[When will mammography screening reduce breast cancer mortality?]

Abstract

Background: Norwegian authorities have decided to start a mass mammography screening programme. One third of the population has been included in a pilot study from 1996. The Cancer Registry of Norway maintains that there will be at least a 30% reduction in mortality, but that an effect may not be detected until the ten-year follow-up.

Methods: We assume that the effect of mammography screening is constantly increasing over a ten-year period before maximum effect is reached. We also assume that the effect of screening in the age group 50-74 years is 80% of maximum effect. We simulate Norwegian breast cancer mortality rates under the assumption that mammography screening reduces breast cancer mortality in the age group 50-69 years with 15% and 30% effects, respectively. We also simulate 30% and 50% effect in the pilot study.

Results: If the effect in the Norwegian population is 30%, one may expect to see a significant decline after five years; however, if the effect is only 15%, one has to wait for a longer time. If the effect is 50%, as the Cancer Registry of Norway has argued, one should see a significant effect in the pilot study after six years.

Conclusions: We think it a contradiction to argue that mammography screening reduces breast cancer mortality by 30%, but that one has to wait ten years to observe an effect on national mortality rates. We suggest that the breast cancer mortality rate in the pilot study is estimated. We also argue that observed reductions of less than 10% in Sweden, Finland and England are strong evidence that the effect of mammography today is far less than 30%.