Night-shift Work and Risk of Prostate Cancer

Results From a Canadian Case-Control Study, the Prostate Cancer and Environment Study

Christine Barul; Hugues Richard; Marie-Elise Parent

Disclosures

Am J Epidemiol. 2019;188(10):1801-1811. 

In This Article

Abstract and Introduction

Abstract

Night-shift work involving disruption of circadian rhythms has been associated with breast cancer risk. A role in prostate cancer is also suspected, but evidence is limited. We investigated the association between night-shift work and prostate cancer incidence in the Prostate Cancer and Environment Study (PROtEuS), a population-based case-control study conducted in 2005–2012 in Montreal, Quebec, Canada. Participants were 1,904 prostate cancer cases (432 high-grade cancers) and 1,965 population controls. Detailed work schedules for each job held for at least 2 years (n = 15,724) were elicited in face-to-face interviews. Night-shift work was defined as having ever worked ≥3 hours between midnight and 5:00 AM ≥3 nights/month for ≥1 year. Unconditional logistic regression was used to estimate odds ratios and 95% confidence intervals for the association between night-shift work and prostate cancer, adjusting for age, ancestry, and education. No association was found between overall prostate cancer and night-shift work metrics, including ever exposure, duration, intensity, cumulative exposure, rotating shifts, and early-morning shifts. For none of the exposure indices was there evidence of heterogeneity in odds ratios between low- and high-grade cancers. Sensitivity analyses restricting exposures to ≥7 nights/month or considering screening history yielded similar results. Our findings lend no support for a major role of night-shift work in prostate cancer development.

Introduction

Prostate cancer is the most frequently diagnosed solid tumor among men in industrialized countries.[1] The global burden of the disease is still rising, and more than 2 million men are predicted to be affected worldwide by 2040.[1] Factors other than screening probably contribute to geographical disparities in incidence.[2] Age, African ancestry, and a first-degree family history of prostate cancer are the only confirmed risk factors for the disease. Identifying modifiable factors which could be targeted by public health measures for prostate cancer prevention remains a considerable research challenge. A role for Western-related environmental influences, including diet, other lifestyle factors, and workplace exposures, has been suggested in various studies investigating the spatiotemporal trends in prostate cancer[2,3] and disease risk in Asian migrant populations.[4]

Night-shift work is increasingly suspected to be involved in cancer development through a number of compelling mechanisms, including disruption of circadian rhythms. Other mechanisms have been advanced as well.[5] Initially proposed for breast cancer, the night-shift work hypothesis was recently extended to prostate cancer. In its 2007 evaluation, the International Agency for Research on Cancer (IARC) classified shift work that involves circadian rhythm disruption as probably carcinogenic to humans on the basis of sufficient evidence from animal models but limited evidence in humans.[6] Inadequate and inconsistent exposure assessment was felt to play a key role in the uncertainty around the overall body of evidence on night-shift work and cancer. In response to this, an IARC Working Group issued recommendations to standardize the aspects or domains of night-shift work that were captured across studies in order to better understand what specific aspects of night-shift work are carcinogenic.[7] In its 2018 review,[8] the National Toxicology Program concluded that persistent night-shift work that causes circadian rhythm disruption is known to be a human carcinogen. A peer-review panel judged that the evidence for breast cancer was sufficient, while for prostate cancer it was found to be limited. For the latter, several issues were raised, including the small number of methodologically strong investigations, poor characterization of night-shift work exposure across studies, and the fact that few studies have evaluated disease aggressiveness or the role of screening.

The possible role of night-shift work in cancer development continues to be of high scientific and public health interest. Our objective in the present study was to investigate the association between night-shift work, early-morning shifts, and the risk of prostate cancer, applying recommendations for a rigorous exposure assessment protocol.

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