AGGRESSIVENESS OF SCREEN-DETECTED BREAST CANCERS

It is not clear whether screening for breast cancer works as public he arth policy and whether early indicators of effect predict an ultimate reduction in mortality. The malignant potentials of 248 breast cancer s detected by the screening service in Finland were compared with thos e of 490 control cancers diagnosed before the screening service was es tablished. Aggressiveness was assessed by DNA flow cytometry and clini cal status by cancer size and node involvement. After the first screen ing round, the results of DNA flow cytometry were the same in cancers diagnosed by screening and in controls; these findings are consistent with the hypothesis that the biological aggressiveness of breast cance r remains constant as the cancer progresses. The proportion of patient s with node-negative and small T1 cancers after the first screening wa s higher among the screened population than among controls, indicating earliness of diagnosis among those screened. Cancers diagnosed in the first round had a low malignant potential, as indicated by the DNA fl ow-cytometry and by clinical stage. Lower aggressiveness of cancers fo und by screening than of control cancers would indicate overdiagnosis or length-biased sampling, but not earliness of diagnosis. Screening w ith mammography is practised as a public health policy in Finland. The results predict that the mortality reduction found in randomised tria ls can be repeated with a screening service.