While still very effective in preventing colorectal cancer and deaths from the disease, limits of the test may be larger than previously thought. Patients need to know that having colonoscopy does not guarantee that they won’t get colorectal cancer.
Experts now say that screening colonoscopy may reduce death from colorectal cancer by 60 to 70 percent and may not keep patients from dying from cancers on the right side of their colons at all.
A new Canadian study found that some people who died of colorectal cancer had a colonoscopy in the years before their cancer diagnosis. A previous completed colonoscopy reduced chances of dying from colorectal cancer by two thirds in patients with cancers on the left side of their colons but did not decrease risk of death among those with right-sided cancers.
Working from the Ontario Cancer Registry, researchers identified 10,300 people who died from colorectal cancer in the seven years between 1996 and 2003. They included only people aged 52 to 90 to ensure that the study would cover patients who had been eligible for colorectal cancer screening in the time before their diagnosis.
They then matched them to 51,500 controls, similar in age, sex, and social economic status, in order to perform a case-control study to determine how much colonoscopy decreased risk of death from colorectal cancer.
Among the 10,300 people who died from colorectal cancer, 719 or 7 percent had a colonoscopy 6 months or more before they were diagnosed with cancer. Eight in ten of those colonoscopies were complete in reaching the cecum at the top of the colon, one in four included removal of polyps. Among the 51,500 controls, 4000 or 9.8 percent had a colonoscopy, 1 in 5 had polyps removed.
Patients with previous colonoscopy whose cancers were on the left side of their colons had a 65 percent reduction in the risk of dying from their cancer, but there was no similar reduction in right-sided cancer death.
In analyzing their results, the researchers speculated that some reasons for colonoscopy not reducing risk for deaths from cancers on the right side of the colon were:
- Poor bowel cleansing before colonoscopy.
- Not completely evaluating the entire right colon.
- Different biology for right-sided lesions including more flat or sessile polyps.
- Different molecular basis for the cause and development of right-sided cancer.
Nancy Baxter, M.D., and her colleagues concluded,
Our study and others like it provide unique insight into colonoscopic approaches to prevent CRC death. In an Ontario-wide sample, colonoscopy is associated with a reduced risk for death from CRC arising from the left colon but not from the right colon. Although improvements in the quality of screening colonoscopy may narrow this difference, differences in tumor biology may limit the potential to prevent right-sided colon cancer deaths with current endoscopic technology.
In an associated editorial, David Ransohoff M.D. discussed the study, its limitations, and what it may mean as doctors talk to patients. He recognized the problems with case control studies and the fact that medical records were not available to assess bowel preparation and colonoscopy completions. However, he was also concerned that patients may be misled into thinking that colonoscopy protects more than it actually does. He wrote,
These concerns and the authors’ results should make us worry that we might mislead our patients (and ourselves) by saying that colonoscopy reduces the risk for CRC death by 90%. Based on the considerations discussed earlier on case–control studies of sigmoidoscopy and randomized clinical trials of fecal occult blood testing that show CRC mortality reduction after colonoscopy (done because of a positive fecal occult blood testing result), a reasonable estimate—and what we should probably tell our patients—might be closer to a 60% to 70% reduction of the risk for death from CRC with high-quality colonoscopy. A 60% to 70% mortality reduction is not as good as 90%, but it should not be considered disappointing. It would be remarkably high compared with screening for other types of cancer, such as breast (with a 25% cancer mortality reduction at best) or prostate (with no proven cancer mortality reduction) cancer.
SOURCE: Baxter et al., Annals of Internal Medicine, Early Release Article, December 16, 2008.
What This Means for Patients
New results from the Canadian study should not keep you from having a screening colonoscopy.
Colonoscopy does prevent death from colorectal cancer. However, it has limits. Colonoscopy will not completely protect you from getting colorectal cancer or dying from it.
Protect yourself further:
- Be aware of the symptoms of colon and rectal cancer and insist on having them evaluated with another colonoscopy even if you have had a clear one in the past.
- Follow colonoscopy prep instructions carefully and contact your doctor if the prep is not working. Some gastroenterologists recommend splitting the prep into two parts — and doing the second part the day of the colonoscopy procedure to be sure the right colon is fully cleaned.
- Choose an experienced gastroenterologist to do your colonoscopy, one who does several procedures a day and spends at least eight minutes withdrawing the scope. If your doctor doesn’t find polyps in 25 percent of men and 15 percent of women examined, you may want to look for a more skilled doctor.