Objectives In 2007, the International Agency for Research on Cancer categorised shift-work that involves circadian disruption as probably carcinogenic to humans. oestrogen and progesterone are believed to play a central role in the development of breast cancer. Although several plausible biologic mechanisms of a relationship between night work and breast cancer have been postulated, the particular breast cancer subgroups have not been adequately examined. The objective of this study was to investigate whether night work is related to breast cancer receptor status.
Methods The effect of night work on the risk of oestrogen receptor- (ER) and progesterone receptor- (PR) defined breast cancers was evaluated in 513 nurses, diagnosed between 1996 and 2007, and 757 frequency-matched controls, all selected from a cohort of Norwegian nurses, using polytomous logistic regression. Odds ratios for the exposure “duration of work with minimum 6 consecutive night shifts” were compared for tumour subgroups with respect to the common control group.
Results Statistically significant associations were observed between the highest exposure to night work (≥ 5 years with ≥ 6 consecutive night shifts) and breast cancer, the largest increase observed for PR+ tumours (odds ratio: 2.4, 95% confidence interval: 1.3, 4.3, P-trend = 0.01). No significant odds ratio heterogeneity was found for the night work variable between the different receptor-defined tumour subgroups when using 4 exposure categories. When dichotomising the exposure variable (ever/never worked ≥ 6 consecutive night shifts), a borderline statistically significant heterogeneity was seen between PR+ and PR- tumours in postmenopausal women.
Conclusions The association observed between long duration with many consecutive night shifts and PR+ cancers, suggests that progesterone may play an important role in the detrimental effects of night work.
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