Women may have an increased risk of developing breast cancer if they have a close male relative who has battled prostate cancer, a new study indicates.

While it is already known that people have an increased risk of developing either of these diseases separately if there is a family history of them, little research has been carried out to determine whether there is a relationship between the two diseases within families.

US scientists decided to investigate this further. They looked at over 78,000 women taking part in a study that began in the 1990s and finished in 2009. At the start of the study, none of the women had breast cancer. During the study period, over 3,500 developed the disease.

The scientists found that women had a 14% increased risk of developing breast cancer if they had a first-degree male relative - father, brother or son - with a history of prostate cancer.

Furthermore, if a woman had a family history of both breast and prostate cancer, her risk of developing breast cancer jumped by 78%.

"The increase in breast cancer risk associated with having a positive family history of prostate cancer is modest, however, women with a family history of both breast and prostate cancer among first-degree relatives have an almost two-fold increase in risk of breast cancer," commented Dr Jennifer Beebe-Dimmer of the Wayne State University School of Medicine in Detroit.

She believes that when doctors are assessing patients for a risk of cancer, they should take a complete history of all family cancers, even if they are in the opposite sex.

"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."

Details of these findings are published in the journal, CANCER.