The Impact of Breast Density Notification on Interval Cancer Rates

Background: Increased breast density is associated with increased breast cancer risk, particularly interval-detected cancers. This study describes interval cancer detection rates by breast density notification status and timing of diagnosis (0–12 months vs. 13–24 months). Methods: 401,254 screening records for 235,333 clients attending a population-based screening program in 2016–2019 were extracted from the BreastScreen Western Australia database. Crude and age-standardised interval cancer rates (ASR), aggregated over three-year periods, were calculated per 10,000 client-years and reported by breast density notification status, screening round, and timing of diagnoses. Results: Overall, interval cancer rates were 2–6 times higher in clients notified they had dense breasts compared to those who were not flagged as having dense breasts. The 2016–2018 ASRs were higher in the first year (vs second year) for clients notified they had dense breasts (ASRs: 28.2 (95% CI: 12.9–66.0) vs 20.6 (95% CI: 13.2–55.8) per 10,000 client-years, respectively), particularly notified clients screening for the first time (ASRs: 32.7 (95% CI: 6.9–111.2) vs 18.2 (95% CI: 5.9–92.0), respectively). The corresponding rates for 2017–2019 showed a similar trend but of lesser magnitude (ASRs: 17.6 (95% CI: 12.9–34.2) vs 15.7 (95% CI: 11.0–35.5) for 0–12 vs 13–24 months, respectively), regardless of screening round. Conclusion: Interval cancer rates were at least similar or higher in the first 12 months compared to 13–24 months post screening for clients notified they had dense breasts, contrary to national reports of interval cancer detection rates. The clinical implications are two-fold: performance metrics for screening programs that notify women of their breast density may need refinement, namely interval cancer rate targets within 0–12 months, and breast density notification could lead to earlier diagnoses of interval-detected breast cancers.