Colonoscopy Fails to Identify Many Colorectal Cancers

Colonoscopy, a standard screening method for colorectal cancer, misses the majority of cancers on the right side of the colon and about one-third of cancers on the left side of the colon, according to the results of a study published in the Annals of Internal Medicine.[1] The researchers estimated that the screening method may reduce colorectal cancer mortality by 60-70%, rather than 90% as previously estimated.

Colorectal cancer is the second leading cause of cancer-related deaths in the United States. A colonoscopy is a procedure that involves the insertion of a lighted tube through the large intestine to allow a physician to examine the entire colon for abnormal areas. A colonoscopy is the preferred method for detecting early colon cancer and is recommended every 5-10 years beginning at age 50 for average-risk individuals.

Despite these recommendations, however, the effectiveness of colonoscopy in preventing colorectal cancer deaths remains unclear. Researchers in Canada conducted a study to determine the association between colonoscopy and reduction in deaths from the disease. The study included 10,292 case patients who died of colorectal cancer matched to 51,460 control patients who did not. The researchers identified the case patients through the Ontario Cancer Registry and the controls through the Registered Persons Database. Control subjects were matched to case patients on a 5:1 ratio based on age, gender, and socioeconomic status. In total, 7% of case patients had undergone colonoscopy compared with 9.8% of control subjects.

The results of the analysis indicated that colonoscopy missed about one-third of colorectal cancers on the left side of the colon and nearly all of the cancers on the right side of the colon. The reasons for this discrepancy are unclear. Some research has indicated that serrated polyps tend to grow in the right side of the colon and that these types of polyps develop into cancer much more quickly than other polyps. Furthermore, flat and indented polyps tend to cluster in the right colon. As a result colonoscopy might never be accurate in diagnosing cancers on the right side of the colon.

While researchers have long estimated that colonoscopy reduces colorectal cancer deaths by 90%, the results of this analysis indicate that the reduction might be closer to 60%. More research is needed to understand the implications of this analysis. In the meantime the researchers caution that the results of this study should not change the colonoscopy screening recommendations. A 60% reduction in mortality is still an important benefit; in comparison, mammography reduces breast cancer mortality by only 25%.

Research will likely be ongoing to further evaluate the benefits and shortcomings of colonoscopy.


[1] Baxter NN, Goldwasser MA, Paszat LF, et al. Association of colonoscopy and death from colorectal cancer: A population-based, case-control study. Annals of Internal Medicine. 2009;150:1-8.

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