Kolstad et al. claim in a letter: “warnings against long-term night shift work to prevent breast cancer are not evidence based” [1]. However, 24 scientists convened by the International Agency for Research on Cancer have concluded, based on sufficient evidence in experimental animals, limited evidence in humans, and strong bio-mechanistic support, that “shiftwork that involves circadian disruption is probably carcinogenic to humans”, (group 2A) [2]. This 200 pages effort by IARC was most definitely ‘evidence based’.

Kolstad et al. base their conclusion solely on a regression analysis of 9 studies. A major impact to their analysis is a large survey of nurses without any information on shiftwork [3]. Further, two of the included studies provide only information on shiftwork for the last 10 or 15 years, respectively, before diagnosis [4, 5]; it is therefore inappropriate to include these two studies in an attempt to construct a ‘dose response’ over decades of work. Despite these severe limitations, they calculate a pooled odds ratio of 1.02 per year (95% CI: 0.92 to 1.13) by increasing years of shiftwork; this would yield a relative risk of 1.49 for 20 years of shiftwork. Overall, this estimate is similar to results from a meta-analysis (RR = 1.40; 95% CI: 1.19-1.65) based on comparison of day-workers with the most extreme shiftwork exposure category in 8 of the same studies as used by Kolstad [6]. Finally, and perhaps most importantly, lack of statistical significance should not, as done by Kolstad, be interpreted as evidence for no effect, particularly when the point estimate (1.49 for 20 years) is far from 1.0.

About 15-20% of the workforce has shiftwork [2]. Therefore, clarifying of carcinogenicity is very important, and should be based on a profound synthesis of all existing documentation, which is extensive [2], but neglected by Kolstad et al. [1].

Johnni Hansen & Richard G. Stevens

References:

1 Kolstad HA, Erlandsen M, Frost P, et al. Should we warn against night shifts to prevent breast cancer? Occup Environ Med 2010;67 (11):797.

2 International Agency for Research on Cancer. IARC monographs on the evaluation of carcinogenic risks to humans. Painting, firefighting, and shiftwork. Lyon: International Agency for Research on Cancer 2010:563-766.

3 Kjaer TK, Hansen J. Cancer incidence among large cohort of female Danish registered nurses. Scand J Work Environ Health 2009;35 (6):446-53.

4 O’Leary ES, Schoenfeld ER, Stevens RG, et al. Shift work, light at night, and breast
cancer on Long Island, New York. Am J Epidemiol 2006;164 (4):358-66.

5 Davis S, Mirick DK, Stevens RG. Night shift work, light at night, and risk of breast cancer. J Natl Cancer Inst 2001;93 (20):1557-62.

6 Viswanathan AN, Schernhammer ES. Circulating melatonin and the risk of breast and endometrial cancer in women. Cancer Lett 2009;281 (1):1-7.