For women weighing whether to have a mammogram for early detection of breast cancer, the findings of some recent studies can seem especially confusing.

This month, a team of researchers at Brigham and Women’s Hospital and Harvard Medical School published a review of recent mammography studies. They concluded that, for women in their 40s, the benefits of mammograms aren’t as great as they’re often touted to be, and the potential downsides – such as the likelihood of having a repeat screening or biopsy that doesn’t find cancer – are greater than for older women.

The benefits of mammography vary with age. The review found that for women in their 40s, mammography can decrease the risk of dying of breast cancer by 15 percent. That may sound like a substantial drop, but, it needs to be viewed in light of very small number of women in their 40s who die of the disease. The risk of a woman in the 40-49 age range dying of breast cancer is about 0.245 percent. With mammography, her risk would decrease to 0.205. This means that annual mammograms for women in their 40s could save four lives for every 10,000 women screened.

Another recent study challenged the notion that deciding to have a mammogram is a simple, uncomplicated process for many women. That study, known as the Canadian National Breast Screening Study, found that annual mammography screening did not reduce deaths from breast cancer for women ages 40-59. It also found that screening can have negative consequences: one in five of the cancers that were treated after being detected by mammography would have posed no threat to the patients’ health.

As thorough and well-designed as the study was, it hardly represents the last word on the usefulness of mammography. Its findings conflict with those of a dozen other studies that have found that mammograms do indeed save lives. And it conflicts with the recommendations of many experts, including the U.S. Preventive Services Task Force, which estimates that breast cancer screening reduces the relative risk of death by about 15 percent in women ages 40-59.

To help sort out the conflicting and sometimes confusing data, we asked Eric Winer, MD, director of the Breast Oncology Program at the Susan F. Smith Center for Women’s Cancers at Dana-Farber, to offer some guidelines for women to follow and to discuss why different studies often reach different conclusions. His response is below:

“As treatment for breast cancer becomes more individualized, screening for the disease needs to be individualized as well. At the Dana-Farber/Brigham and Women’s Cancer Center, we endorse the U.S. Preventive Health Task Force recommendations, which are yearly or every-other-year mammograms for women over the age of 50 (until about 75). Women between the ages of 40 and 49 should discuss the pros and cons of mammography screening with their health care provider.

“Besides the well-known cancer-susceptibility genes BRCA1 and BRCA2, inherited abnormalities in an array of other genes can also raise breast cancer risk. Women who have had certain benign breast conditions may be at an increased risk as well, as may women with a family history of breast cancer. Women with gene mutations that predispose them to breast cancer typically begin regular screening at an early age, which usually involves breast MRI in addition to mammography. For other women who are at high risk, routine screening mammography may not be sufficient, and other approaches, including MRI screening, are being studied.”