Breast cancer stage and size detected with film versus digital mammography in New South Wales, Australia: a population-based study using routinely collected data.

Background Digital mammography has replaced film mammography in breast screening programs globally, including Australia. This led to an increase in the rate of detection, but whether there was increased detection of clinically important cancers is uncertain. Methods In this population-wide retrospective cohort study in New South Wales, Australia spanning 2004-2016 and including 4,631,656 screens, there were 22,965 cancers in women screened with film (n=11,040) or digital mammography (n=11,925). We examined the change in tumour characteristics overall and how these rates changed over time, accounting for changes in background rates using an interrupted time-series. Comparisons were made to unscreened women (n=26,326) during this time. Results We found increased detection of in-situ cancer (3.36 per 10,000 screens), localised invasive, and smaller-sized breast cancers attributable to the change in mammography technology, while screen-detected intermediate-sized and metastatic breast cancers decreased. Rates of early-stage and intermediate-sized interval cancers increased, and late-stage (-1.62 per 10,000 screens) and large interval cancers decreased. In unscreened women, there were small increases in the temporal trends of cancers across all stages. Conclusion At least some of the increased detection of smaller early-stage cancers may have translated into a reduction in larger and late-stage cancers, indicating beneficial detection of cancers that would have otherwise progressed. However, the increased detection of smaller early-stage and small cancers may also have increased overdiagnosis of lesions that would otherwise have not caused harm. Impact Robust evaluation of potential benefits and harms is needed after changes to screening programs.