The Swedish study found that starting women on mammography at age 40 rather than age 50 was associated with a 26 percent reduction in risk of death from breast cancer -- a finding that raises new questions about what women 40-49 should do about mammography screening. The study comes just a week after another study, also from a Scandinavian country, found that screening mammography contributed only a 10 percent reduction in mortality.
Researchers led by Hakan Jonsson of Umea University in Umea, Sweden, reported the findings at a press briefing in advance of a presentation at the American Society of Clinical Oncology Breast Cancer Symposium. The study also was released simultaneously online in the journal Cancer.
The number of women age 40 to 49 who would need to be screened to save a single life in the Swedish study was 1,252 -- a lower figure than that seen in prior studies. But it still was higher than for older age groups given the higher breast cancer mortality later in life, Jonsson noted.
The number needed to screen is the critical issue, according to Dr. Jennifer Obel, ASCO Communications Committee member and moderator of the press briefing.
"One thing is certain," she told reporters. "Mammography has been shown in study after study to reduce the risk of breast cancer in women 40 to 49. What we're talking about is the debate about how many women need to be screened to save one life."
Dr. Donna Plecha, director of breast imaging at Case Western Reserve University in Cleveland, agreed that is the bottom line despite the back-and-forth that has been confusing for patients.
The debate erupted last year when the U.S. Preventive Services Task Force (USPSTF) recommended that women under age 50 don't need routine screening mammography; whereas its earlier stance was in accord with American Cancer Society guidelines suggesting mammography every one to two years for all women age 40 and older.
Congress subsequently undermined the USPSTF recommendation by inclusion in the health care reform law a requirement that insurers cover mammograms for women 40 to 49.
Jonsson pointed out, though, that the body of research upon which the USPSTF based its recommendations included eight largely older randomized trials, most of which ended more than 20 years ago.
By contrast, his study included 600,000 women in Sweden age 40 to 49 and took place since national guidelines were implemented in 1986 in Sweden calling for screening mammography -- making it the largest epidemiological study of mammography in this age group.
The varying benefits seen in different studies and lack of consensus on timing of screening mammography has made some clinicians hesitant to alter their practice.
"The patients are naturally confused, as we practicing doctors are, hence the caution on changing recommendations too quickly," said Dr. Albert Levy of Mount Sinai School of Medicine in New York City.
But breast cancer researchers, radiologists and primary care physicians contacted by ABC News overwhelmingly agreed that the benefits of mammographic screening in women ages 40 to 49 outweigh the risks.
Physicians use mammography, along with self-examination, as the first line of defense in combating breast cancer.
"At this time, screening mammography is the best tool we have," said Dr. Donna Plecha, director of breast imaging at University Hospitals Medical Center in Cleveland. "The earlier we catch breast cancer, the easier it is to cure."
"The risk of breast cancer is reduced in women under 50, [but] when it occurs, the impact on patient longevity, productivity and both the family and society is enormous," said Dr. Gary Lyman, director of comparative effectiveness and outcomes research at Duke University in Durham, N.C., emphasizing the need to catch breast cancer early in younger women.
Though current USPSTF recommendations do not urge universal screening for women under the age of 50, physicians seem to have a different take.
"The facts are that the age of 50 has no biological or scientific importance when it comes to mammography screening," said Dr. Daniel Kopans, professor of radiology at Harvard Medical School and senior radiologist in the Breast Imaging Division at the Massachusetts General Hospital in Boston. "There is clear benefit for screening beginning by the age of 40, and age 50 should be dropped as a threshold."
Screening mammography does carry some costs and risks such as false positive results, noted Dr. Carol Lee, chairwoman of the American College of Radiology's Breast Imaging Commission.
However, she said, "In my opinion, the benefits far outweigh the potential harms, and screening should be performed annually starting at age 40 for women at average risk for breast cancer."
Despite the strong opinions of those who favor earlier screening, though, some breast cancer experts still believe that the benefits of mammography have been overstated.
"So many other studies that have been done, where the methodology is clear and very well done, have shown no benefit in this age group," said Fran Visco, president of the National Breast Cancer Coalition, who called mammograms in this age group "outdated and ineffective." "We need to focus on issues that will prevent breast cancer and that will truly save many lives."
"Analyses of screening have shown that there is a high rate of over-diagnosis, with many women undergoing radiation and surgery per woman saved, as well as many false positive mammograms," said Dr. Lee Green, professor of Family Medicine at the University of Michigan. "It's not enough just to know how many women were saved, we have to know how many were hurt to save them."
Currently, there is no sign that the USPSTF plans to change its recommendations.
Dr. Ned Calonge, USPSTF panel chair and chief medical officer of the Colorado Department of Public Health and Environment, had not read the study yet, but had concerns about the way the study was done. He said the study will be included in the USPSTF's review of new studies. However, he was skeptical that it would result in any changes in their recommendations.
Meanwhile, since the USPSTF recommendations came out, several medical journals and prominent epidemiologists have supported the notion of backing away from regular screening of 40-year-old women. As it stands, the American Cancer Society recommends annual screening of women 40 and older, while the National Cancer Institute recommends screening for women 40 and older every one to two years.
The researchers behind the most recent study, meanwhile, weren't willing to make a blanket recommendation for screening to start at age 40 for all women based on these results.
Rather, Obel emphasized that message is still individualized discussions with patients to determine what is the right starting time for them.