The findings, which come from a study of more than 227,000 women diagnosed with breast cancer, are somewhat surprising because experts have long put breast cancer on a list of risk factors for colon and rectal cancer, right alongside known risk factors like having a first-degree relative with colon or rectal cancer and having a history of polyps.
"In this analysis, clearly breast cancer isn't a risk factor -- and if anything, there is the suggestion that there is decreased risk in certain subgroups," says Craig Newschaffer, PhD, of Johns Hopkins School of Hygiene and Public Health in Baltimore. He is the lead author of the study, which appears in the March 17 issue of the British medical journal The Lancet.
Newschaffer and colleagues found that women diagnosed with breast cancer between 1974 and 1995 had a 5% lower risk of colon cancer and a 13% lower risk of rectal cancer when compared with women in the general population who had never had breast cancer.
While the researchers don't know for sure how breast cancer might protect against colon and rectal cancer, they say it could have to do with the fact that women who have had breast cancer are monitored more closely for other problems. Another possibility is that after having survived breast cancer, women may make healthy lifestyle choices like eating better and exercising -- which are just the things that can protect from colon and rectal cancer later in life.
Subscribing to this line of thought is Theodora Ross, MD, PhD, a professor of medicine at the University of Michigan Comprehensive Cancer Center in Ann Arbor, who assessed the study for WebMD.
"These patients are doing something right," she says, but adds that women with family histories of colon or rectal cancer and those with familial breast cancer -- which involves genetic mutations -- are probably at higher risk than other breast cancer patients for colon and rectal cancer and should be monitored closely.
Newschaffer agrees that women should not assume that because they have had breast cancer they're off the hook.
"I don't want to convey the message that breast cancer implies immunity from colon and rectal cancer," he says. "Breast cancer survivors should be just as vigilant as all women in pursuing healthy lifestyles that are associated with colon cancer prevention and in following the recommendations for colorectal screening."
Colorectal cancer is the third most common nonskin cancer among women in the U.S. Approximately 40% of women diagnosed with the disease will die from it.
The American College of Gastroenterology says people with an average risk for colon and rectal cancer should have a colonoscopy to check the entire length of the colon for cancer every 10 years after age 50. Other tests that can detect the disease include a stool test to look for evidence of bleeding and a flexible sigmoidoscopy, which lets doctors get a close look at the rectum and sigmoid colon.