Occupational exposures and male breast cancer: A nested case-control study in the Nordic countries

Our plain-English summary of Occupational exposures and male breast cancer: A nested case-control study in the Nordic countries, by M. Talibov, J. Hansen, S. Heikkinen, et al (The Breast, 2019).

The study

• Authors: M. Talibov, J. Hansen, S. Heikkinen, J.I. Martinsen, P. Sparen, L. Tryggvadottir, E. Weiderpass, E. Pukkala
• Journal: The Breast
• Published: 2019;48:65-72
• Cited by: 11 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Background: Male breast cancer (MBC) is a rare and understudied disease. There is limited evidence on association between environmental and occupational agents and MBC. Some similarities in risk factors may be shared with female breast cancer. We evaluated solvents, metals, exhaust gases and other agents in relation to MBC within the large Nordic Occupational Cancer Study (NOCCA).

Methods: The study included 1469 MBC cases and 7345 controls from Finland, Iceland and Sweden, matched for the date of birth, sex and country. Cases were identified through national cancer registries. Data on occupation and other demographic indicators were collected from census records and population registries. Overall, 24 occupational exposures were assessed. Exposure estimates were assigned by linking job titles to job-exposure matrices (NOCCA-JEM). Odds ratios (OR) and 95% confidence intervals (95% CI) were estimated by using conditional logistic regression models.

Results: Significantly decreased overall OR was observed for physical workload (OR = 0.78, 95% CI 0.67-0.91). Protective effect of physical workload was stronger by increasing exposure level with significant dose-response relationship (p-trend<0.01). Non-significantly increased ORs were observed for trichloroethylene, iron, lead, chromium, welding fumes and wood dust, and decreased ORs for asbestos, silica dust and perchloroethylene. However, these results were not consistent across all analyses.

Conclusions: The current study showed 20-25% protective effect for physical workload at work, while no strong evidence for other agents was observed.

Copyright © 2019 Elsevier Ltd. All rights reserved.

In plain English

This nested case-control study drew on the Nordic Occupational Cancer Study (NOCCA) cohort. It compared 1469 men diagnosed with breast cancer between 1961 and 2005 in Finland, Iceland and Sweden with 7345 controls matched by country, sex and year of birth. Exposures were estimated from job titles in census records using a job-exposure matrix, not measured for each person. They included asbestos (the article does not name fiber types), solvents, metals, exhaust gases and physical workload. For men ever exposed to asbestos, the odds ratio was 0.83 (95% CI 0.73 to 0.95) when asbestos was analyzed alone. In the multiple-exposure model, which analyzed the agents together but kept highly correlated agents in separate models, it was 0.89 (95% CI 0.73 to 1.10), not statistically significant. Physical workload was linked to lower odds (0.78, 95% CI 0.67 to 0.91), with stronger effects at higher levels. After correction for multiple comparisons, only physical workload and ultraviolet radiation (in single-agent analysis) stayed significant. The authors concluded that physical workload showed a 20 to 25% protective effect, with no strong evidence for other agents. They noted possible exposure misclassification.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Case-control study nested within a registry-based cohort (NOCCA), with exposures assigned by job-exposure matrix
• Who was studied: 1469 men with incident breast cancer diagnosed 1961 to 2005 in Finland, Iceland and Sweden, plus 7345 controls (five per case) randomly selected from the NOCCA cohort and matched for country, sex and year of birth. About 77% were from Sweden, 21% from Finland and 2% from Iceland.
• Exposure: 24 occupational agents, including asbestos (unit given as fibres per cubic centimetre; fiber type not specified), six organic solvents, metals, diesel and gasoline exhaust, silica dust, wood dust, welding fumes, night-shift work, magnetic fields, ultraviolet radiation and physical workload. Cumulative exposure was estimated by linking census job titles to the NOCCA job-exposure matrix.
• Main results:
  • Asbestos, ever exposed (326 cases, 1889 controls): odds ratio 0.83 (95% CI 0.73 to 0.95) in the single-exposure model, statistically significant only in that model
  • Asbestos, ever exposed, in the multiple-exposure model (agents analyzed together, adjusted for socioeconomic status, with highly correlated agents placed in separate models): odds ratio 0.89 (95% CI 0.73 to 1.10), p = 0.29
  • In the multiple-exposure analysis, highly correlated agents (Spearman correlation above 0.80) were put into separate models
  • Highest asbestos exposure category (above 17.4): 30 cases and 192 controls. Odds ratio 0.76 (95% CI 0.52 to 1.13; p-trend 0.01) in the single-exposure model and 0.74 (95% CI 0.39 to 1.38; p-trend 0.17) in the multiple-exposure model
  • Physical workload was linked to significantly lower odds of male breast cancer: odds ratio 0.76 (95% CI 0.67 to 0.85) in the single-exposure model and 0.78 (95% CI 0.67 to 0.91) in the multiple-exposure model. The effect grew stronger with higher exposure (p-trend less than 0.01).
  • After correction for multiple comparisons (Holm's method), only physical workload (both models) and ultraviolet radiation (single-exposure model only) remained statistically significant
  • The positive and negative associations for other agents, including asbestos, were mostly not consistent across the sensitivity analyses
• The authors' conclusion: The authors concluded that most associations were inconsistent across analyses, but that a 20 to 25% protective effect of physical workload was significant in all analyses, with an exposure-response trend.
• Limitations the authors state:
  • Potential exposure misclassification, because there was no lifetime job history and subjects were assumed to stay in the job recorded at census
  • The job-exposure matrices assign the same average exposure to everyone in an occupation, although exposure within an occupation may vary substantially
  • Smoking, alcohol, leisure-time physical activity and obesity could not be controlled in the main analyses. Lifestyle adjustment was possible only for the Finnish data.
  • Some findings may be due to chance, because many comparisons were made

Topics

Medical terms in this study, explained in our Lexicon: Occupational Exposure, nested case-control study, Odds Ratio, confidence interval, Dose-response relationship.

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