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The new study looked at Sweden’s mammography program. Since 1986, the country has required that screening be offered to women over 50 but left it up to each county to decide whether to offer it to younger women. About half of counties did, and researchers compared breast cancer death rates in areas where it was and wasn’t offered.

They counted breast cancer deaths of women who had been diagnosed in their 40s and died within 16 years of followup.

There were 803 such deaths among women in counties that offered screening versus 1,238 in counties that didn’t offer it, although the number of women in each group and the amount of time they were followed differed. Researchers did not express the results in terms of death rates, which would have made comparing these groups much easier.

The results translated to a 26 percent lower risk of dying of breast cancer for those offered screening. There was a 29 percent lower risk for women who actually had mammograms, said lead researcher Hakan Jonsson of Umea University in Umea, Sweden.

Researchers did not have information on, and the study therefore could not account for, any differences in general health and other factors that could have affected the number of deaths.

“It very well may be that blue-collar counties may have higher rates,” Brawley said.

A comparison of breast cancer death rates in the same counties in the 15 years before screening began found 6 percent fewer deaths in the areas offering screening, suggesting this group was healthier, Jonsson said. But this difference was not so large that it would significantly affect the study’s findings, he said.