Breast cancer screenings may not lead to fewer deaths but may lead to overdiagnosis, US researchers suggest.

In areas of the US with high levels of screening, more tumors were diagnosed - but breast cancer death rates were no lower than in areas with fewer screenings, researchers report.

"The mortality results that we observed are far from definitive," cautioned Charles Harding, the study's lead author from Seattle, Washington.

"The most dramatic finding of our study is the immediately evident - and substantial - evidence of breast cancer overdiagnosis," he told Reuters Health in an email.

Each year, about 230,000 US women are newly diagnosed with breast cancer, according to the National Cancer Institute.

While screening guidelines vary, the government-backed US Preventive Services Task Force says average-risk women should have mammograms every other year between ages 50 and 74. Getting screened before age 50 should be an individual decision, according to the Task Force.

For the new study, researchers analyzed breast cancer screenings, cancer diagnoses, tumor characteristics and deaths in 547 US counties.

The data came from nearly 16 million women living in those counties in 2000. All were at least 40 years old. The percentage who had screening mammograms ranged from 39 percent to 78 percent, depending on where they lived.

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.