About 1 in 3 adults in the United States between the ages of 50 and 75 -- about 23 million people -- have not been screened for colorectal cancer and are at a high risk of developing the malignancy, federal health officials reported Tuesday.

The probability of colorectal cancer increases with age, but about 28 percent of those who should be screened have never undergone an exam, according to the Centers for Disease Control and Prevention. Experts say fear over invasive testing procedures and popular myths about screening keep patients away.

Close to 150,000 people in the United States develop the potentially lethal cancer annually.

"Colorectal cancer is the second-leading cancer killer for both men and women," CDC director Dr. Thomas Frieden said in an interview Tuesday. "In fact, it's the leading killer of nonsmokers in this country."

About 50,000 people in the United States die annually of the disease, often needlessly, Frieden said.

The U.S. Preventive Services Task Force, a panel of medical experts who advise the federal government on preventive health measures, recommends regular screening for people 50 and older. Exams can be done via colonoscopy, sigmoidoscopy or fecal occult-blood test.

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Sigmoidoscopy examines only a quarter of the large intestine, however, said Dr. Bethany DeVito, a gastroenterologist at North Shore University Hospital in Manhasset.

The fecal test, DeVito added, spots blood in the stool invisible to the naked eye, but offers few assurances.

"The gold standard is the colonoscopy," said Dr. Dean P. Pappas, co-chief of colorectal surgery at Winthrop-University Hospital in Mineola.

In the CDC report, 62 percent of people who underwent screening chose colonoscopy.

"We have patients who have normal fecal occult-blood tests but still develop colon cancer," Pappas said.

Pappas said people often avoid getting checked because of screening stigmas.

He blamed three myths he wants dispelled: avoiding screening because of no family history of colorectal cancer; having no symptoms and having regular bowel movements.

"That trifecta makes people feel comfortable and it shouldn't," he said.

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Dr. Rajiv Datta of South Nassau Communities Hospital in Oceanside said he examined two patients Tuesday who had avoided screening because they didn't have symptoms. Changes in bowel habits may signal a problem, he said.

"Both ignored the symptoms until they saw blood in their stool," Datta said.

Many avoid screening, Datta said, because they dislike colonoscopy's prep procedure, referring to drinking a polyethylene glycol solution to clear the colon. And the screening itself, he said, is done under anesthesia, which may induce fear.

DeVito said colorectal cancer is the only malignancy in which screening offers an immediate cure. When a polyp is found during a colonoscopy, it can be removed on the spot, she said.