A large, nationwide Swedish study presented during the 2010 Breast Cancer Symposium found that women who received breast mammography screening between ages 40 and 49 had a 26 percent lower risk of dying from breast cancer than those who did not have mammography.

"The benefits from routine screening mammography for younger women have been argued since the 1980s," said Hakan Jonsson, Ph.D., associate professor of cancer epidemiology at Umea University in Umea, Sweden, who led the study. "We're hoping that large population-based studies will help provide more answers."

Studies have shown a clear reduction in breast cancer mortality with routine mammography screening for women aged 50 and over, and more modest, but significant reduction for women aged 40 to 49. Many health organizations, including ASCO, continue to support the availability of screening for women 40 to 49 based on consultation with their doctor.

Since 1986 the Swedish government has mandated screening for all women ages 50 to 69, but left it up to individual counties to decide on offering to screen women ages 40 to 49. Roughly half of all Swedish counties chose to invite women ages 40 to 49 for mammography screening, and about half offered screening to only women who were at least 50 (current screening recommendations include women 40 to 74). In an effort to estimate the effectiveness of mammography screening on breast cancer mortality, Jonsson and his colleagues compared breast cancer deaths between counties where women 40 to 49 years of age were invited to screening (study group) and areas where women below age 50 were not invited (control group).

The researchers calculated the number of deaths related to breast cancer by looking at the national cancer register. Deaths were counted as part of the study if their diagnosis was between ages 40 to 49 and the death occurred within an average 16-year follow-up period. They found a 26 percent reduction in breast cancer mortality in the group which was invited to receive screening compared to the control group and 29 percent reduction for women who actually were screened. The mortality reduction was larger for women 45-49 than for women 40-44.

While there are currently 600,000 women age 40-49 in Sweden, over the course of the study - which was not randomized - more than 1 million women were involved. Among women invited for screening, there were 619 breast cancer deaths in the study group and 1,205 in the control group during the follow-up period between 1986 and 2005.

The researchers would like to retrospectively examine subgroups of women for known breast cancer risks, such as having a first-degree relative with breast cancer or giving birth for the first time at a late age, to see if there are differences in the effectiveness of mammography screening in detecting cancer and in mortality.