Women with a history of breast cancer are not at greater risk for developing colorectal cancer than women in the general public, according to a study headed by a researcher at the Johns Hopkins School of Public Health. In fact, some women with breast cancer may be less likely to develop colorectal cancer compared to women who have not been diagnosed with breast cancer. The findings, reported in the March 16 issue of The Lancet, challenge the notion that breast cancer increases the risk of colorectal cancer.

Breast cancer history should not be thought of as a risk factor for colorectal cancer, says Craig J. Newschaffer, Ph.D., lead author of the study and assistant professor of epidemiology at the Johns Hopkins School of Public Health. Overall we found that women with breast cancer were five percent less likely to have colon cancer and 13 percent less likely to develop rectal cancer compared to women in the general public, adds Dr. Newschaffer.

For the study, Dr. Newschaffer and his colleagues used the Surveillance Epidemiology and End Result (SEER) database to select nearly 227,000 women who were diagnosed with first incident breast cancer between 1974 and 1995. The researchers then searched the database for cases of colon and rectal cancer among the women with breast cancer and compared them to women in the general public. The comparisons were then broken down by the womans age at the time of breast cancer diagnosis, development stage of the cancer, follow-up care after diagnosis, and ethnic background.

Researchers found a slight overall decline in colorectal cancer cases among women with breast cancer compared to the general public, but the decline was most evident among women who were diagnosed with breast cancer after age 65, women who were diagnosed with breast cancer at its earliest stage of development, women who were white, and women diagnosed in the later years of the study from 1990 to 1994.

We are not sure why we saw a reduced risk of colorectal cancer among women with breast cancer. One possibility is that some of these women may receive more thorough medical care or make beneficial lifestyle changes after being diagnosed with breast cancer, which reduces their risk. Another possibility is that these women were exposed to factors that increased their risk of breast cancer but protected against colorectal cancer, explains Dr. Newschaffer.

Dr. Newschaffer cautions doctors and patients not to misconstrue the findings of the study. He warns that colorectal cancer is often deadly and is the third most common non-skin malignant disorder among women.

Breast cancer does not provide immunity from colorectal cancer. Women with breast cancer should continue to be screened for colorectal cancer and make lifestyle changes to reduce their risk, just like everyone else, adds Dr. Newschaffer.