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Regular colonoscopies may cut colorectal cancer risk by 40 percent

Colonoscopies save lives, but they're not perfect 02:07

Forty percent of all colorectal cancers could be prevented if people underwent regular colonoscopy screenings, according to a new study from the Harvard School of Public Health.

According to the National Cancer Institute, 102,480 people will be diagnosed with colon cancer in the U.S. this year, and another 40,340 will learn they have rectal cancer. About 50,830 people are expected to die from colorectal cancer this year, making it the second-leading cause of cancer deaths in the U.S.

Colorectal cancer usually starts from abnormal growths known as precancerous polyps, so experts recommend that people get screened regularly. One way to get examined for the disease is to have a colonoscopy. During the procedure, doctors use a tool called an endoscope with a tiny camera attached to take a look at the entire large intestine and part of the small intestine. The patient often has to be sedated. Doctors can remove any questionable polyps if they see them during the exam.

"Colonoscopy is the most commonly used screening test in the U.S., but there was insufficient evidence on how much it reduces the risk of proximal colon cancer and how often people should undergo the procedure," co-senior author Dr. Shuji Ogino, associate professor in the Department of Epidemiology at Harvard School of Public Health, said in a press release. "Our study provides strong evidence that colonoscopy is an effective technique for preventing cancers of both distal [left] and proximal [right] regions of the colorectum."

Furthermore, the study found that a less-invasive alternative to colonoscopy -- a sigmoidoscopy -- is not as effective at preventing cancer in the proximal or right side of the colon.

The Centers for Disease Control and Prevention recommends that people begin getting colonoscopies soon after they turn 50, and continue to do so every 10 years. Those with a history of colorectal polyps or other bowel issues may need to be screened before age 50. For sigmoidoscopies, the recommendation is every one to five years.

CBS News chief medical correspondent Dr. Jon LaPook, who is also a gastroenterologist, says it's important that doctors take their time and look carefully for abnormalities while performing a colonoscopy. Potentially dangerous polyps can be easy to miss if they lie flat against the wall of the colon.

For the latest study, Harvard researchers looked at data from 88,902 patients who were involved in two long-term studies. The participants filled out questionnaires every two years between 1988 and 2008 about their health and whether they had undergone colonoscopy and sigmoidoscopy procedures.

During the study period, 1,815 people developed colorectal cancer and 474 died from the disease.

Researchers discovered that people who underwent colonoscopies or sigmoidoscopies as recommended had a reduced risk of getting colorectal cancer or dying from it.

Colonscopies had a stronger link when it came to lower incidence and cancer deaths. Only colonoscopies were linked to a decreased risk of getting cancer that started in the proximal colon, the upper part of the colon that includes the cecum, appendix, ascending colon, hepatic flexure, transverse colon and splenic flexure. Sigmoidoscopy was not associated with any reduced risk of upper colon cancers.

Ogino told that doctors have been seeing an increase in the rate of proximal cancers. He said people are living longer, and the disease tends to be a cancer that affects the older population.

"Sigmoidoscopies can prevent distal colon cancer but not proximal cancers," he explained. "If you want to be on the safe side, I would recommend a colonoscopy."

The researchers concluded that 40 percent of colorectal cancers -- including 61 percent of distal cancers and 22 percent of proximal cancers -- could have been prevented with routine screening.

Most people whose colonoscopies came up negative had a significant reduction in their risk of getting colorectal cancer for up to 15 years after the procedure. However, the researchers did find that people who had a history of adenoma (a benign tumor that originates from a gland that can become cancerous over time) or a family history of the disease should be screened more often.

In addition, colorectal cancers diagnosed within five years of a colonoscopy were shown to have a different molecular structure compared to colorectal cancers diagnosed more than five years after the procedure. The older cancers were typically harder to detect or remove.

If you want to prevent colorectal cancer, Ogino suggested avoiding obesity and exercising. Eating sweets and sugary snacks was linked to an increased risk of colorectal cancer in a July study. Other research has shown that exercise can help lower the risk of getting several obesity-related cancers, including cancers of the esophagus, pancreas, colon, breast (after menopause), endometrium (lining of the uterus), kidney, thyroid and gallbladder.

Eating more vegetables, cutting down on red meat and avoiding drinking or smoking was also shown to decrease risk of colorectal cancers, Ogino said.

One more thing that might protect you from getting colorectal cancer is to get some vitamin D, which you can get from sun exposure. "I would recommend going to the beach in the summer," Ogino said with a chuckle.

The study appears in the Sept. 19 edition of the New England Medical Journal.

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