NATIONAL HARBOR, MD. – Colonoscopy led to significant reductions in colorectal cancers located in both the distal colon and proximal colon, based on a large analysis of Medicare data from 1998 to 2002.
The finding that colonoscopy provided protection in the proximal colon is important because "several recent studies [have] yielded mixed results on the protective effect of colonoscopy in the proximal colon," said Dr. Yize Wang of the Mayo Clinic in Jacksonville, Fla.
To assess the impact of colonoscopy and flexible sigmoidoscopy on colorectal cancer risk, Dr. Wang and colleagues analyzed cancer rates in adults aged 67 to 80 years. The study group included 12,266 patients who had an outpatient colonoscopy, 6,402 who underwent sigmoidoscopy, and 41,410 individuals who did not have a screening procedure and served as controls.
Patients with inflammatory bowel disease, a history of polyps, or a history of colorectal cancer were excluded. The study patients were followed until the end of 2005, or until a diagnosis of colorectal cancer, or death.
A total of 58 colorectal cancers (CRCs) were diagnosed in the colonoscopy group during the follow-up period, compared to 66 CRCs in the patients who underwent flexible sigmoidoscopy and 634 CRCs in the control group.
In a multivariate analysis, colonoscopy was associated with a significant 73% reduction of distal colorectal cancer (hazard ratio, 0.27) and a significant 54% reduction of proximal colorectal cancer (HR, 0.46), compared with unscreened controls.
Sigmoidoscopy was associated with a significant 60% reduction of distal colorectal cancer (HR, 0.40), but no significant reduction in proximal colorectal cancer, compared to unscreened controls; this was expected because the procedure does not examine the entire colon.
A total of 771 (12%) of the patients who first underwent sigmoidoscopy went on to have a colonoscopy within 12 months. When the investigators excluded data from these patients, who had sigmoidoscopy followed by colonoscopy, the study results on cancer risk were essentially unchanged.
The findings were limited by the retrospective nature of the study, as well as the confounding effect of the diagnostic procedure chosen and the selection bias in the use of screening endoscopy, Dr. Wang said.
In addition, the study "does not reflect recent advancements in endoscopy equipment and techniques," he noted.
However, the results from this large study population confirm that colonoscopy remains the preferred screening test for colorectal cancer, said Dr. Wang.
The study was funded by a research grant from the American College of Gastroenterology. The researchers said they had no financial conflicts to disclose.