Trends in stage-specific breast cancer incidence in New South Wales, Australia: insights into the effects of 25 years of screening mammography.

PURPOSE: Screening mammography aims to improve breast cancer (BC) prognosis by increasing the incidence of early-stage tumours in order to decrease the incidence of late-stage cancer, but no reports have investigated these potential effects in an Australian population. Therefore we aimed to identify temporal trends in stage-specific BC in New South Wales (NSW), Australia, between 1972 and 2012.

METHODS: An observational study of women who received a diagnosis of BC from 1972-2012 as recorded in the NSW Cancer Registry, a population-based registry with almost complete coverage and high rates of histological verification. We analysed trends in stage-specific incidence before screening and compared them to periods after screening began. Our primary group of interest was women in the target age range of 50-69 years, though trends in women outside the target age were also assessed.

RESULTS: Screening was not associated with lower incidence of late-stage BC at diagnosis. Incidence for all stages remained higher than prescreening levels. In women aged 50-69 years, the incidence of carcinoma in situ (CIS), localised and regional BC has more than doubled compared to the prescreening era, with incidence rate ratios ranging from 2.0 for regional (95% CI 1.95-2.13) to 121.8 for CIS (95% CI 82.58-179.72). Before the introduction of screening, there was a downward trend in distant metastatic BC incidence, and after the introduction of screening there was an increase (IRR 1.8; 95% CI 1.62-2.00). In women too young to screen the incidence of late-stage BC at diagnosis also increased, whereas localised disease was stable.

CONCLUSIONS: The incidence of all stages of BC has increased over the past 40 years, with the greatest rise seen during the established screening period for women aged 50-69 years. Our findings suggest that some of the expected benefits of screening may not have been realised and are consistent with overdiagnosis.