1236 GMT February 24, 2020
These findings, from the large Women's Health Initiative, reinforce the results of a 1994 study in the Journal of the National Cancer Institute, the authors write, Reuters Health reported.
"This is not the first study to examine this relationship, but it is one of the larger to date, if not the largest study," said lead author Jennifer L. Beebe-Dimmer of Karmanos Cancer Institute in Detroit.
Cancer is a disease of the DNA, she said, and family clustering indicates that breast and prostate cancers may have genes in common, she said.
Beebe-Dimmer and her colleagues used data for more than 78,000 women in the Women's Health Initiative who were over age 50 and cancer-free when the study began in 1993. At the start they had comprehensive physical exams and gave detailed personal and family medical histories.
Most women remained in the study for more than 10 years.
By 2009, there had been 3,506 new breast cancers in the original group.
Overall, more than 11,000 women had a first-degree relative — mother, sister or daughter — with breast cancer, and this was more common for those who were eventually diagnosed themselves. Twenty percent of women with breast cancer had first-degree relatives with the disease, compared to nearly 15 percent of those who did not develop breast cancer.
There was a similar, but very slight, association with prostate cancer, the researchers reported in Cancer.
More than 11 percent of women who developed breast cancer reported a first-degree relative with prostate cancer, compared to about 10 percent of women without the disease. Having a father, brother or son with prostate cancer increased the risk of breast cancer by about 14 percent.
Compared to women with no family history of breast or prostate cancer, those with a family history of both were 80 percent more likely to develop breast cancer, the authors found.
"We know that the major breast cancer susceptibility genes BRCA1 and BRCA2 are also linked to prostate cancer," Beebe-Dimmer told Reuters Health by email. That may explain some of the clustering, she said.
Researchers have been reporting on familial links between breast and prostate cancer for 40 years, said Mary-Claire King of the University of Washington School of Medicine in Seattle, in email to Reuters Health.
"It is good to see the link confirmed" in the Women's Health Initiative, said King, who was not involved in the new research.
"Both of these cancers are relatively common, so that it is possible when cancers are diagnosed in multiple family members it may be due to chance," Beebe-Dimmer said. "It may also be an exposure to something in the environment."
The decision to increase breast cancer screening will depend on how many male relatives have been diagnosed with prostate cancer and at what age, she said, with more diagnoses at young ages being particularly telling.
"Knowledge of breast cancer family history is still extremely important," Beebe-Dimmer said. She would not recommend BRCA1 or 2 genetic testing for women with a family history of prostate cancer but no history of breast or ovarian cancer.