Breast cancer screening with mammography leads to a significant reduction in risk for breast cancer deaths, the long-term followup to a large study in Sweden indicates.
The results suggest one breast cancer death is prevented for every 1,000 to 1,500 mammograms, said one of the co-authors, Robert Smith, director of cancer screening at the American Cancer Society.
"This is the longest followup of a breast cancer screening randomized clinical trial that's ever been done, and it shows that the benefit that was seen early on not only persists, but it has gotten stronger over time," he said in an interview from Atlanta.
The paper drew on a seven-year study of 133,065 women aged 40 to 74 living in two Swedish counties and randomized into two groups - one that received an invitation to screening, and another that received usual care.
At the end of the earlier study, there were 30 per cent fewer breast cancer deaths among all women invited for screening.
After 29 years of followup, it was found that the estimated 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.
"The mortality reductions that were seen at the first round of followup have been stable over this long period of time," Smith said.
The findings, published in the journal Radiology, show that breast cancer screening is a good buy from a public health perspective, he said.
"We estimated if the screening had continued for 10 years, let's just say on average every other year, that we would need to screen 300 women to save one life, which means that mammography is extraordinarily cost effective."
In 2009, a panel of U.S. science advisers recommended against routine screening before age 50, indicating the benefits were small and there were potential problems in screening all women over 40 - such as unnecessary stress due to false positives, biopsies and overtreatment. The study group said the decision should be left to individual women and their doctors.
Smith said the task force is unlikely to change its position based on this new study because it looks at studies as an aggregate and it "doesn't really want to pick winners and losers in trials."
His preference is to find cancers early and reduce deaths compared with avoiding harms based upon short-term estimates, he said.
Smith also pointed to improvements in mammography since the Swedish study began - digital replacing film, two views instead of one, lower doses and better image quality.
"There also have been improvements in therapy, so the combination of improvements in mammography and therapy are resulting in lives saved," he said.
Martin Yaffe, a physicist and breast cancer researcher at the University of Toronto and Sunnybrook Health Sciences Centre, wasn't involved in the research but said the new study shows that after 29 years, the results are "strong, stable and constant."
"There's a consistent result, and of course as the number of deaths mount, the absolute benefit, I guess, in terms of fewer deaths due to breast cancer becomes more and more apparent."
In Canada, he noted that federal recommendations have been to start routine screening at age 50.
"I personally have disagreed with those recommendations for some time because I think there is good evidence of a mortality reduction from screening as of age 40," he said in an interview.
"When younger women develop breast cancer and die earlier, I think the impact on society and certainly on them and their families is much greater than when there's a death of an older woman."