Across the ages, there was no decline in screenings, the researchers found.
Among women 40 to 49, the rates rose slightly, from 46.1 percent in 2008 to 47.5 percent in 2011. Among women aged 50 to 74, the rates also rose, from 57.2 percent in 2008 to 59.1 percent in 2011.
The study, supported by Brigham and Women's Hospital, is published in the April 19 online edition of the journal Cancer.
Pace said conflicting recommendations from different organizations could have generated much confusion among both doctors and patients.
She added, "Another possibility would be that some providers and patients would simply be in disagreement [with the task force recommendation]."
In the 2009 recommendations, the task force said women 40 to 49 should discuss the pros and cons with their doctor, then decide whether to get screened. The task force took into account the lower incidence of breast cancer in younger women, as well as the downsides of screening, such as false positives, in which cancer is suspected but not found. False positives can lead to unnecessary testing, expense and emotional strain, experts say.
But even if a woman's doctor advises reducing the number of mammograms or waiting until age 50, "patients can self-refer for mammography," Pace said.
"It's an emotionally charged decision for women and doctors as well," she added.
"I'm not surprised by this," said Dr. Joanne Mortimer, co-director of the breast cancer program at the City of Hope Comprehensive Cancer Center, who reviewed the findings.
She, too, speculated there could be many reasons behind the findings. "It takes years for doctors to change their practice," she said, adding that many doctors may still not be comfortable with the new guidelines.
Doctors could also be reluctant to suggest delayed screenings for younger women or expanding the interval between tests for older women, Mo
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