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.