Canadian study finds mammography screening reduces breast cancer mortality by 40%

A pan-Canadian study of breast cancer screening has suggested large-scale mammography screening programs can substantially reduce breast cancer mortality, according to results published in the Journal of the National Cancer Institute.

Lead author Andrew Coldman, PhD, from the University of British Columbia School of Population and Public Health, and colleagues gathered data from nearly 2.8 million women participating in screening programs across seven of Canada’s 12 provinces between 1990 and 2009.

“The study was designed to compare the mortality experience of women who participated in an organized screening program with an estimate of what would have occurred had they not enrolled, based upon the experience of women in the same jurisdiction who did not,” the study noted.

For the research, each participating province linked its screening program database to its provincial cancer registry and mortality database. This identified cases and deaths from breast cancer that occurred within the study time frame.

Province-specific breast cancer rates were computed by five-year age ranges and calendar periods between 1990 and 2009.

Researchers found that participation in large-scale screening programs produced a 40 percent lower death rate from cancer than expected in all participating provinces and did not vary greatly with age of participant.

“The value of breast cancer screening has attracted a large number of polarized comments,” Coldman and colleagues wrote. “The present study found statistically significantly lower breast mortality rates among screening participants of all ages in multiple regions.”

A substudy conducted found no evidence that self-selection biased the mortality results.

These results are in direct contrast to findings released by the Canadian National Breast Screening Study (CNBSS) published in British Medical Journal in February. In that study, 666 cancers were found in a mammogram group versus 524 found in a no-screen group.

Because more cancers were diagnosed by mammogram, but a similar number of women died of breast cancer, the study authors concluded that there had been over-diagnosis. They found that 22 percent of breast cancer cases detected through screening were over-diagnosed, concluding that changes to guidelines calling for annual mammograms should be considered.

In an editorial that accompanied the Pan-Canadian study, Russell Harris, MD, of the University of North Carolina, Chapel Hill, wrote that different methodologies led to very different results in the two cancer studies.

“The CNBSS was a [randomized controlled trial], assuring similarity of compared groups by randomization,” Harris wrote. “The provincial study, on the other hand, is an observational study that pulls together data from each of the seven screening programs, provincial cancer registries and provincial mortality databases in an attempt to compare observed breast cancer mortality among ‘participants’ with expected mortality  if they had not been screened.”

Harris contends that while the provincial study makes important points about needing observational evidence of cancer detection, the CNBSS’ randomized controlled trial method produces better evidence about mortality in screening programs.

In his essay, Harris calls for an end to the competing nature of observational studies like Coldman and colleagues and the CNBSS.

“Otherwise,” Harris wrote, “we can expect to be buffeted by continual duels between studies with results determined as much by methodology as by the true effects of screening.”