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.
- shift work
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Contributors LF, JH, CS, EW and JS designed and carried out the original study. SE-Z conducted the data analysis. SE-Z drafted the manuscript, and all authors critically revised it and approved the final version.
Funding The Breast Cancer Environment and Employment Study was funded by a National Health and Medical Research Council Australia (NHMRC) Project Grant #572530. SE-Z was supported by a Cancer Council Western Australia Cancer Epidemiology Initiative grant. JS is funded by the National Breast Cancer Foundation as a principal research fellow.
Competing interests None declared.
Patient consent for publication Not required.
Ethics approval Study approval was obtained from the The University of Western Australia’s Human Research Ethics Committee and the Western Australian Department of Health.
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement No data are available.