The findings, however, raise concerns about a move by some doctors toward less frequent screenings for colon cancer, the nation's third leading cause of cancer deaths.
The study looked at sigmoidoscopy, in which a flexible, lighted tube is inserted in the rectum to examine about half of the large intestine, where most dangerous tumors form. Medical guidelines recommend this exam every five years after age 50 along with a yearly test for blood in the stool.
Recent studies have shown that a more thorough and costly test, the colonoscopy, does a superior job of detecting tumors because it examines the entire large intestine. It is generally recommended every 10 years after age 50, and some doctors say this alone should be the routine screening method.
The new study involved 9,317 men and women with an average age of about 66 who had a repeat sigmoidoscopy exam three years after receiving normal results. The findings appear in Wednesday's Journal of the American Medical Association.
Cancer or precancerous tumors called adenomas were found in 292 people, or 3.1 percent. Seventy-two people had advanced adenomas - tumors that had grown to a potentially dangerous stage - and six had full-fledged cancer, for a combined rate of 0.8 percent.
That rate is similar to previous estimates and is not high enough to warrant changing the current recommendations for sigmoidoscopy, said lead author Dr. Robert Schoen of the University of Pittsburgh.
But he said the study raises serious concerns about switching solely to colonoscopies every 10 years. The findings suggest potentially dangerous tumors would be found in at least 2.5 percent of people returning for a repeat colonoscopy after 10 years.
In an accompanying editorial, Dr. Robert Fletcher of Harvard Medical School agreed that the results do not support changing current recommendations for five-year intervals between normal sigmoidoscopies - partly because the exams themselves carry a slight risk of perforating the colon and also because not all precancerous tumors will develop into cancer.
By Lindsey Tanner