Interval breast cancer: Analysis of occurrence, subtypes and implications for breast cancer screening in a model region.

Abstract Purpose Interval breast carcinomas (IBC) constitute a subgroup of malignancies in women participating in a breast cancer screening programme, yet diagnosed outside of a screening appointment. Tyrol is an Austrian screening model region with a dedicated IBC board. We analysed IBC subtype distribution, demographic and biological parameters and implications for screening programmes. Method 161 patients with an IBC diagnosed from 2014 to 2017 were retrospectively analysed and grouped into true, occult, minimal-signs, and false negative (due to reading or technical error) IBCs cases by three independent readers. The influence of demographic and disease-related covariates were assessed. Results The median interval from screening to diagnostic diagnosis was 12.8 months (range 1.1 to 23.9 months). Most cases were true (36.02%), occult (31.06%) and false-negative IBCs due to reading errors (29.81%). Interobserver agreement was rated as ‘high’ between all readers. Higher breast density was associated with true and occult IBCs. The rate of invasive subtypes was highest in true IBCs. Regardless of smaller tumour size in true and occult IBCs, doubling time was lower and ki-67 index higher in true and occult compared to false-negative IBCs. Conclusions True and occult IBCs present with a more aggressive biological phenotype and are associated with younger age and higher breast density. Additional yearly ultrasound examinations in women at risk may aid in the earlier detection.