Colonoscopy less effective for cancers in the proximal colon
Patients who undergo a complete negative colonoscopy have a reduced incidence of colorectal cancer, according to a study published in the August issue of Clinical Gastroenterology and Hepatology. However, in the proximal colon, the incidence reduction of colorectal cancer following complete negative colonoscopy differs in magnitude and timing.

The reduction of colorectal cancer was observed in about half of the 14 follow-up years and for the most part occurs after just seven years of follow-up, according to the study authors.

The relative rate of colorectal cancer overall and the relative rate of distal colorectal cancer in the study group remained significantly lower than the control population. The relative rate of proximal colorectal cancer was significantly lower than the control population in half of the follow-up years, mainly after seven years of follow-up.

Using linked administrative databases, researchers identified a cohort of 110,402 Ontario residents aged 50 to 80 years old who had a negative complete colonoscopy between Jan. 1, 1992 and Dec. 31, 1997. Cohort members had no prior history of colorectal cancer, inflammatory bowel disease or recent colonic resection. Each individual was followed through Dec. 31, 2005 and the relative rate of overall colorectal cancer, distal colorectal cancer and proximal colorectal cancer was compared with the remaining Ontario population.

According to the study’s author Linda Rabeneck, MD, MPH, of the University of Toronto and Institute for Clinical Evaluative Sciences, the findings raise a question about the effectiveness of colonoscopy in usual clinical practice and suggest that the effectiveness of colonoscopy is reduced for cancers arising in the proximal colon.

“Whether this is due to colonoscopy quality, or whether it is due to tumor biology is the key issue that we need to address,” she noted.