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Mammograms May Not Reduce Breast Cancer Deaths
Breast cancer screenings may not lead to fewer deaths but may lead to over-diagnosis, according to new research involving investigators at Harvard University and the Dartmouth Institute for Health Policy and Clinical Practice.
In areas with high levels of screening, more tumors were diagnosed –– but the death rates for breast cancer were no lower than in areas with fewer screenings, the authors reported. Their study was published in JAMA Internal Medicine.
“The most dramatic finding of our study is the immediately evident –– and substantial –– evidence of breast cancer over-diagnosis,” lead author Dr. Charles Harding told Reuters Health.
Each year, approximately 230,000 women are newly diagnosed with breast cancer in the U.S., according to the National Cancer Institute.
While screening guidelines vary, the U.S. Preventive Services Task Force says average-risk women should have mammograms every other year between the ages of 50 and 74. Getting screened before age 50 should be an individual decision, according to the Task Force.
For the new study, the researchers analyzed county-level data on breast cancer screenings, cancer diagnoses, tumor characteristics, and deaths in nearly 16 million women living in the U.S. in 2000. All of the women were at least 40 years old. Screening mammograms had been performed in 39% to 78% of the women, depending on the county in which they lived. Overall, more than 53,000 of the women were diagnosed with breast cancer in 2000.
The researchers reported that the number of breast cancer diagnoses increased with the number of screenings, but that the amount of breast cancer deaths over the next 10 years remained the same. Overall, a 10% increase in breast cancer screenings was linked to a 16% increase in breast cancer diagnoses. The number of screening mammograms performed did not affect the number of breast cancer deaths, however.
Most of the additional cancers detected on screening were small tumors. The authors did not detect an increase in the diagnosis of large –– and presumably more-advanced –– tumors.
The findings suggest that breast cancer screenings lead to over-diagnosis because they mainly catch smaller tumors, the researchers noted.
“We were troubled that we did not see evidence of a mortality benefit from screening, especially because there was no relationship between screening and advanced-stage cancer, either,” Harding said. “We feel that our study raises important questions about the benefits of mammography screening, but it certainly does not answer them.”
The researchers also warned in their paper that their findings may be limited by so-called “ecological bias,” which can occur when assumptions are made about individuals based on data from a large group.
Drs. Joann Elmore and Ruth Etzioni of the University of Washington agreed in an accompanying editorial that the study’s results may have been influenced by ecological bias. For example, there was no way to know whether the women who received the mammograms were the same women who were diagnosed with the disease.
“Prior ecological studies of mammography conducted at the larger state level with a wider range of mammography frequencies showed a decline in breast cancer mortality associated with more screening,” they wrote.
Sources: Reuters; July 6, 2015; JAMA Internal Medicine; July 6, 2015; and JAMA Internal Medicine Editorial; July 6, 2015.