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Women whose father, brother or son have had prostate cancer may have a 14 percent higher risk of developing breast cancer, said Jennifer Beebe-Dimmer, a researcher at the Barbara Ann Karmanos Cancer Institute at Wayne State University in Detroit.
Those women with a family history of both prostate and breast cancer were at a 78 percent greater risk of developing breast cancer, the researchers found. And the risk was greater for black women than whites.
The researchers found a link, not a direct cause-and-effect relationship, between family history of prostate cancer and women's breast cancer risk. However, this is "one of the largest studies, if not the largest study, to examine the association," Beebe-Dimmer said.
The 14 percent increase "is a modest increase in risk," she said. Of the two, "breast cancer family history is likely still more important in terms of risk assessment," she added.
The findings underscore the need for women to know their complete family medical history, "particularly cancer diagnosed among first-degree relatives," including fathers, brothers and sons, Beebe-Dimmer said.
Doctors should ask about all cancers in the family, even in members of the opposite sex, she suggested.
"Communication of this information to the physician is important in assessing future risk of breast cancer and may impact screening recommendations," Beebe-Dimmer said.
For the study, published online March 9 in the journal Cancer, Beebe-Dimmer's team evaluated more than 78,000 women enrolled in the Women's Health Initiative Observational Study between 1993 and 1998. At the study start, all were free of breast cancer. When follow-up ended in 2009, more than 3,500 breast cancer cases had been diagnosed. The researchers looked to see which patients' family members had either breast or prostate cancer.
Cancers of the breast and prostate are among the most common invasive cancers found in the United States, Beebe-Dimmer said. In 2014, an estimated 233,000 new breast cancer cases and 233,000 new prostate cancer cases were diagnosed in the United States, according to background information in the study.
Beebe-Dimmer can't say for sure what may be driving the potential link between prostate and breast cancers. "It may be genetic, it may be shared environment," she said. The two cancers have similar causes in that both are driven by exposure to the sex steroid hormones estrogen and testosterone, she explained.
Dr. Courtney Vito, assistant professor of surgical oncology at City of Hope Cancer Center in Duarte, Calif., was not involved with the study. She said the "article raises an interesting point."
Some of the possible increased risk, Vito said, may be due to the presence of the BRCA1 and BRCA2 gene mutations. However, that information was not available to the researchers. Men with BRCA1 and 2 mutations have a higher risk of prostate cancer, and women with the mutations have higher risks of breast and ovarian cancer, researchers have found.
These mutations "may explain some of this relationship but not all," Vito said.
Discoveries about new genes that may function in concert with the BRCA genetic mutations might shed more light on the possible link, she said.
Vito and Beebe-Dimmer agreed that more study is needed to figure out to what extent genes and shared environment contribute to risk for both breast and prostate cancer.
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