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A history of breast cancer does not increase the risk of colorectal cancer and, in fact, may actually reduce the risk for some women, according to the results of a study recently published in The Lancet.

Colorectal cancer is the second leading cause of cancer death in the United States. The disease strikes both men and women, with 130,000 cases diagnosed each year. Approximately 56,000 people die from colorectal cancer each year.

Historically, researchers have considered a history of breast cancer to be a risk factor for colorectal cancer despite a lack of supporting evidence. Research has long been focused on identifying risk factors for this disease and improving prevention strategies. About 75% of all new cases of colorectal cancer occur in people who have no known risk factors other than age.

Researchers from the Johns Hopkins School of Medicine and Public Health and the Jefferson Medical College recently conducted a study to evaluate the relationship between breast cancer and the subsequent development of colorectal cancer. The researchers used data from the Surveillance Epidemiology and End Results (SEER) database to estimate the risk of colorectal cancer in women who were diagnosed with breast cancer between 1974 and 1995. The study included 227,165 women. The researchers then searched the records for evidence of primary invasive colorectal cancers and compared the observed colorectal cancer risk with that expected in the general population.

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The results from the study indicated that women with previous breast cancer were 5% less likely to develop colon and 13% less likely to develop rectal cancer than women in the general population. The risk reductions were more significant for women who were 65 or older when diagnosed with breast cancer, white women, women with local stage breast cancer and women who were diagnosed with breast cancer during the latter portion of the study (1990-1994).

The researchers concluded that breast cancer does not increase the subsequent risk of colorectal cancer. The researchers were unable to explain the decreased risk of colorectal cancer in certain subgroups, but theorized that the risk reduction could be the result of exposures that increase breast cancer risk but protect against colorectal cancer. Despite the apparent reduced risk in some groups, all women are encouraged to undergo colorectal cancer screening and to avoid the risk factors for this disease. (The Lancet, Vol. 357, pp.837-840, 2001)