drawing from an insurance claims database for the province of Ontario, this study estimated that colonoscopy’s reduction in colorectal cancer deaths is 60%—a far cry from the widely advertised 90% seen in most public education campaigns that encourage screening.
Several large ongoing randomized trials are expected to provide more definitive information about colonoscopy’s role as a screening procedure. In the meantime, let’s hope that more attention is given to the harms of both procedures. Here’s Dr. Atkin’s 1999 paper that quantifies the small but definite risks of colonoscopy, including death, perforation, hemorrhage while undergoing polyp removal, and cardiovascular events. It probably influenced her decision to study sigmoidoscopy as the first-choice screening procedure.
For more information
A head-to-head comparison study of all colorectal cancer screening methods has yet to be published. Until then, we do not know which method saves the most lives while inflicting the least harm. Here’s the Colon Cancer Alliance’s list of all the screening methods with the advantages and disadvantages of each.
See this 2003 study about the risk of perforation associated with sigmoidoscopy and colonoscopy.
Maryann Napoli, Center for Medical Consumers(c)ISSN 2155-1480