Family history of breast cancer continues to significantly increase chances of developing invasive breast tumours in ageing women -- those ages 65 and older, according to research published in JAMA Internal Medicine.

The findings could impact mammography screening decisions later in life.

The large study of more than 400,000 women is the first to specifically look at family history as a breast cancer risk factor in two groups of women, age 65-74 and 75 and older, says the research team, led by Dejana Braithwaite, PhD, associate professor of oncology at Georgetown University School of Medicine and a member of Georgetown Lombardi Comprehensive Cancer Center.

"Family history of breast cancer does not decline as a breast cancer risk factor as a woman ages. The relationship didn't vary based on whether a first-degree relative's diagnosis was made in a woman age 50 or younger, or older than age 50," Braithwaite says. "This means that women with that first-degree family history -- breast cancer in a mother, sister, or daughter -- should consider this risk factor when deciding whether to continue mammography screening as they age."

Currently, the U.S. Preventive Services Task Force (USPSTF) recommends mammography screening every two years between ages 50 and 74 for women at average risk.

After age 75, the evidence is insufficient to assess risk and benefit of mammography, according to USPSTF's most recent update in 2016.

The American Cancer Society recommends yearly mammograms in women age 45, and then biennial screening at age 55 and on "as long as a woman is in good health."

"As breast cancer screening guidelines change from age-based to risk-based, it is important to know how standard risk factors impact breast cancer risk for women of different ages," said Karla Kerlikowske, MD, senior author of the new study and a member of the UC San Francisco Helen Diller Family Comprehensive Cancer Center.

"The goal of our work is to provide evidence that helps inform breast cancer screening guidelines for older women," Braithwaite says. "Older women who are in good health and have a first-degree family history may consider a screening mammogram even as they age beyond the screening recommendations for average risk women."

Researchers from Washington, California, Wisconsin, Vermont, New Hampshire and North Carolina participated in the research by examining 1996-2012 records from the Breast Cancer Surveillance Consortiums (BCSC) registries in their regions.

The team found that while age is the strongest risk factor for breast cancer -- any adult woman in the general population has a baseline 12 percent risk of developing the disease -- first-degree family history can almost double that risk.

Overall, a first-degree family history leads to an absolute increase in 5-year risk of breast cancer ranging from 1.2 to 10.3 percentage points depending on breast density and age.

For example, in women 65-74 years old with scattered areas of dense tissue in their breasts, the team found an increased 5-year risk of breast cancer that ranged from 15.1 percent in women without a family history of the disease to 23.8 percent in women whose first degree female relatives had developed breast cancer.

Similarly, among women 75 years or older with the same scattered breast density, 5-year cumulative risk of breast cancer increased from 15.9 percent for women without a family history to 23.1 percent for women with a family history.

Researchers also discovered that breast density, one of the strongest risk factors for breast cancer, did not attenuate the association of family history of breast cancer and breast cancer risk in the women studied as a whole.

But when broken into age groups, fatty breasts added a little risk to women age 65-74 years with a family history; in the older cohort, the association was flipped -- dense breasts added slight risk.

Source: Georgetown University Medical Center