Interval Cancers in a Large, Prospective Breast Tomosynthesis Screening Trial

PURPOSE
To assess interval cancer rate in a large, prospective digital breast tomosynthesis (DBT) screening trial in comparison with a concurrent screening population and to assess tumor characteristics of interval cancers in DBT-screening.
METHOD AND MATERIALS
The prospective ****Trial, comparing digital breast tomosynthesis (DBT) with digital mammography (DM) in 14,848 women has shown a significantly increased sensitivity with DBT. Interval cancer rate in the trial was compared with a concurrent screening population; i.e. women participating in DM screening at the same screening site during the same time period (2010-15, n=100,273 screens). Interval cancers and concurrent screens were identified through linkage with the Radiology Information System, the **Cancer Registry and the National Quality Register Breast Cancer. Confidence intervals (CI) 95 % were calculated for rates and difference between rates. Tumor characteristics were retrieved from pathology reports and invasive cancers classified according to St Gallen subtypes.
RESULTS
In total, there were 22 interval cancers in the ****Trial. The interval cancer rate was 1.5 per 1000 screens [22/14,848] (95% CI 0.9-2.2) in the ****Trial and 1.8 per 1000 screens [179/100,273] (95% CI 1.5-2.1) in the concurrent population. Although the interval cancer rate was lower in the trial, the difference of 0.3 was not statistically significant (95 % CI -0.5-0.9). Among the interval cancers in the ****Trial, the mean cancer size was 17 mm (range 2-37 mm), 2 were DCIS (grade 2 and 3), 5 luminal A-like, 9 luminal B-like HER2-, 2 luminal B-like HER2+ and 4 triple negative.
CONCLUSION
The slightly lower interval cancer rate in the trial might indicate that DBT-screening leads to the detection of clinically relevant cancers. Still, a relatively large proportion of the interval cancers had unfavorable prognostic characteristics.