No association between night shiftwork and mammographic density

Background Increased mammographic density is one of the strongest risk factors for breast cancer. Night shiftwork and its related factors, which include light at night, phase shift and sleep disruption, are believed to increase breast cancer risk however, their effects on mammographic density have barely been studied. Methods This study included 1821 women enrolled in the Breast Cancer Environment and Employment Study between 2009 and 2011. Mammographic density was measured using the Cumulus software program. The association of night shiftwork factors with square root transformed absolute dense area (DA) and percentage dense area (PDA) were modelled using linear regression adjusted for confounders. Results Ever doing graveyard shiftwork (between 24:00 and 05:00 hours) was not associated with PDA (β=-0.10; 95% CI -0.27 to 0.08)) and DA (β=-0.12; 95% CI -0.33 to 0.09)). No association was found between night shiftwork related factors (light at night, phase shift and sleep disturbance) with PDA or DA. Conclusions Shiftwork and its related factors are not associated with mammographic density. Using high-quality, comprehensive shiftwork data from a large population-based breast cancer case-control study, this study suggests that mammographic density does not play a role in the relationship between shiftwork and breast cancer risk.