O-384 The association between night shift work and risk of breast cancer: results from the Nightingale Study

Abstract

<h3>Introduction and Objectives</h3> The International Agency for Research on Cancer confirmed night shift work as a ‘probable’ carcinogen in an updated evaluation (2019). Noting that evidence from human studies is still inconclusive, in particular due to the lack of prospective cohort studies with detailed exposure assessment. We prospectively examined different metrics of night work (e.g. duration, lifetime mean no. nights per month and cumulative no. nights) and risk of breast cancer, and whether the association was dependent on age or change in body weight. <h3>Methods</h3> Overall, 55,350 female nurses completed extensive questions on shift work and were followed for incident breast cancer. Cox regression yielded multivariable-adjusted breast cancer incidence hazard ratios (HRs) and 95% confidence intervals (CIs) for night work groups vs never night work, and Wald tests were used to assess potential interaction with age and change in body weight. <h3>Results</h3> During 5,5 years of follow-up, 687 cases of incident invasive breast cancer were registered. The age adjusted HRs were 1.26 (95% CI 1.01–1.57) for ever vs never night work, and 1.32 (95% CI 1.01–1.74) for the tertile with the greatest cumulative no. nights (≥991 nights) vs never night work. No trends were observed with increasing levels of different exposure metrics. There was a statistically significant interaction between night work and weight gain on risk of breast cancer; among women with 12–18 kg weight gain since age 18, the HR for ever vs never night work was 2.45 (95% CI 1.28 to 4.68). <h3>Conclusion</h3> Higher duration, mean no. nights per month and cumulative no. nights were associated with a moderately increased risk of invasive breast cancer. Particularly for night shift workers with weight gain of 12–18 kg, the risk was increased. Our results point towards a potential interaction between change in body weight and night shift work on breast cancer risk.