[Norwegian breast cancer mortality rates and validity of Swedish mammographic studies].
BACKGROUND
The claim that screening for breast cancer with mammography reduces breast cancer mortality is mainly based on the results from the Swedish two-county trial (WE study), where the effect was reported to be 30% for the age group 50-69 years. The two-county trial has recently been criticised for inadequate randomisation and for not following the study protocol.


METHODS
We do some simple calculations to study whether the WE study is robust for an alternative statistical analysis. We use stage-specific breast cancer mortality in the Norwegian population as the baseline mortality rate in Sweden. Then we study the expected reduction in overall breast cancer mortality in the WE study while we vary the mortality rate in stage 1 and the stage distribution.


RESULTS
We show that a 30% reduction in overall mortality rate is in conflict with observed decline in mortality in stage 1 and the expected stage migration. One either has to decrease mortality in stage 1, or increase the reduction of tumours with distant metastases, or both, to much higher levels than those reported in Sweden to get a 30% reduction in overall mortality of breast cancer.


CONCLUSIONS
Our study adds further evidence to the proposal that the WE study is biased and not valid.
Zahl
P H
PH
Avdeling for samfunnsmedisin Statens institutt for folkehelse Postboks 4404 Nydalen 0403 Oslo. per-henrik.zahl@folkehelsa.no
Kopjar
B
B
Maehlen
J
J
D000368:Aged; D015982:Bias; D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000401:mortality*; D005260:Female; D006801:Humans; D008327:Mammography; D008403:Mass Screening; D008875:Middle Aged; D015233:Models, Statistical; D009664:Norway / Q000453:epidemiology; D016032:Randomized Controlled Trials as Topic; D015996:Survival Rate; D013548:Sweden / Q000453:epidemiology