Colonoscopy prevents colon cancer? New study shows, not as well as we thought. (Video included.)

by James Hubbard, M.D., M.P.H.

We thought colonoscopy could prevent 90 percent of colon cancers.  We were wrong.  If the results in the new Annals of Internal Medicine hold up, it’s more like 60 percent.  A colonoscopy is still excellent for a screening test, but far from perfect, we discover.

The investigators did a case-control study.  That’s when you find people with a diagnosis and work your way backwards. In this case, the researchers found people with colorectal cancer and looked back to see whether they’d had a normal colonoscopy six months to seven years before their diagnosis. They compared them to similar people who didn’t have colorectal cancer.

What the investigators found surprised them so much, they asked that the data be looked at a second time.

Overall, colonoscopies helped prevent colorectal cancer, but the odds of helping were very different depending on the cancer’s location. (Scroll down for a video on the study.)

The colon is shaped roughly like an upside-down U.  The right side connects to the bottom of the small intestine and continues up, across, and down the left side, to connect to the rectum.  The data revealed the odds of preventing left-sided colon cancer by doing a colonoscopy were about 70 percent.  But odds of preventing cancer on the right side were no better than those who did not have the colonoscopy.

According to this study, a colonoscopy did not improve your odds of detecting cancer on the right side of your colon a bit. Zilch.

Why was this? The investigators concocted three nonexclusive theories.

  1. Some of the doctors may have been unable to complete the procedure all the way to the end of the right side.
  2. It’s more difficult to clean out all fecal material, and keep sticky mucus from accumulating back quickly, on the right side–therefore, more difficult to detect lesions.
  3. We know some colon cancers grow in a flatter, more difficult to see, pattern instead of the well-known stalk type.  Perhaps these are more prevalent on the right side.

Although this was a well-done study, the case-control type has a lot of weaknesses.  No doubt, someone will now do a stronger, prospective one in which people who have had normal colonoscopies will be studied in real-time to see how many develop colorectal cancer.

Although doctors don’t expect the virtual colonoscopy to be any better at detection, there are other new tests, such as one that detects cancer DNA in the stool, that might combine with the colonoscopy for better sensitivity.

No matter, please don’t use this study as an excuse not to get recommended screening.  It still saves lives. Just not as many as we thought.

Image courtesy, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health.
Video courtesy, American College of Physicians.

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4 Responses to “Colonoscopy prevents colon cancer? New study shows, not as well as we thought. (Video included.)”

  1. James Hubbard’s My Family Doctor Blog » Blog Archive » Hormones, breast cancer and other medical misconceptions Says:

    [...] HomeMedical TopicsAsk the Doctor!Meet the ExpertsRecipesDiscussionsBlogVideos Home » Blogs « Colonoscopy prevents colon cancer? New study shows, not as well as we thought. (Video included.) [...]

  2. Tom Rooney Says:

    Dr, Hubbard,
    This is an unbelievable study. I’ve now had 2 of these procedures in my 56 years and not being 100% sure seems a little scary.

    Tom Rooneys last blog post..Laughing For Your Health

  3. Other Blogs of Note This Week Says:

    [...] look at My Family Doctor – Colonoscopy prevents colon cancer? New study shows, not as well as we thought. (Video included.) – Pretty much self explanatory from the title, Dr. Hubbard gives a peak into the drawbacks of this [...]

  4. James Hubbard’s My Family Doctor Blog » Blog Archive » Thank you! (Plus, find new health, mom and baby-boomer blogs) Says:

    [...] HomeFitnessBody.com: “Other Blogs of Note This Week” (linked to blog post about colonoscopy study) [...]

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