An estimate of overdiagnosis 15 years after the start of mammographic screening in Florence.

AIM: Several studies have tried to quantify overdiagnosis of breast cancer with mammography screening, but estimates vary widely. The aim of this study is to evaluate the degree of overdiagnosis of breast cancer 15 years after the introduction of service screening in Florence (Italy).
METHODS: We selected 61,568 women aged 50-69 years at the beginning of service screening (1990) and we used the cancer registry data to follow up them for breast cancer incidence. The measure of overdiagnosis is the ratio of cumulative incidence of breast cancer in the invited group (observed) at least 5 years after the last screening to that expected in the absence of screening.
RESULTS: Under the assumption of a 1.2% annual trend in pre-screening incidence in women aged 60-69 years at the start of service screening, the ratio of observed to expected cumulative cases was 1.01 (95%CI: 0.95-1.07), but assuming no incidence trend, an unlikely scenario, the estimate of overdiagnosis rose to 1.13 (95%CI: 1.07-1.19).
CONCLUSION: Overdiagnosis of breast cancer in Florentine service screening can be estimated only for women aged 60-69 years at the start of service screening, for it is only for this group that a sufficient follow-up period is available after the last screening. Although the estimate of overdiagnosis is very sensitive to pre-screening trend estimates, our data show that 15 years after the introduction of mammographic service screening the degree of overdiagnosis was nearly zero and more than likely lower than 13% in this age group.