Objectives Identifying work-related risks for female breast cancer is often challenged by confounding hormonal and reproductive factors. Examining risk in men may help us understand occupational breast cancer risk factors, but few studies have been able to do this due to the rarity of male breast cancer. We used a large cohort of 2+ million Ontario workers to determine if evidence of male breast cancer by occupation can inform our understanding of female breast cancer.

Methods Our study uses data from the Occupational Disease Surveillance System (ODSS), established through administrative data linkage, to follow 2,190,246 Ontario workers derived from WSIB lost time claims data (1983–2014). Breast cancer cases were identified in the Ontario Cancer Registry (OCR, 1964–2016). Cox-proportional hazard models were used to estimate age-adjusted hazard ratios (HR) and 95% confidence intervals (CI) with an internal reference group of all other workers in the cohort.

Results A total of 17, 865 and 492 breast cancer cases were identified in working women and men, respectively. By occupation, elevated rates were observed for management/administration, social sciences, teaching and related, and medicine and health. Specifically, elevated rates were observed for elementary and secondary school teachers (HRw=1.27, 95% CI 1.19–1.35; HRm=2.16, 95% CI=1.02–4.55), nurses (HRw=1.08, 95% CI=1.04–1.13; HRm=4.73, 95% CI=3.02–7.41) and other medicine/health workers (HRw=1.18, 95% CI=1.03–1.34; HRm=6.50, 95% CI=2.69–15.7).

Conclusion The ODSS enabled the analysis of a large cohort of working men and women with breast cancer. Findings show that at-risk groups were similar among both sexes, demonstrating the importance of occupation for breast cancer risk, although the HRs were much higher for men. Further research is needed in potential job-related factors such as sedentary behaviour, stress, shift work, and for some occupations, radiation exposure.