To maximize the benefits and lower the risks of routine mammograms for breast cancer screening, it's best to base decisions on each woman's individual risk level, as well as her values and priorities, researchers concluded after reviewing 50 years of studies on mammograms.

Recommendations about who should get mammograms and how often remain controversial. Mammography is performed routinely with the goal of catching breast cancer early, but it is unclear if early diagnosis is necessary or even effective in reducing deaths from breast cancer.

Moreover, regular screening may harm women by leading to unnecessary treatment of benign tumors that would not have caused medical problems if left untreated.

In the review study, published today (April 1) in the journal JAMA, researchers attempted to tease out the overall benefits and harms of mammograms, as well as what each woman stands to gain or lose from undergoing routine mammograms. [9 Risk Factors for Breast Cancer]

"What I tell my patients is that the mammogram is not a perfect test," said study researcher Dr. Nancy Keating, an associate professor of health care policy at Harvard Medical School and associate professor of medicine at Brigham and Women's Hospital. "Some cancers will be missed. Some people will die of breast cancer regardless of whether they have a mammogram, and a small number of people that might have died of breast cancer without screening will have their lives saved."

Based on their results, the researchers estimated that among every 10,000 women in their 40s who undergo yearly mammography for 10 years, about 190 will be diagnosed with breast cancer. Of those 190, the researchers estimated that about 5 will avoid death from breast cancer due to the screening test. Meanwhile, about 25 of the 190 would die of breast cancer regardless of whether or not they had mammograms. The rest will survive, thanks largely to advances in breast cancer treatment.

The researchers also estimated that about 36 of the 190 women who received annual mammography for 10 years and were diagnosed with breast cancer would receive unneeded surgery, chemotherapy or radiotherapy.

Among women in their 40 and 50s who undergo 10 years of annual mammograms, 61 percent will get at least one false positive mammogram, which would entail additional scans and unnecessary biopsies.

In 2009, the U.S. Preventive Services Task Force reversed its previous recommendation of mammography every one to two years beginning at age 40, and recommended routine screening every two years starting at age 50.

However, recent data suggests that use of mammography in the United States has not changed following the updated recommendations, the researchers said.

The new study also showed that older women gained more from mammography than their younger counterparts. The reduction in mortality was about 15 percent for women in their 40s, and 32 percent for women in their 60s.

In general, breast cancer risk rises with age, and women in their 50s and 60s likely receive more benefit relative to their risk of being harmed by a mammogram, the researchers said. For women ages 40 to 49 who get regular mammograms for 10 years, 5 of 10,000 will be saved because of mammograms. For women ages 50 to 59 years, 10 of 10,000 will be saved, and for women ages 60 to 69 years, 42 of 10,000 will be saved, according to the study.

Keating noted that factors other than age come into play when deciding whether to undergo routine mammograms. Women who have limited life expectancy due to a medical problem are also highly prone to get harmed by overdiagnosis, as older women are, she said.

Other factors, including family history, whether and when a woman has given birth, and lifestyle, also change how much a woman's risk of breast cancer, and how much she might gain or lose by getting mammograms.

"While we need more research on mammography's benefits and harms today, existing data suggest that we have been overestimating the benefits of mammography and underestimating the harms over the years," said Dr. Lydia Pace, study co-author and research fellow in women's health at Brigham and Women's.

"It is really important to have informed discussions with our patients to help them understand the chances that a mammogram will benefit them as well as the possible downsides of getting a mammogram, so that they can incorporate their own values and preferences in making the right decision for themselves."

Email Bahar Gholipour. Follow us @LiveScience, Facebook & Google+. Original article on Live Science.