Objective: To analyze phenotypic classification and other risk factors for interval breast cancer, focusing on true interval and false negative cancers.
Methods: A nested case-control study was performed among 115 cancers detected between two screening mammograms (interval cancers) and 115 screen-detected cancers diagnosed between 1995 and 2008 in a population-based breast cancer screening program in Barcelona (Spain). Bivariate and multivariate analyses were performed to compare patient and tumor molecular characteristics among all interval cancers, true intervals and false negatives, and screen-detected cancers.
Results: A total of 42.5% of interval cancers were true interval tumors and 16.2% were false negatives. High breast density and triple negative phenotype were more frequent in true interval cancers than in screen-detected cancers (57.6 and 34.1%, respectively for breast density, p = 0.023; 28.1 and 7.5%, respectively for triple negative phenotype, p = 0.028), while no statistically significant differences were observed between false negatives and screen-detected cancers. The main adjusted factors associated with true interval cancers compared with screen-detected cancers were high breast density and triple negative phenotype (OR = 3.1, 95% CI, 1.03-9.24 and OR = 8.9, 95% CI, 2.03-38.62, respectively).
Conclusion: A more aggressive molecular phenotype and high breast density were identified in breast tumors that truly arise in the interval between screenings.