Mammograms may not reduce breast cancer deaths

Overall, more than 53,000 of the women were diagnosed with breast cancer in 2000.

The researchers report in JAMA Internal Medicine that the number of breast cancer diagnoses rose with the number of screenings, but the amount of breast cancer deaths over the next 10 years remained the same.

Overall, a 10 percentage point increase in breast cancer screenings was tied to a 16 percent 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. There wasn’t an increase in diagnosis of large – and presumably more advanced – tumors.

The findings suggest breast cancer screenings lead to overdiagnosis because they mainly catch smaller tumors, the researchers say.

“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.

“However, our findings are quite tentative for mortality because the data are very noisy,” he added. “We feel that our study raises important questions about the benefits of mammography screening, but it certainly does not answer them.”

More research is needed, he said.

The researchers also warn in their paper that their findings may be limited by so-called ecological bias, which can occur when assumptions are made about individuals from data of a large group.

Dr. Joann Elmore and Ruth Etzioni of the University of Washington in Seattle agree in an editorial that the study’s results are limited by the potential of ecological bias.

For example, there is no way to know if the women who received the mammograms were the same women who went on to be diagnosed with the disease, they write.

Also, Elmore and Etzioni caution, other unmeasured factors may account for the lack of difference in breast cancer deaths.

“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,” Elmore and Etzioni added.

SOURCE: and JAMA Internal Medicine, online July 6, 2015.