Women who have a first-degree relative with prostate cancer may be 14% more likely to be diagnosed with breast cancer after the age of 50, according to the research.
The research team, including Dr. Jennifer L. Beebe-Dimmer of the Barbara Ann Karmanos Cancer Institute and Wayne State University School of Medicine in Detroit, MI, publishes its findings in the journal Cancer.
Breast cancer is estimated to affect approximately 1 in 8 women in the US at some point during their lifetime. According to the American Cancer Society, around 231,840 new cases of invasive breast cancer will be diagnosed in the US this year, and more than 40,000 women will die from the disease.
It is well established that a family history is a significant risk factor for breast cancer, particularly for women who have a first-degree relative with the disease. Mutations in the BRCA1 or BRCA2 genes are the most common causes of hereditary breast cancer, which can be inherited from a mother or father.
Prostate cancer - which is estimated to affect 1 in 7 men in their lifetime - is another form of cancer that is more likely to develop in individuals with a family history of the condition. Having a father or a brother with prostate cancer can more than double a man's risk for the disease.
But according to Dr. Beebe-Dimmer and colleagues, very little is known about how a family history of prostate cancer can influence a woman's risk of breast cancer.
78% higher breast cancer risk with family history of breast and prostate cancers
To investigate this association, the researchers analyzed data from 78,171 women who were recruited to the Women's Health Initiative Observational Study between 1993 and 1998.
All women were free of breast cancer at study baseline and were followed for breast cancer development until 2009. During the follow-up period, 3,506 women were diagnosed with the disease.
From assessing self-reported questionnaires that the women completed at study baseline - which disclosed their family history of cancer - the team found that women who had a first-degree relative with prostate cancer - such as a brother, father or son - were 14% more likely to be diagnosed with breast cancer after the age of 50.
What is more, women who had a family history of both breast cancer and prostate cancer were 78% more likely to be diagnosed with breast cancer after the age of 50. This risk was highest for African-American women.
The researchers say their results suggest it is imperative that doctors check patients' full family history of cancer. Dr. Beebe-Dimmer says:
"These findings are important in that they can be used to support an approach by clinicians to collect a complete family history of all cancers - particularly among first-degree relatives - in order to assess patient risk for developing cancer.
Families with clustering of different tumors may be particularly important to study in order to discover new genetic mutations to explain this clustering."
A strength of this study is the large population of women included, according to the researchers, which allowed a more accurate estimation of breast cancer risk among those with a family history of breast cancer, prostate cancer or both.
However, the study is subject to some limitations. For example, the findings were based on a self-reported family history of cancer, which may have been flawed, though the researchers point out that self-reported cancer family histories - particularly among first-degrees relatives - are generally accurate.
Medical News Today recently reported on a study in which researchers from the UK detailed a potential way to make breast cancer tumors more sensitive to treatment.