by ecancer reporter Clare Sansom
 

A number of studies have suggested that people who carry out night shift work over a long period are likely to be at higher risk of developing several different cancers.

Most of these have focused on breast cancer, and some meta-analyses have suggested that women who work night shifts may be at as much as 40-50% higher risk of developing this disease than those who do not.

The mechanism through which disruption of the normal circadian rhythm – which necessarily takes place in shift work – may act as a carcinogen is still unknown.

It has been suggested, however, that the extra light that night shift workers are exposed to may decrease their production of the hormone melatonin, which is thought to be mildly protective against cancer.

Studies of the correlation between shift work and cancer have been hampered by a lack of precision in defining a night shift.

Furthermore, there have been few attempts to date to stratify the risk by cancer subtype and, in particular, to determine whether the increased risk of breast cancer varies between hormone receptor positive and negative tumours.

Kristan Aronson of Queen’s Cancer Research Institute, Ontario, Canada and her colleagues have now carried out a comprehensive case-control study of the correlation between night shift work and breast cancer risk that was robust enough to stratify by hormone receptor status and to test the correlation in different occupational groups.

The researchers recruited a total of 1134 cases and 1179 controls into the study from Vancouver, British Columbia, and Kingston, Ontario.

All cases were women who had been diagnosed with breast cancer under the age of 80; age-matched controls were recruited from breast cancer screening programmes.

All cases and controls completed a questionnaire that covered health, family history and lifestyle characteristics and included a comprehensive occupational history.

This occupational history was used to determine whether, and to what extent, the study participants had taken part in work that included rotating or permanent night shift schedules.

Participants who had undertaken night shifts were classified by the duration of this work and by occupation.

A majority of the women who reported long periods of night shift work were working or had worked in health-related occupations such as nursing

All participants were further classified by menopausal status, and estrogen receptor (ER) and progesterone receptor (PR) status were determined for most of the cases.

Multivariable logistic regression was used to determine the odds ratio (OR) and 95% confidence interval (CI) for the relationship between different lengths of night shift work and breast cancer risk.

Approximately one-third of both cases and controls had some history of night shift work, although the proportion of women who had undertaken night shift work for very long periods (30 years or more) was noticeably higher in the cases than in the controls.

This observation was confirmed in the multivariate analysis, with a significant positive association observed between at least 30 years of night shift work and breast cancer (OR = 2.23; 95% CI = 1.12 to 4.45) but no significant association with shorter periods of shift work.

The precision of this result, however, was compromised to some extent by the relatively small number of women who reported over 30 years of night shift work.

Slight changes in the parameters used to classify night shift work made little difference to the results, and there were no significant differences between women working in health-related professions and others.

Evaluation of the risk associated with a history of night shift work by oestrogen and progesterone receptor status appeared to show a slightly stronger association between long periods of night shift work and hormone receptor positive tumours, although this result failed to reach statistical significance due to the small numbers of women in each category.

In summary, this study confirmed that night shift work is associated with an increased risk of developing breast cancer, although this association is only significant for women whose career histories include very long periods of such work.

Further studies of the mechanisms through which this risk arises and of the working patterns that confer the greatest risk will be useful for the further development of workplace health and safety policies.

Reference

Grundy, A., Richardson, H., Burstyn, I. and 6 others (2013). Occup Environ Med, published online ahead of print 1 July 2013. doi:10.1136/oemed-2013-101482