Surveillance of interval cancers (IC) provides a measure of breast screening efficacy. Increased breast density is a predictor of breast cancer risk and of the risk of IC in screening. Improving screening sensitivity in women with dense breasts, through adjunct ultrasound (US), may potentially reduce IC; however this has not been proven. We report on first-year IC in a retrospective cohort of 8865 women who had 19,728 screening examinations (2001-2006): women with non-dense (D1-D2) breasts received mammography (M) screening, and women with dense (D3-D4) breasts also received ultrasound. Data linkage with both hospital discharge records and cancer registry databases was used to identify IC. Underlying cancer rates (cancers observed within 1-year from screening) were 6.3/1000 screens in the D1-D2 group and 8.3/1000 screens in the D3-D4 group. Cancer detection rate (CDR) was 5.98/1000 in all screening examinations; in D3-D4 breasts ultrasound had an additional CDR of 4.4/1000 screens. There were 21 first-year IC, an overall interval cancer rate (ICR) of 1.07/1000 negative screens: 0.95/1000 in women < 50 years and 1.16/1000 screens in women ≥ 50 years. ICR by breast density were 1.0/1000 negative screens in D1-D2, and 1.1/1000 negative screens in D3-D4. Interval cancers were early stage (in situ or small invasive) cancers, almost all were node-negative. Screening sensitivity was 83.5% for mammography alone in D1-D2 breasts relative to 86.7% for mammography with ultrasound in D3-D4 breasts. Our study shows that including ultrasound as adjunct screening in women with D3-D4 breasts brings the IC rate to similar levels as IC in non-dense breasts--this suggests that additional cancer detection by ultrasound is likely to improve screening benefit in dense breasts, and supports the implementation of a randomised trial of adjunct ultrasound in women with increased breast tissue density.
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