Regular mammogram screening could cut deaths by up to a third, the longest-running breast cancer screening study suggests.

The study followed 130,000 Swedish women aged 40 to 74 in two counties who were randomly divided into two groups. Half were invited to have breast screening and the others received usual care. In the study, women between ages 40 and 49 were screened every two years, and women 50 to 74 were screened about every three years. Eric Gaillard/Reuters

"Breast cancer can take many years to develop so to tell if screening is effective, we need to see how women fare in the long-term," said the study's senior author, Prof. Stephen Duffy of the University of London.

"In this study, we've continued to monitor women for nearly three decades and we've found that the longer we look, the more lives are saved," he added in a statement.

Robert Smith, director of cancer screening at the American Cancer Society, said the results suggest one breast cancer death is prevented for every 1,000 to 1,500 mammograms.

The screening phase of the trial lasted about seven years. Women between 40 and 49 were screened every two years, and women 50 to 74 were screened about every three years.

Screening age debate

The Public Health Agency of Canada recommends women start getting mammograms once they hit age 50.

The advantage of screening outweighs the short-term stress of a false positive — when a woman who's had a screening exam is called back because there is something suspicious that doesn't turn out to be cancer, said Martin Yaffe, a physicist at Toronto's Sunnybrook Hospital and an advisor to the Canadian Breast Cancer Foundation.

"Certainly women at age 50 should have mammograms but there is good evidence that suggests women should get routine mammography at approximately age 40," Yaffe said in an interview with CBC News.

After 29 years of follow-up, the study's authors estimated the number of women needed to undergo screening every two or three years over a seven-year period to prevent one breast cancer death ranged from 414 to 519.

Smith, a co-author of the study in the journal Radiology, said the findings show that screening is cost effective, and a good buy from a public health perspective.

In 2009, breast screening recommendations from the U.S. Preventive Services Task Force, an influential advisory group, recommended against routine mammograms for women in their 40s and said women in their 50s should get mammograms every other year instead of every year.

Breast cancer experts and advocacy groups argued the recommendation for fewer screenings would result in more deaths from breast cancer.

The study used an older form of mammography than what is common now, the researchers said.

With files from The Canadian Press