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Swedish Trials Suggest Modest Benefit For Screening Mammography (PP 904, 909)

March 13, 2002

New data with longer follow-up from four Swedish trials published in this week's issue of THE LANCET suggests there may be a modest benefit from screening mammography for women aged 55 years or over.

Considerable debate surrounds the issue of screening mammography and its possible benefits. A research letter by Ole Olsen and Peter Gotzsche (Lancet 2001; 358: 1340-42) concluded that there was no reliable evidence from randomised trials to support the benefit of screening mammography. Lennarth Nystrom and colleagues from the University of Umea, Sweden, present new data updating the overview of four Swedish randomised controlled trials on mammography screening up to and including 1996. The study also contains data from the trials that have not been presented before.

The trials include data for about 247,000 women, who were followed-up by record linkage to the Swedish Cancer and Cause of Death Registers. The relative risks for breast cancer death and death from all causes were calculated for women given screening mammography and those not invited for screening (the control groups in the four trials).

Average follow-up time for the trials was 15.8 years. There were 511 breast cancer deaths in
1 864 770 women-years in the groups given mammography, and 584 breast cancer deaths in 1 688 440 women-years in the control groups-resulting in a significant 21% reduction in breast cancer mortality for women given screening mammography. The reduction was greatest (33%)in the age group 60-69 years at entry to the trials. There were statistically significant effects in the age groups 55-59, 60-64, and 65-69 years, but there was only a small relative risk reduction (5%) in women aged 50-54 years. The benefit in terms of cumulative breast cancer mortality started to emerge at about 4 years after randomisation and continued to increase to about 10 years. There was no significant difference in death from all causes between screening and control groups.

Lennarth Nystrom comments: "The advantageous effect of breast screening on breast cancer mortality persists after long-term follow-up. The recent criticism against the Swedish randomised controlled trials is misleading and scientifically unfounded."

In an accompanying Commentary (p 904), Karen Gelmon from the British Columbia Cancer Agency, Vancouver, Canada, concludes: "These data confirm that women who are otherwise well, especially those aged 55-69 years, and who are concerned about breast cancer, should be encouraged to attend screening. Partly because of the controversy around mammography, less than half of such women, the age group most likely to benefit, access this service on a regular basis in many western countries. The focus of screening should be to ensure these women attend about every two years. However, as breast cancer accounts for only about 4% of all deaths annually, even a 21% reduction in breast cancer mortality is barely measurable when all-cause mortality is the endpoint."

Lancet