Night shift work and breast cancer risk in healthcare workers: a systematic review and meta-analysis

Background: Healthcare workers (HCWs) are often exposed to night shift work, which may increase cancer risk. Aims: We conducted a systematic review and meta-analysis on night shift work and breast cancer risk in HCWs. Methods: We systematically searched PubMed/Embase, evaluated the risk of bias (RoB) of the included studies, and estimated pooled relative risks (RR) using random-effects models. Results: Twelve studies were included (12 132 breast cancer cases); 60% of these were rated as 'definitely' or 'probably' high RoB in the exposure characterization domain. Compared to never exposure, pooled RRs were 1.05 (95% confidence interval [CI] 0.96-1.14, 12 studies, I2 = 39%) for ever, and 1.11 (95% CI 0.96-1.28, 9 studies, I2 = 62%) for ≥10 years, 1.25 (95% CI 1.01-1.55, 7 studies, I2 = 59%) for ≥20 years, and 1.68 (95% CI 0.77-3.65, 3 studies, I2 = 79%) for ≥30 years of night shift work. Pooled RRs were higher in case-control/nested studies and studies assessing lifetime occupational history. The RR for ≥20 years of exposure lost statistical significance in influence analysis and approached unity after correction for possible publication bias. Conclusions: Long-term night shift work may increase breast cancer risk in HCWs, but the association is far from established. If this association were real, night shift work would be responsible for a substantial number of breast cancers in HCWs.