The current models used assess the risk on the basis of what is known of family history of breast cancer and the researchers say the models are poor predictors for women with few female relatives over the age of 45.

Dr. Jeffrey Weitzel of the City of Hope cancer center in Duarte, California says the theory relies on a woman having a family with a history of breast cancer and does not account for smaller families.

Dr. Weitzel says the model means some women will not know if there is a risk of their own cancers returning or not and women with few older female relatives on either side of their family are not well served by the available models.

The researchers found that of a total of 1,543 breast cancer patients enrolled in a cancer registry between 1997 and 2007, 306 were diagnosed before age 50 and reported no known first- or second-degree relatives with breast or ovarian cancers.

Further questioning regarding family history revealed half of them (153) were determined by researchers to have too few close female relatives to accurately assess risk, but genetic testing revealed that these women had a 14% risk of having a mutation, compared with a 5% risk among women with a larger number of female relatives who survived to their mid-40s without developing breast cancer.

Inherited gene mutations feature in only about 5% of all breast cancers, but women with BRCA1 or BRCA2 mutations have about a 50% to 85% lifetime risk of developing the disease, compared with a roughly 10% lifetime risk among the general population of women.

BRCA carriers are also at a greater risk of developing breast cancers before the age of 50, and also have a higher risk for ovarian cancer.

It is known that identifying women with BRCA mutations early reduces their risk of death from breast or ovarian cancer, but the mutations are rare and testing all women or even testing all breast cancer patients is often not a practical option.

Although most women who develop breast cancer before the age of 40 are considered for genetic testing, doctors often rely on the prediction models to determine whether women diagnosed in their 40s should be screened if they have no known family link to the disease.

The American Cancer Society now recommends annual breast screening with magnetic resonance imaging (MRI) along with mammography for high-risk women, as determined by existing family-risk models.

The study is published in the current issue of The Journal of the American Medical Association.