Breast cancer screening of women aged 70-74 years: results from a natural experiment across Australia.

There is a lack of evidence regarding the optimal age at which to cease mammographic screening for breast cancer. This ecological study compared Australian state and territory level screening participation rates and cancer outcomes from 1996 to 2005 to identify the extent to which screening women aged 70-74 results in smaller, earlier stage breast cancers. With each 10 % absolute increase in screening participation, there was no significant difference in cancer incidence, but the incidence of large cancers was 8 % lower (IRR = 0.92, 95 % CI 0.90-0.94, p < 0.001); there was some evidence of reduced nodal involvement at diagnosis (IRR 0.97, 95 % CI 0.95-0.99, p = 0.004) but this estimate was sensitive to assumptions regarding missing data. Increased mammographic screening of women aged 70-74 years reduces the incidence of large (>15 mm) cancers-and possibly cancers with nodal involvement-without a concomitant increase in overall cancer incidence.