Thoughts on screening: Colonoscopy vs. Sigmoidoscopy

An article in Medscape Medical News highlights a study in which researchers compared two popular screening options: the colonoscopy and the sigmoidoscopy.

The article, titled "Colonoscopy linked to Lower Risk for Proximal Colon Cancer," says that the risk for colon cancer in both the left and ride sides of the colon dropped after older patients had a colonoscopy.

Flexible sigmoidoscopy was associated with a lower risk of colon cancer on the left side only, researchers from the Mayo Clinic said.

The study was presented recently at the American College of Gastroenterology 2011 meeting.

The article continues:

"This finding is the latest salvo in the ongoing battle to determine the best way to screen for colon cancer. The study’s goal was to determine whether risk for colorectal cancer fell more after colonoscopy compared with flexible sigmoidoscopy or no lower endoscopy among older patients.

"I think that the most surprising part is that we actually show for colonoscopy that there is risk reduction for the proximal colon," Yize Richard Wang, MD, PhD, a gastroenterology fellow at the Mayo Clinic in Jacksonville told Medscape Medical News. "Previous studies don't show that."

The analysis draws on Medicare patient data from 1998 to 2005. The results showed that upon follow-up, colorectal cancers were found in:

  • 0.5% of the 12,266 patients who initially underwent colonoscopy;
  • 1.0% of the 6402 patients who initially underwent flexible sigmoidoscopy; and
  • 1.5% of the 41,410 patients in the control group, who did not undergo screening lower endoscopy.

"We do believe colonoscopy is still the preferred method to screen and prevent colon cancer," said Dr. Wang, who was the study’s lead author.

For the study, Dr. Wang and colleagues reviewed a sample of the national Surveillance, Epidemiology, and End Results (SEER) database for patients aged 67 to 80 years at first outpatient colonoscopy or flexible sigmoidoscopy between 1998 and 2002.

They excluded patients with inflammatory bowel disease, history of polyps, or a family history of colorectal cancer. The study period was limited to allow a minimum of 3 years of follow-up after colonoscopy.

The study sample was composed of patients who had a first colonoscopy, flexible sigmoidoscopy, or no lower endoscopy screening. About one third of the flexible sigmoidoscopies and two thirds of the colonoscopies were performed by gastroenterologists. All patients were followed until colorectal cancer was diagnosed, death, or December 31, 2005.

The study is the first based on US data on screening colonoscopies, Dr. Wang said. In 1998, Medicare began covering such colonoscopies for high-risk patients. In 2001, it added coverage for screening of average-risk patients. Potential inclusion of diagnostic colonoscopy probably biased results toward negative findings (ie, no risk reduction associated with colonoscopy), Dr. Wang told Medscape Medical News.

Earlier studies on the relationship of colonoscopy to colon cancer reduction had mixed results, Dr. Wang said. Studies from Canada have indicated that colonoscopy reduces colon cancer by 40% — or the same rate as sigmoidoscopy — and suggested that colonoscopy did not reduce deaths from right-sided colon cancers.

Dr. Wang noted that the Canadian data were based on diagnostic, not screening, colonoscopies. "So these patients who had diagnostic colonoscopies may be at a higher risk of cancer in the first place," he said. For diagnostic colonoscopies, there was protection against distal colon cancer but not proximal colon cancer.

---Andrea Uhde Shepherd, Executive Director

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