Objective Night work and shift work are among the most prevalent occupational exposures. It is postulated that night work can result in a higher risk of breast cancer. There is uncertainty in evidence from previous research to implicate night work in causing breast cancer. We did a systematic review to assess the strength of association between exposure to night shift work and breast cancer incidence.
Methods Multiple databases and non-electronic sources were systematically searched to identify case control and cohort studies involving females in night shift work. The comparison was non-shift or day work and the outcome was incident breast cancer. We assessed studies for risk of bias using a content specific piloted checklist on 10 domains of interest. We performed random effects meta-analysis and meta-regressions of study-specific incremental relative risks to determine the risk of cancer associated with a 5 year and 300 night shift increases in exposure. Sensitivity analyses were performed to test model assumptions.
Results We included 16 studies (12 case control and four cohorts). Ten studies were conducted in Western Europe, four in USA and two in China. Almost half of the studies were on nurses. None of the studies were at a low risk of bias however five were at a moderate risk of bias. Studies with appropriate exposure assessment were lacking, with only one measuring exposure in an objective way prospectively. Twelve studies (nine case controls and three cohorts) provided data for the random effects meta-regression of dose response using generalised least square estimates.
Conclusions Several new studies have become available since the last review in 2008. However, exposure assessment in existing studies is still far from optimal. Results of the meta-analysis will be presented at the conference.