Colonoscopy isn’t perfect.

I think of this when I see our friend Sarah. About five years ago her husband died of colon cancer even though he had a “negative” colonoscopy two years before. Everyone blamed the poor doc who did the procedure, but it turns out that he might not have been at fault. Colonoscopy reduces the chance of dying from this cancer, but doesn’t entirely eliminate it.

I was reminded of this by a recent report in the Journal of Clinical Oncology. The study, done in Germany, found that the chances of developing colon cancer within ten years of a negative colonoscopy were about one-fourth to one-third that of people who didn’t have the procedure. That is good, but not great.

Another study, this time from Canada, (Annals of Internal Medicine, January, 2009) came up with similar results. Only this study looked at the risk of dying from colorectal cancer after having a negative colonoscopy, like my friend Sarah’s husband. Their numbers were similar to the German group. The risk of dying from colorectal cancer, if you had a negative colonoscopy, was one-third that of people who never had a colonoscopy. Still good, but not great. And, if by any chance, the cancer started in the right side of the colon, the part of the colon farthest from the rectum, colonoscopy did not lower the chance of dying from colorectal cancer. People with cancers developing in the right side of the colon were not helped. This was the case with Sarah’s husband.

This is confusing, but let me explain. The colon is a fairly long tube that runs up the left side of the abdominal cavity and then crosses over to the right side. Cancers that develop on the left side are easy to spot. The colonoscopy tube has no problem getting there and the cancers generally start as polyps, which stand up and are easy to spot. But cancers on the right side are a problem. They are hard to find because they tend to be flat and not stand up like polyps. Also, it is hard to get that part of the colon really clean so that the colonoscopist can see everything that is there. And finally, snaking the colonoscope up that far is challenging.

Whatever the reason, the doctor who examined Sarah’s husband’s colon may have done a good job. But, since the tumor was on the right side, the odds were stacked against him. The procedure isn’t that good for cancers on the right side of the colon and perhaps gives people a false sense of security so when they have symptoms they ignore them.

This doesn’t help Sarah, but at least her husband’s death was not due to negligence, but rather to bad luck. His cancer started on the wrong side.