For women older than 50 who have been confused by conflicting advice on how frequently to get a mammogram, some new science is here to guide their decisions.

An ambitious research effort published last week in the Annals of Internal Medicine set out to tailor women’s breast cancer screening practices to match their actual risk of breast cancer. It concludes that a woman’s breast density should influence the frequency with which she is screened for breast cancer, in addition to such long-recognized breast cancer risk factors as age, ethnicity, personal history of abnormal breast findings and a family history of breast cancer.

For a small portion of those women, that makes a once-a-year mammogram — twice as often as the current standard — the best bet.

Underwritten by the National Cancer Institute, the new study combined the data-collection efforts of breast-cancer epidemiologists with three separate teams of cancer modelers. In the study, women were separated into subcategories based on four levels each of individual risk and breast density.

The research recommends that women older than 50 with dense breast tissue who have higher-than-normal risk of developing breast cancer should get annual mammograms. These women, however, represent a small minority of women in that age group — less than 1 percent of all women between 50 and 74.

Many women, however, could go as long as three years between mammograms without increasing their risk of death from breast cancer, the study found.

For women with average risk and low breast density, the models showed that there was no difference in deaths averted from breast cancer whether they were screened every two years or every three years. Women who had a mammogram every three years, however, had fewer unnecessary follow-up procedures, including biopsies.

Only in the last decade or so have radiologists, oncologists and women’s health specialists begun to appreciate the role of breast density in a woman’s breast cancer risk. While radiologists have long warned that dense breast tissue made cancerous masses harder to spot, mounting evidence has found that cancer is also more likely to gain a foothold in dense breast tissue.