I am a staunch advocate of screening by means of colonoscopy for the early detection and treatment of colorectal cancer, the second leading cause of cancer death in the U.S. When told that my colonoscopy was negative 2 years ago—that is, that no cancer had been found—I thought I was pretty much home free. After all, the procedure was said to reduce deaths from colorectal cancer by about 90 percent.

Now my complacency is shaken by an article in a recent issue of Annals of Internal Medicine which found that colonoscopy did not reduce death rates from tumors of the right side of the colon (the part of the colon that is furthest away from the rectum and runs down the right side of the abdomen) any better than did sigmoidoscopy, a less invasive procedure that can only visualize the left side of the colon.

Complete colonoscopies reduced the likelihood of dying from colorectal cancer in the left side of the colon, but did not lower the mortality from tumors in the right side of the colon. In this study, left-side cancers were almost twice as common as cancers in the right side of the colon.

It is not certain why right-side cancers are identified less frequently. However, bowel preparations may be less effective for the right colon, since that side is farther along the length of the colon. This may emphasize the need for a thorough bowel cleansing by the bowel prep, which in my experience was by far the most unpleasant part of the colonoscopy undertaking. Right-side tumors can also be more difficult to visualize during colonoscopy, and there may be biological differences between the cancers in the right and left sides of the colon.

In an accompanying editorial, Dr. David Ransohoff of the University of North Carolina said that a screening colonoscopy appears to reduce the risk of dying from colorectal cancer by 60 percent to 70 percent, rather than the oft-cited 90 percent. Nonetheless, he pointed out that these benefits are "remarkably high compared with screening for other types of cancer, such as breast ... or prostate."

Dr. Ransohoff also noted that 40 percent of the colonoscopies in the study were done by surgeons, 16 percent by internists, and only 31 percent by gastroenterologists. Although he suggested that gastroenterologists might have detected more of the right-sided tumors, this suggestion may have been influenced by the fact that Dr. Ransohoff is himself a gastroenterologist.

Despite the sobering findings of this study, I still believe that everyone should talk with his or her doctor about getting a screening colonoscopy. (Just this morning, I learned that a close college friend, who is now a practicing physician, is terminally ill with colon cancer.) It is of course important to recognize that colonoscopy is an invasive procedure with the risk of serious complications in about 1 in 2,000 procedures.

And, as I described in a previous entry, the U.S. Preventive Services Task Force has recommended that colonoscopy should not be done to screen for colon cancer in most people who are 75 or older, and should never be done for those who are 85 or older.