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Adapted from the NCI Cancer Bulletin, vol. 6/no. 1, January 13, 2009 (see the current issue ²).

In a large case-control study performed by researchers at the University of Toronto, patients who had undergone colonoscopy at least once were significantly less likely to die of colorectal cancer (CRC) arising in the left side of the colon (the part of the colon closest to the rectum) than patients who had never had a colonoscopy. However, colonoscopy did not reduce mortality from CRC in the right side of the colon, according to results published in the December 15, 2008, Annals of Internal Medicine (see the journal abstract ³).

Using four Ontario-based databases, the investigators matched 10,292 patients who had died of CRC with 51,460 controls who did not have CRC. Each patient was matched to five controls by age, sex, income, and location of residence, and all controls were alive at the date of the patient’s death.

Because the investigators could not distinguish screening colonoscopies from diagnostic colonoscopies using the database records, they excluded all colonoscopies done within six months before a diagnosis of CRC.

Seven percent of case patients and almost 10 percent of controls had undergone colonoscopy. “Case patients were less likely than controls to have undergone any attempted colonoscopy…or complete colonoscopy,” stated the authors. Both attempted and complete colonoscopy reduced the likelihood of death from left-sided CRC, but not right-sided CRC.

The reasons for the lack of efficacy in preventing right-sided CRC are not clear, but may include less effective bowel preparation or colonoscopy technique in the right colon or differences in the biology and natural history of cancer arising in the right colon, explained the authors.

These results “seem consistent with a CRC mortality risk reduction of about 60 percent to 70 percent for the left colon and highlight the fact that we have very little data…about efficacy in the right colon,” said Dr. David Ransohoff of the University of North Carolina at Chapel Hill, in an accompanying editorial ⁴.

Glossary Terms

A study that compares two groups of people: those with the disease or condition under study (cases) and a very similar group of people who do not have the disease or condition (controls). Researchers study the medical and lifestyle histories of the people in each group to learn what factors may be associated with the disease or condition. For example, one group may have been exposed to a particular substance that the other was not. Also called retrospective study.

Examination of the inside of the colon using a colonoscope, inserted into the rectum. A colonoscope is a thin, tube-like instrument with a light and a lens for viewing. It may also have a tool to remove tissue to be checked under a microscope for signs of disease.