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LONDON (Reuters) - Regular mammographic screening for breast cancer saves the lives of two women for every one who is given unnecessary treatment, scientists said on Wednesday, in a study which adds to a global row over screening programs.The British researchers said their work, which contradicts some recent studies on screening programs but confirms others, showed the benefits outweigh the harm screening can cause by picking up tumors that would not have presented a problem."Unfortunately, we haven't yet got a flawless screening test, and some cases that are picked up wouldn't have needed treatment," said Stephen Duffy of Queen Mary, University of London, who led the study."But for every case like this, screening saves two women who would have otherwise died from breast cancer," he said.Duffy's findings contradict the results of a Nordic study published last week in the British Medical Journal which found no evidence that routinely screening women for breast cancer had any effect on death rates.The report by Duffy and his team were published in the Journal of Medical Screening, which has a much lower Thomson-Reuters' Journal Citation Reports "impact factor" -- a measure of the impact journals tend to have in their fields -- than the British Medical Journal.The findings will also further fan a row which erupted in the United States in last November after public health officials on the U.S. Preventive Services Task Force questioned whether annual screening mammograms for women under 40 actually saved lives and suggested raising the screening age to 50.Cancer doctors and advocacy groups decried the move, saying the changes would mean more women die of breast cancer.Breast cancer is the most common cancer in women worldwide, accounting for around 16 percent of all female cancers. It kills around 519,000 people globally each year.Although experts are at odds over whether routine screening is worth the trouble and expense, most wealthy nations have settled on a plan for regular screening after age 40 or 50 to try to find tumors when they are small and more easily cured.Critics of screening programs say they can be more harmful than helpful if the extra hospital time and costs they require, coupled with the stress and worry of false alarms, are not outweighed by the benefit of preventing more deaths.Duffy and colleagues conducted two studies into the risk-benefit balance of screening programs.One study predicted the number of women who would have died from breast cancer in Britain if the breast cancer screening program had not been launched in 1988, and another looked at the number of breast cancer deaths among 80,000 women in Sweden, comparing those offered screening with those who were not.The results showed a "substantial and significant reduction in breast cancer deaths" from mammographic breast cancer screening with "between 2 and 2.5 lives saved" for every overdiagnosed case.Lesley Walker, director of cancer information at the charity Cancer Research UK, said the study showed screening saves lives.She said it gave women access to "high quality information" to help them make decisions with their doctors."It's standard practice to have these discussions which help women make the choice that's right for them, so overdiagnosis need not be a reason to feel worried about going for screening," she said in a statement.