Breast Cancer Among Nurses: Is the Intensity of Night Work Related to Hormone Receptor Status?

The aim of this study was to investigate whether night work is related to breast cancer receptor status. The effect of night work on the risk of estrogen receptor– and progesterone receptor–defined breast cancers was evaluated in 513 nurses diagnosed with breast cancer between 1996 and 2007 and in 757 frequency-matched controls, all of whom were selected from a cohort of Norwegian nurses. Odds ratios for the exposure “duration of work with a minimum of 6 consecutive night shifts” were compared for tumor subgroups with respect to the common control group through the use of polytomous logistic regression. Statisticallysignificant associations were observed between breast cancer and work durations of ≥5 years with ≥6 consecutive night shifts, with the highest risk observed for progesterone receptor–positive tumors (odds ratio = 2.4, 95% confidence interval: 1.3, 4.3; P-trend = 0.01). When the exposure variable was dichotomized (ever/never worked ≥6 consecutive night shifts), a borderline statistically significant heterogeneity (P= 0.05) was seen between progesterone receptor–positive and progesterone receptor–negative tumors in postmenopausal women. The association observed between consecutive night shifts and progesterone receptor–positive cancers suggests that progesterone could play an important role in the detrimental effects of night work. breast cancer; case-control studies; chronobiology phenomenon; estrogen; Norway; nurses; progesterone; receptors Abbreviations: CI, confidence interval; ER, estrogen receptor; OR, odds ratio; PR, progesterone receptor. The impact of night work on breast cancer risk has been evaluated in many recent studies. In 2007, the International Agency for Research on Cancer categorized “shift-work that involves circadian disruption” as probably carcinogenic to humans (1), and 9 of 13 published studies (2–14) on night work and breast cancer risk have indicated that night work represents a risk factor for breast cancer. In a previous study of Norwegian nurses, an association was found between breast cancer risk and night work, as characterized by a long