New data from the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial give fresh insight into the appropriate screening intervals for colorectal cancer after a negative exam. The interim report published in the July 2 issue of the Journal of the American Medical Association* is the largest study to date of repeat sigmoidoscopy screening after an exam.

The current accepted interval for sigmoidoscopy, a technique in which the rectum and lower colon are examined with a lighted instrument called a sigmoidoscope, is 5 years after a negative exam. This recommendation is based primarily on indirect evidence. Exactly how often to repeat sigmoidoscopy is an evolving field of research. Whether data from the new study, which measures the incidence of growths or polyps three years after an initial exam, will play a role in changing the current five-year interval is not clear.

"We have very little observational data on what to expect on repeat screening after a negative exam," said Robert E. Schoen, M.D., MPH, from the University of Pittsburgh in Pittsburgh, Pa., and first author of the study. "Our data are the first representative estimate of what can be expected on repeat examination three years after a negative sigmoidoscopy in the general population."

In a study involving 9,317 participants, researchers measured the incidence of benign, precancerous, and cancerous growths in the lower 24 inches of the large intestine three years after an initial screening when no polyps or growths were found. The participants were screened using sigmoidoscopy. Those with a polyp or growth were referred to their physicians for further follow-up, usually a colonoscopy, a procedure in which the rectum and entire colon can be visualized and any growths removed for analysis.

The researchers found that 13.9 percent (1,292 out of 9,317) of the participants had a polyp or growth. The majority were benign. However, 2.3 percent of the participants (214 out of 9,317) had nonadvanced adenomas (precancerous lesions), and 0.8 percent (78 out of 9,317) had an advanced adenoma or cancer. These growths were present in the rectum or lower colon, which is the portion of the large intestine that can be visualized using sigmoidoscopy. A handful of previous smaller studies of repeat sigmoidoscopy ranging from approximately one to five years in selected populations reported an incidence rate for adenomas of around 5 percent to 6 percent, and an advanced adenoma rate of less than 1 percent.

Men and women in the PLCO trial were screened twice with sigmoidoscopy - at the initial visit and at either the third or fifth annual visit. Those enrolled before December 1995 received sigmoidoscopy at enrollment and three years later; those enrolled after December 1995 undergo sigmoidoscopy initially and five years later. The 9,317 participants in this study were screened three years after their initial screening.

Most public health organizations such as the U.S. Preventive Services Task Force, the American Cancer Society, and the American Gastroenterological Association recommend that men and women at average risk begin colorectal cancer screening at age 50. Approximately 75 percent to 80 percent of colorectal cancers occur among people at average risk. The tests recommended include:

  •  Fecal occult blood test (FOBT): A test used to check for hidden blood in the stool. Sometimes cancer or polyps can bleed, and FOBT is used to detect small amounts of bleeding. The recommended screening interval is every year.
  •  Sigmoidoscopy: An exam of the rectum and lower colon using a lighted instrument called a sigmoidoscope. Recommended screening interval is every five years.
  •  Colonoscopy: An examination of the rectum and entire colon using a lighted instrument called a colonoscope. The recommended screening interval is every 10 years.
  •  Double contrast barium enema (DCBE): A series of X-rays of the colon and rectum. The patient is given an enema with a solution that contains barium, which outlines the colon and rectum on the X-rays. The recommended screening interval is every five years.

People with a personal or family history of colorectal cancer or adenoma, or an illness predisposing them to colorectal cancer (e.g., inflammatory bowel disease), are considered at increased risk. They are advised to begin screening at an earlier age and may need to be tested more frequently. **

The recommended interval for colonoscopy screening is every 10 years after a negative exam. Although this technique is more sensitive than sigmoidoscopy for detecting cancer and large polyps, the costs are considerably higher and influence its use as a screening tool.

Data supporting a five-year interval for flexible sigmoidoscopy and a 10-year interval for colonoscopy come from several sources: case control studies showing that sigmoidoscopy was associated with a reduction in colorectal cancer deaths up to 10 years from the last screening exam; a few small studies of repeat sigmoidoscopy or colonoscopy ranging from one to five years after a negative exam that found few instances of advanced lesions; and studies on the natural history of polyps suggesting that it takes years for a polyp to evolve into cancer.

"Although the appropriate interval for repeat screening for sigmoidoscopy and colonoscopy is an evolving field of research, it is important that people at average risk undergo one of the recommended screening procedures beginning at age 50," said Peter Greenwald, M.D., Dr.P.H., director of the Division of Cancer Prevention at the National Cancer Institute (NCI) in Bethesda, Md. "Screening at an earlier age is recommended for those at higher risk."

The PLCO Cancer Screening Trial is a large randomized trial sponsored by NCI evaluating whether certain cancer screening practices reduce the number of deaths from prostate, lung, colorectal, and ovarian cancer. The 155,000 participants nationwide were randomized from 1993 through 2001, and will be followed for at least 14 years from the time they enrolled. Participants randomized for screening receive screening for three of the four cancers being evaluated, depending on gender. It will be years before researchers know if screening reduces cancer mortality among PLCO participants.

There is evidence from previous randomized trials that periodic fecal occult blood testing reduces risk of death from colorectal cancers by 15 percent to 33 percent. To date, there have been no completed randomized trials testing whether screening reduces the risk of colorectal cancer death for any of the other colorectal cancer screening tests.

**For further information on colorectal cancer screening guidelines, refer to two government Web sites:
Center for Disease Control: http://www.cdc.gov/cancer/colorctl/colorect.htm#prevention
U.S. Preventive Services Task Force: http://www.ahrq.gov/clinic/3rduspstf/colorectal/colorr.htm

For information about cancer, visit NCI's Web site at http://www.cancer.gov

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