Colonoscopy effectively screens for both left- and right-sided colorectal cancer, study finds

Screening colonoscopy helps reduce incidence of both left- and right-sided colorectal cancer, according to a recent study.

Clinical studies have shown that screening colonoscopy helps reduce incidence of colorectal cancer by detecting precancerous lesions, but the test's effectiveness in community settings has been less clear, especially for right-sided disease. German researchers performed a population-based case-control study to examine the association between previous colonoscopy and colorectal cancer risk. Data were collected via the medical record and self-report on lifetime history of risk factors and preventive factors for colorectal cancer. The researchers calculated the odds ratios for colorectal cancer associated with undergoing colonoscopy in the previous 10 years, adjusted for several factors including sex, age, and family history. The study appears in the Jan. 4 Annals of Internal Medicine.

Overall, researchers gathered data on 1,688 case-patients who had colorectal cancer (41.8% women) and 1,932 controls (42.7% women), all of whom were age 50 or older (mean age, 69 and 70 years, respectively). Those who had had colonoscopy in the previous 10 years had a 77% lower risk for colorectal cancer. Adjusted odds ratios were 0.23 (95% CI, 0.19 to 0.27) for any colorectal cancer, 0.44 (95% CI, 0.35 to 0.55) for right-sided colorectal cancer and 0.16 (95% CI, 0.12 to 0.20) for left-sided colorectal cancer. Colonoscopy was associated with a strong reduction in risk for all stages of cancer and for all ages, with the exception of right-sided colorectal cancer in those 50 to 59 years of age. Risk reduction was greater among those 70 and older for right-sided but not left-sided disease.

The authors noted that their results could have been affected by residual confounding and selection bias because of the study's observational design. However, they concluded that colonoscopy, followed by polypectomy, is associated with substantial reduction in risk for colorectal cancer in the community. Although risk reduction in their study was strongest for left-sided disease, it was also significant for right-sided disease at more than 50%.

An accompanying editorial pointed out that while some factors related to colonoscopy's effectiveness, such as patients' age and sex, can't be changed, others, such as laxative preparation, can be controlled. More research should be done to determine why colonoscopy is not as effective in the right colon, the editorialist noted, but "it would be a mistake to conclude that 'less effective' is the same as 'ineffective.'" The current study "offer[s] reassurance that colonoscopy can provide substantial protection against right- and left-sided [colorectal cancer]," the editorialist concluded.