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

Results

The main characteristics of cases and controls are presented in Table 1. Most subjects were of French descent. As expected, a greater proportion of cases than of controls were of sub-Saharan African ancestry and had a positive family history of prostate cancer. Controls were generally more educated and were older by 1 year, on average, than cases owing to the slightly longer time required to secure interviews. Reported night-shift jobs involved mainly protective services (16.4% of jobs), materiel handling (6.0%), and motor transport operations (5.7%). More specifically (Table 2), night-shift work occurred most frequently among firefighters (88.3% of jobs), persons in distilling, subliming, and carbonizing occupations (81.8%), deck officers (80.0%), and airline pilots and flight workers (68.8%).

Associations between night-shift work and overall prostate cancer risk are shown in Table 3. Compared with men who had never engaged in night-shift work, there was no clear evidence that those who had were at increased risk of prostate cancer for any of the metrics evaluated, including categories representing the highest exposures. Some risk estimates were slightly above 1, but confidence intervals included the null value in all metric categories, and no dose-response patterns emerged. A modest increase in risk was observed among men who had always worked on night-shift schedules involving forward rotation (odds ratio (OR) = 1.23, 95% confidence interval (CI): 0.96, 1.58).

In additional analyses, we investigated whether associations with the different metrics varied according to tumor aggressiveness (Table 4). For low-grade cancers, some elevated risks were apparent in the highest category of intensity of rate of rotation (OR = 2.10, 95% CI: 0.99, 4.47), based on small numbers, but other associations were not elevated. No clear patterns emerged for aggressive tumors. P values for heterogeneity between low- and high-grade cancers varied between 0.15 and 0.94.

Odds ratios for overall prostate cancer based on the timing of the last night-shift job were 1.07 (95% CI: 0.82, 1.40) when the last night-shift job occurred within 20 years of the index date and 0.95 (95% CI: 0.65, 1.38) when it was further in the past. Among current/recent night-shift workers (last night-shift job within 2 years of the index date), the odds ratio was 1.28 (95% CI: 0.85, 1.91). Timing of exposure was not associated with tumor aggressiveness (data not shown).

In additional analyses, we examined associations with early-morning shifts (Table 5). We found no increased risks with duration or intensity of exposure or with cumulative exposure. There was no heterogeneity in odds ratios between tumor grades.

We conducted several sensitivity analyses. Increasing the cutoff defining exposure to night-shift work from ≥3 nights/month to ≥7 nights/month did not substantially alter results (Web Table 1).

When restricting controls to men who had been screened for prostate cancer during the 2 years prior to interview, risk estimates for overall prostate cancer and according to tumor aggressiveness remained largely unchanged for night-shift work or early-morning shifts (Web Table 2 and Web Table 3).

Complete-case analyses without imputations generated findings consistent with those from our main analyses. Results based on tertiles of exposure, which allowed for greater numbers of subjects in individual categories, were also consistent with those of our main analyses (data not shown).

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