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WORKING FOR A HEALTHY FUTURE
INSTITUTE OF OCCUPATIONAL MEDICINE . Edinburgh . UK www.iom-world.org
Shift Work and Breast Cancer: What’s the evidence and what can we do to reduce the risks?
John CherrieResearch Director
Summary…
• Breast cancer in the UK• Shift work involving night work• The epidemiological evidence relating to
shift working• Intercontinental flights• Other data• The IARC evaluation• What can we do about this problem?• Other messages…
The statistics in the UK…
• 49,564 women and 397 men were diagnosed with in breast cancer in 2010
• 11,633 deaths from invasive breast cancer
• 85% of women survive their invasive breast cancer for five years or more
• The lifetime risk of developing breast cancer is estimated to be 1 in 8 for women
Risk factors…
• Age• Reproductive factors, e.g. age at menarche, age
at first birth, parity, breast feeding, age at menopause
• Hormones, both endogenous and exogenous• Family history• Body weight and physical activity• Alcohol and diet• Height
Prevalence of shift work in the EU…
Data for shift work involving night work
From the IARC monograph
Tends in night shift work…
http://www.eurofound.europa.eu/surveys/smt/ewcs/ewcs2010_02_06.htm
EU15 data
The body of evidence for shift work…
Three studies published in 2001…
Hansen J. Light at night, shiftwork, and breast cancer risk. J Natl Cancer Inst 2001;93:1513–5.
A recent systematic review…
Kamdar BB, et al. Night-shift work and risk of breast cancer: a systematic review and meta-analysis. Breast Cancer Res Treat 2013:1–11.
Risk of breast cancer for
“ever” versus “never”
night-shift workers
A recent systematic review…
• Pooled relative risk:• ever night-shift work exposure 1.21 • short-term night-shift workers (<8 years) 1.13 • long- term night-shift workers (>8 years) 1.04
• Review highlights flight attendants (with international or overnight work) and nurses working night-shifts long-term were at increased risk of breast cancer
Semiconductor workers in Scotland…
• An initial cohort analysis showed a small non-significant excess of breast cancer • 20 deaths vs 15 expected
• Detailed case-control study and further follow-up• Incidence of breast cancer still not significantly
increased
Darnton A, et al. An updated investigation of cancer incidence and mortality at a Scottish semiconductor manufacturing facility with case-control and case-only studies of selected cancers. Occup Environ Med 2012;69:767–9.
Hazard OR (95% CI)
Nightshift 1.2 (0.4 – 3.5)
Rotating shifts 1.9 (0.5 – 6.9)
Circadian disruption 1.2 (0.4 – 3.5)
Flight attendants…
Tokumaru O, Haruki K, Bacal K, Katagiri T, Yamamoto T, Sakurai Y. Incidence of cancer among female flight attendants: A meta-analysis. Journal of Travel Medicine 2006;13:127–32.
(A) Pukkala et al;
(B) Haldorsen et al;
(C) Rafnsson et al;
(D) Reynolds et al;
(E) Linnersjö et al;
(F) Tokumaru et al.
Understanding the risks…
Hansen J, Stevens RG. Case-control study of shift-work and breast cancer risk in Danish nurses: Impact of shift systems. Eur J Cancer. 2011 Aug. 16. 1–8.
Understanding the risks…
Alternative hypotheses…
• Light at night suppresses the production of melatonin
• Phase shift – animal studies have shown links between clock gene function and the induction of tumours
• Sleep deprivation• Shift workers lifestyles that increase cancer risk, • Shift workers lay have lower vitamin D levels
because of less exposure to daylight
Fritschi, L. et al., 2011. Hypotheses for mechanisms linking shiftwork and cancer. Medical Hypotheses, 77(3), pp.430–436.
The IARC evaluation…
Based on “limited evidence in humans for the carcinogenicity of shift-work that involves nightwork”, and “sufficient evidence in experimental animals for the carcinogenicity of light during the daily dark period (biological night)”, the Working Group concluded that “shift-work that involves circadian disruption is probably carcinogenic to humans” (Group 2A).
Straif K, Baan R, Grosse Y, Secretan B. Carcinogenicity of shift-work, painting, and fire-fighting. Lancet Oncology 2007.
British cancer burden estimates…
Rushton L, Bagga S, Bevan R, Brown TP, Cherrie JW, Holmes P, et al. Occupation and cancer in Britain. Br J Cancer 2010;102:1428–37.
5.3% (4.6 – 6.6%)
Men = blueWomen = red
British cancer burden estimates…
So what can we do?
Ban women from night shifts? ✗
Restrict the number of years women work nights ?
Have rapidly rotating shifts (1–2 consecutive nights), which causes less disruption of circadian rhythms?
✔
Use forward- rather than backward-rotating shifts? ✔
Use bright lighting in the workplace? ✗
Offer earlier or more intensive mammography screening to female night shift workers
✗
Melatonin supplementation? ?
Increase awareness and information for female workers ✔
Bonde JP, et al. Work at night and breast cancer – report on evidence-based options for preventive actions. Scand J Work, Env and Health 2012;38:380–90.
