Objectives Previous studies suggest that flight attendants have a higher incidence of breast cancer than the general population; however, the reason remains unclear. We evaluated the relation of breast cancer incidence with estimates of cosmic radiation dose and metrics of circadian rhythm disruption among a cohort of 6092 female former US flight attendants.

Method Cohort members (or their proxy) completed a computer assisted telephone interview that collected data on incident cancers and non-occupational risk factors for breast cancer. Incident cancers were also identified through linkage with state cancer registries. Life table analyses were conducted to compare breast cancer incidence among the cohort to that in the general population and to evaluate exposure-response relations.

Results Breast cancer incidence was increased compared to the general population (observed 343; standardised incidence ratio 1.37; 95% confidence interval 1.23, 1.52). Among flight attendants, breast cancer was not significantly associated with ten-year lagged cumulative estimates of absorbed cosmic radiation dose, time spent working during normal sleep hours, or time zones crossed in the cohort overall or in women who gave birth two or fewer times. A significant positive association was observed between breast cancer incidence and these exposures only in the small subset of women who gave birth three or more times.

Conclusions Our data suggest that the effect of occupational exposures on breast cancer risk among flight attendants may be modified by non-occupational risk factors for breast cancer. Future modelling analyses will further evaluate the role of occupational exposures and non-occupational risk factors.