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.