Interval carcinomas in a breast cancer screening program (2007-2018): Characteristics and prognosis.

Background and aims: To analyze the radiologic and histologic characteristics of screening and interval cancers diagnosed in the period comprising 2007 through 2018 in a total of six rounds of a population-based breast cancer screening program.

Material and methods: We analyzed 1395 carcinomas detected at screening and 300 interval carcinomas diagnosed in women aged 50-69 years old who underwent digital mammography every two years during the study period. Screening mammograms were read once. To classify the interval carcinomas, we retrospectively reviewed (blind reading followed by unblinded reading) at the end of each round, recording the radiologic findings, breast density, histologic characteristics, phenotype, and surgical treatment.

Results: The interval carcinomas were classified as true interval cancers in 156 (52%) cases, false-negatives in 62 (20.5%), minimal signs in 39 (13%), occult lesions in 29 (9.5%), and impossible to classify in 14 (5%). Retrospectively, the most common radiologic findings in the false-negative cases were mass/asymmetry (64%), calcifications (16%), and distortion (13%); the most common radiologic findings in the cases with minimal signs were mass/asymmetry (58%) and calcifications (31%). There were significant differences in the histologic characteristics between cancers detected at screening and interval cancers: T1a-b [9% of the interval cancers vs. 34% of those detected at screening, P < .001]; T1c [30% of the interval cancers vs. 44% of those detected at screening P < .001], T2 or greater [61% of the interval cancers vs. 22% of those detected at screening P < .001], and the degree of axillary involvement [45% of the interval cancers vs. 27% of those detected at screening, P < .001]. There were also significant differences between cancers detected at screening and interval cancers in the proportion of cases with more aggressive subtypes (HER2+ and triple-negative): [38.5% of the interval cancers vs. 23% of those detected at screening, P < .001]. A significantly higher proportion of interval cancers were treated with mastectomies [80% vs. 67% of those detected at screening, P < .001].

Conclusions: About 20% of interval cancers were evident on screening mammograms. The most common radiologic finding in interval cancers was asymmetry/mass. Interval cancers are diagnosed at a more advanced stage than cancers identified at screening, so they sre more often treated by mastectomy. Reviewing interval cancers is essential for quality control in screening programs.