July 1, 2013

expert reaction to study on long term shift work and breast cancer

A study in Occupational and Environmental Medicine found working night shifts for 30 or more years doubles the risk of developing breast cancer, and is not confined to nurses as previous research had indicated. A before the headlines analysis was sent out to accompany this roundup.

Dr Jane Green, Clinical Epidemiologist at the University of Oxford, said:

“This seems to be a well-conducted study with the benefit of more detailed occupational history than in many studies.  The finding of an increased risk of breast cancer in women with a long history of shift work adds to similar results from some previous studies, but does not change the existing consensus: that while there is some evidence to associate increased risk of breast cancer with very long term shift work, the evidence is not yet sufficient to be sure and certainly not sufficient to give a public health message about working shifts.

“With further work, the increased cancer risk may not be confirmed; and even if it is, could turn out to be explained by differences in known breast cancer risk factors among shift workers vs non-shift workers.  In other words, it might not be the shift work itself that is to blame.  We therefore cannot interpret this study to say shift work causes cancer.

“The authors found no trend in risk by duration of shift work when looking at short, medium and long-term relative risk – as the press release points out, the 1-14 and 15-29 year groups showed no increased risk, which suggests we should be cautious in giving a causal interpretation.  The relative risk estimates for the 30+ year shift work group are – as the authors point out- based on very small numbers of cases, so the true value might be lower- or higher- than the overall 2-fold increase reported.  Finally: this is a case-control study and cannot exclude recall bias as an explanation for the association.

“Overall I’d say this is a good study which adds to existing evidence, but does not tip the balance.”

‘Increased risk of breast cancer associated with long-term shift work in Canada’ by Anne Grundy et al. in Occupational and Environmental Medicine on Monday 1st July.