What is the optimum interval between mammographic screening examinations? An analysis based on the latest results of the Swedish two-county breast cancer screening trial.

Further results are presented from the Swedish two-county breast cancer screening trial. The reduction in the rate of advanced cancers and of breast cancer mortality in the group allocated to screening when compared to the control group has accelerated with a further year of follow-up. Mortality due to other causes and the rate of other cancers remains similar in the two groups. Attention has been focused on the rate at which cancers start re-emerging among women with negative mammograms. Among women over 50 years of age at entry to the study, relatively few interval cancers are seen in the first two years after a screening test; in the third year the rate rises to nearly 50% of the comparable rate in the control group. Among women aged 40-49 years at entry, by contrast, the rate of interval cancers even in the first post screening year is nearly 40% of that in the controls and in the second year nearly 70%. In older women in the group allocated to screening, much of the breast cancer mortality comes from the refusers and little from the interval cancers; in younger women the picture is reversed. The implications for screening policy, including the interscreening interval are discussed.