Quantification of the effect of mammographic screening on fatal breast cancers: The Florence Programme 1990-96

Breast cancer cases diagnosed in women aged 50-69 since 1990 to 1996 in the City of Florence were partitioned into those who had been invited to screening prior to diagnosis and those who had not. All cases were followed up for vital status until 31 December 1999. The cumulative number of breast cancer deaths among the cases were divided by screening and invitation status, to give the rates of cancers proving fatal within a period of 8 years of observation (incidence-based mortality). We used the incidence-based mortality rates for two periods (1985-86, 1990-96), pre and during screening. The incidence-based mortality ratio comparing 1990-96 and 1985-86 was 0.50 (95% CI : 0.38-0.66), a significant 50% reduction. For noninvited women, compared to 1985-86, there was a 41% significant mortality reduction (RR=0.59, 95% CI : 0.42-0.82). The comparable reduction in those invited was a significant 55% (RR=0.45, 95% CI : 0.32-0.61). The incidence ratio of rates of cancers stage II or worse was close to one when the noninvited in 1990-96 were compared with 1985-86 (RR=0.97, 95% CI : 0.78-1.21). Excluding prevalent cases, the rate of stage II+ breast cancer cases was 42% lower in Screened women compared with the noninvited (RR=0.58, 95% CI : 0.45-0.74). This study confirmed that new treatments and the first rounds of the screening programme contributed to reducing mortality from breast cancer.