Current guidelines for a common screening procedure for colorectal cancer may recommend testing more frequently than necessary, according to a new study from the Public Health Sciences Division.
Dr. Polly Newcomb and colleagues suggest that flexible sigmoidoscopy-a procedure that enables doctors to find and remove precancerous lesions in the lower half of the colon-could be given every 10 years or more instead of the currently recommended five-year screening interval. The researchers found the test offers a significant reduction in cancer incidence for 15 years after screening with little decrease in effectiveness over the time period.
Newcomb said the durability of the screening interval adds compelling support to existing evidence that sigmoidoscopy is an excellent cancer-screening tool. In an earlier study, she and colleague Dr. Barry Storer found evidence for similar reductions in death due to distal colon cancer.
"Sigmoidoscopy is a good screening test and these results make it an even better one," she said.
"People may not fully appreciate the difference between reducing incidence vs. mortality. This is a screening test that can actually reduce the risk of ever getting cancer in a most significant way, unlike mammography, which increases the probability of being diagnosed at an earlier stage."
The magnitude of risk reduction was substantial: Individuals who had ever had a screening sigmoidoscopy had a four-fold reduction in the incidence of cancer of the lower, or distal, colon compared with individuals who had never had the test.
The study did not focus on subgroups of people at higher risk for colorectal cancer than the general population, such as those with a family history of the disease. Such individuals would probably benefit from more frequent cancer-screening procedures and should consult their physician on the recommended interval for their special circumstances.
Reducing screening frequency could cut national yearly expenditures for the procedure and may encourage more people to undergo the test, which many avoid for fear of discomfort. The researchers suggest if the current proportion of U.S. adults over 50 who have a sigmoidoscopy every 10 years doubled, the incidence of cancers of the lower colon could be reduced by about 19,000 cases annually.
The study, funded by the National Cancer Institute, appears in the April 16 issue of the Journal of the National Cancer Institute. Co-authors include Storer, a biostatistician in the Clinical Research Division; Libby Morimoto, PHS graduate student; Allyson Templeton, PHS project manager; and Dr. John Potter, PHS division director.
Dr. Scott Ramsey, a physician and economist in PHS, said that in light of the new findings, screening by sigmoidoscopy about every 10 years may be more cost-effective than previously estimated.
"Screening efficacy translates into expected life years saved from screening, and Dr. Newcomb's study suggests we can get the same number of years of life saved with half as much screening," he said.
"Studies that have compared screening by colonoscopy every 10 years to screening by flexible sigmoidoscopy every five years have found that cost-effectiveness for the two procedures are about the same," he said. "These new findings push flexible sigmoidoscopy ahead of the pack in terms of value per screening dollar spent."
What's more, he said, less frequent screening with sigmoidoscopy could lead to more widespread use of the test.
"Compliance is perhaps the biggest issue for colorectal-cancer screening," he said. "People don't like the procedure, and in my practice, it is difficult to persuade them to repeat it in just five years. If guidelines were revised to recommend sigmoidoscopy once every 10 years rather than every five, compliance might improve."
To examine the impact of sigmoidoscopy screening on colorectal-cancer incidence, the researchers collected information on screening history and colorectal risk factors from interviews with a population-based sample of 1,668 patients with either distal (lower half) or proximal (upper half) colorectal cancer and 1,294 healthy individuals.
Compared with individuals who had never had a screening sigmoidoscopy, individuals who had ever had a screening sigmoidoscopy had a four-fold reduction in the incidence of distal colorectal cancers. These reductions appeared to be sustained for more than 15 years.
The study may have been limited by the fact participants self-reported their screening histories as well as the possibility that those who underwent screening may have had other risk-lowering behaviors, although Newcomb said that she and her colleagues did not find evidence that such factors impacted their results.
Randomized trials of sigmoidoscopy screening, in which individuals are randomly assigned to groups that do or do not undergo the test, would provide more definitive evidence of the procedure's efficacy. Although one such study is underway-the multi-site Prostate, Lung, Colorectal and Ovarian Screening Trial-it is examining the effectiveness of sigmoidoscopy screening only at three-or five-year intervals. The results of this study, available in 2013, will not evaluate the benefit of other screening intervals.
This year, about 105,500 Americans will be diagnosed with colon cancer and 42,000 with rectal cancer. Combined, they will cause about 57,100 deaths. About 60 percent of all colorectal cancers occur in the lower half of the colon, the region examined by flexible sigmoidoscopy. Unlike colonoscopy, which examines the entire large bowel for precancerous lesions, flexible sigmoidoscopy examines only the lower half of the colon and the rectum. Both procedures use a thin, flexible tube equipped with a tiny camera. Forceps can be passed through the tube to sample or remove abnormal growths for analysis.
Although many doctors view colonoscopy as the "gold standard" colorectal cancer- screening tool, cost and acceptability have limited its widespread use. Unlike sigmoidoscopy, colonoscopy requires that individuals be sedated, miss a day of work and have someone available to escort them home after the procedure.
Currently, only 34 percent of U.S. adults over ago 50 have had a sigmoidoscopy or colonoscopy within the past five years.
Both procedures alert doctors to the presence of precancerous growths known as adenomatous polyps, which, over time, can develop into an invasive cancer known as colorectal carcinoma. Newcomb's results confirm that such growths can take 15 or more years to develop into cancer.
"The large bowel is uniquely suited for early detection of precursor lesions and cancer because the natural history of the disease is so protracted," she said. "If screening by sigmoidoscopy every 10 years was widely adopted by adults over age 50, the incidence and mortality of colorectal cancer could be substantially reduced."
Recommendations for colorectal-cancer screening
The American Cancer Society suggests that men and women, beginning at age 50, follow one of the screening options below:
For the stool blood test, the take-home, multiple-sample method should be used. If something abnormal is found, a colonoscopy should be performed.