The use of mammography to detect breast cancer prevents fewer deaths than previously thought, a new analysis suggests.

Authors of the study, published this week in the journal Archives of Internal Medicine, concluded that although many women have been helped by breast screening, the number of those who have had their lives saved is lower than generally estimated.

Researchers in the U.S. used the National Cancer Institute's software for analyzing data to estimate the 10-year risk of diagnosis and the 20-year risk of death.

Mammography aims to detect breast cancer when it is localized and curable to prevent advanced disease and breast cancer deaths.

Dr. H. Gilbert Welch and Dr. Brittney A. Frankel from Dartmouth University in Hanover, N.H., estimated that the probability a woman with breast cancer detected with screening avoids dying of breast cancer because of mammography is 13 per cent.

"Breast cancer survivors are particularly common," the study's authors said. Although "perhaps the most persuasive messages promoting screening mammography come from women who argue that the test 'saved my life,'" other possibilities for breast cancer survival exist, they added.

The study's authors were concerned that inflated perceptions on the benefits of mammography "may lead to a self-perpetuating cycle of unwarranted demand for screening, over-diagnosis, over-treatment, and a continually growing population of breast cancer survivors who advocate mammography," Dr. Timothy Wilt and Melissa Partin of the University of Minnesota in Minneapolis said in a journal commentary.

All provincially organized screening programs in Canada encourage women aged 50 to 69 with no symptoms to have regular screening mammograms. The frequency depends on a woman's individual risk factors, such as family history.

A decline in the death rate for Canadian women diagnosed with breast cancer since 1985 is attributed to better screening, mammography and treatments such as tamoxifen that help prevent recurrence.