The incidence of colon cancer, declining since the mid-1980s, plunged a further 30 percent last decade among Americans 50 and older as more people had colonoscopies, a new study found.

The drop in colon-cancer death rates accelerated as well, falling about 3 percent a year between 2001 and 2010, compared with 2 percent a year in the previous decade, according to the American Cancer Society study of government data.

American Cancer Society Chief Medical Officer Otis Brawley estimated that without the widespread screening efforts that began in the 1980s, "we'd be seeing twice as many deaths today. This study celebrates the fact that we've almost halved the mortality rate from colon cancer in the last 35 years."

Still, colon cancer remains the third-most-common cancer and the third leading cause of cancer deaths in the U.S. More than 136,000 new cases, and 50,000 colon-cancer deaths, are expected this year.

The study found the positive trend was most pronounced among older Americans. The rate of colon cancers among those 65 and over dropped about 7 percent a year from 2008 to 2010. Meanwhile, the percentage of Americans who are up-to-date on recommended colon-cancer screening rose from 55 percent to 65 percent during the past decade.

Screening tests for breast and prostate cancer have come under fire in recent years for overdiagnosing malignancies—that is, finding a significant number of early cancers that would never cause harm if left untreated, leading to unnecessary treatment. Colon-cancer screening is less controversial.

"With colon cancer, it's not so much screening to find early cancers but screening to find polyps and remove them, which prevents cancer," said James Church, a colorectal cancer surgeon at the Cleveland Clinic, who wasn't involved in the new study.

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