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One-time screening with flexible sigmoidoscopy reduced colorectal cancer (CRC) incidence and mortality by about 25% over two decades, according to long-term follow-up of a randomized U.K. study.

Among individuals ages 55 to 64 who said they would undergo sigmoidoscopy screening if invited, those invited to screening had a 24% lower incidence of CRC over more than 21 years compared with those who received no further contact (3.18% vs 4.16%; HR 0.76, 95% CI 0.72-0.81), reported researchers led by Kate Wooldrage, MSc, of Imperial College London.

Furthermore, CRC mortality was reduced by a relative 25% in the screening group (0.97% vs 1.33%; HR 0.75, 95% CI 0.67-0.83). Reductions in incidence and mortality were seen both in men and women, and regardless of age, the findings in showed.

"Continued protection against colorectal cancer into the period of life of highest colorectal cancer risk (age ≥80 years) brings great benefit to patients, particularly as life expectancies are longer and colorectal cancer treatment in older patients is more complex," the authors wrote. "That a one-time screening procedure could have such a lasting impact on public health over 21 years is impressive."

"These results contribute important data to inform colorectal cancer screening guidelines and cost-effectiveness analyses, helping to provide more efficient use of health resources and greater benefits to patients," they added.

The U.S. Preventive Services Task Force (USPSTF) lists sigmoidoscopy in its current CRC screening recommendations as one of the three options for direct visualization, along with colonoscopy and CT colonography. Of these options, colonoscopy every 10 years or CT colonography every 5 years have greater estimated life-years gained compared with flexible sigmoidoscopy every 5 years, USPSTF states.

Sigmoidoscopy is less invasive than colonoscopy -- but not widely used for CRC screening in the U.S. -- since it is limited to the rectum, sigmoid colon, and descending colon. It takes less time to perform, usually doesn't require anesthesia, and is less likely to result in damage to the colon or rectum.

The study from Wooldrage and co-authors -- the U.K. Flexible Sigmoidoscopy Screening Trial -- was a randomized controlled trial that began in 1994, recruiting 170,432 adults ages 55 to 64 years who said they would undergo sigmoidoscopy screening if invited. Of those, 57,237 were randomly invited to undergo screening (40,624 actually did) and 113,195 were randomly assigned to a control group. Researchers performed intention-to-treat analyses for the primary endpoints of CRC incidence and death.

Wooldrage and colleagues found a greater reduction in CRC incidence in men (HR 0.70, 95% CI 0.65-0.76) than in women (HR 0.86, 95% CI 0.79-0.93).

They also found that the benefit in reducing CRC incidence and mortality was limited to the distal colon (incidence HR 0.59, 95% CI 0.54-0.64; mortality HR 0.55, 95% CI 0.47-0.64), with no benefit for cancers in the proximal colon.

In an , Michael Bretthauer, MD, PhD, and Nastazja Pilonis, MD, PhD, both of Oslo University Hospital in Norway, wrote that while relative reductions of risk with cancer screening "make compelling media headlines, absolute risk data are necessary to make informed decisions about screening on an individual and society level."

They noted that the absolute risk reduction in terms of CRC incidence was just 0.98% in the study, with a smaller effect in terms of mortality, 0.36%. "It is an individual choice whether these benefits merit being screened," they wrote.

Bretthauer and Pilonis pointed out that sigmoidoscopy was first introduced as a triage screening test.

"The expectation was that sigmoidoscopy would find people with polyps in the rectum and sigmoid, and the assumption was that these people would have a high risk for proximal lesions," they said, adding that colonoscopy would be performed on those persons, but not on those with a negative sigmoidoscopy.

The lack of benefit for cancers of the proximal colon "indicates that sigmoidoscopy might not be a good triage screening strategy," the editorialists wrote.

And while this study established a benefit of sigmoidoscopy screening on distal CRC, Bretthauer and Pilonis suggested that sigmoidoscopy is "outdated and should not be reintroduced," adding that "colonoscopy might be a better replacement, but long-term benefits remain to be discovered."

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