Exposure to asbestos and the risk of colorectal cancer mortality: a systematic review and meta-analysis

Our plain-English summary of Exposure to asbestos and the risk of colorectal cancer mortality: a systematic review and meta-analysis, by K. Kwak, D. Paek, K.E. Zoh (Occupational and Environmental Medicine, 2019).

The study

• Authors: K. Kwak, D. Paek, K.E. Zoh
• Journal: Occupational and Environmental Medicine
• Published: 2019;76(11):861-871
• Type of article: Meta-analysis, Systematic review
• Cited by: 20 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Asbestos exposure is associated with mesothelioma and cancer of the lung, larynx and ovary. However, the association between asbestos exposure and colorectal cancer is controversial despite several systematic reviews of the literature, including a number of meta-analyses. We performed a systematic review and meta-analysis to evaluate quantitatively the association between exposure to asbestos and colorectal cancer. We searched for articles on occupational asbestos exposure and colorectal cancer in PubMed, EMBASE and Web of Science published before April 2018. In total, 44 articles were selected and 46 cohort studies were analysed. The overall pooled risk estimates and corresponding 95% CIs of the association between occupational asbestos exposure and colorectal cancer were calculated using a random-effects model. Subgroup analyses and sensitivity tests were also performed. There was a significantly increased risk of colorectal cancer mortality among workers exposed to asbestos occupationally, with an overall pooled SMR of 1.16 (95% CI: 1.05 to 1.29). The pooled SMR for colorectal cancer was elevated in studies in which the asbestos-associated risk of lung cancer was also elevated (1.43; 95% CI: 1.30 to 1.56). This implies that the risk of colorectal cancer mortality increases as the level of asbestos exposure rises. A sensitivity analysis showed robust results and there was no publication bias. Although the effect size was small and the heterogeneity among studies was large, our findings indicate that occupational exposure to asbestos is a risk factor for colorectal cancer.

© Author(s) (or their employer(s)) 2019. No commercial re-use. See rights and permissions. Published by BMJ.

In plain English

This systematic review and meta-analysis pooled 46 published cohort studies of asbestos-exposed workers, drawn from 44 articles. Together these covered 1642 colorectal cancer deaths in Europe, the USA, Canada, Australia and Asia. Each study compared observed colorectal cancer deaths with the number expected (the SMR). Across all studies the pooled SMR was 1.16 (95% CI: 1.05 to 1.29), a statistically significant increase, though results varied widely between studies. The authors used a lung cancer SMR of 2 or more (at least twice the expected lung cancer deaths) as an indirect stand-in for high asbestos exposure. In the 21 studies at or above that level, the pooled colorectal cancer SMR was 1.43 (95% CI: 1.30 to 1.56). In the 24 studies below it, it was 0.97 (0.83 to 1.13). Estimates for colon/intestine cancer, for rectal cancer, and for studies with smoking data were not statistically significant. Only two studies adjusted for smoking. The authors concluded that occupational asbestos exposure is a risk factor for colorectal cancer. They noted that the effect size was small and that the result should be confirmed in further studies.

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: Systematic review and meta-analysis of published cohort studies, pooled with random-effects models
• Who was studied: 46 cohort studies of asbestos-exposed workers, from 44 articles found by searching PubMed, EMBASE and Web of Science on 2 April 2018. The studies covered 1642 colorectal cancer deaths and were conducted in Europe, the USA, Canada, Australia and Asia between 1963 and 2018.
• Exposure: Occupational asbestos exposure in several industries: textile (10 studies), mining and milling (8), asbestos cement (8), insulation (4), shipbuilding (2) and others. Environmental exposure was excluded.
• Main results:
  • Across all 46 studies, the pooled SMR for colorectal cancer mortality was 1.16 (95% CI: 1.05 to 1.29), with large heterogeneity among studies (I2=62.0%).
  • In cohorts with high lung cancer mortality (21 studies), the pooled colorectal cancer SMR was 1.43 (95% CI: 1.30 to 1.56). High lung cancer mortality meant a lung cancer SMR of 2 or more. The authors used high lung cancer mortality as a substitute for high asbestos exposure.
  • In cohorts with low lung cancer mortality (lung cancer SMR below 2; 24 studies), the pooled colorectal cancer SMR was 0.97 (0.83 to 1.13).
  • Insulation workers (4 studies, all involving amosite) had a pooled SMR of 1.49 (95% CI: 1.26 to 1.75).
  • Studies reporting colon/intestine and rectal cancer separately gave summary SMRs of 1.15 (95% CI: 0.96 to 1.39) for colon/intestine cancer and 1.20 (95% CI: 0.98 to 1.47) for rectal cancer. Neither was statistically significant.
  • Restricted to studies with smoking data, the overall pooled SMR was 1.15 (95% CI: 0.98 to 1.35). This was not statistically significant.
  • Removing each study in turn gave pooled estimates from 1.15 to 1.18, all statistically significant. Egger's test did not indicate publication bias (p=0.645).
• The authors' conclusion: The authors concluded that, although the overall effect size was small and heterogeneity was large, their findings imply that occupational exposure to asbestos is a risk factor for colorectal cancer.
• Limitations the authors state:
  • Heterogeneity among studies was large, meaning results were inconsistent and harder to generalise.
  • Most included studies did not consider other risk factors for colorectal cancer, such as red meat, obesity, alcohol and smoking.
  • Only two studies adjusted for smoking when calculating colorectal cancer risk.
  • Asbestos exposure levels could not be classified accurately, so high lung cancer risk was used as a substitute for high exposure.
  • Peritoneal mesothelioma was often misdiagnosed as colorectal cancer until the early 1960s. The authors found no difference between studies started before and after 1965, but they could not rule out exaggerated colorectal cancer mortality in earlier studies.
  • The overall effect size was small, and the result should be confirmed in further studies.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Colon cancer.

Medical terms in this study, explained in our Lexicon: Colorectal Cancer, Systematic Review, Meta-Analysis, Asbestos Exposure, Mesothelioma, Lung Cancer, Occupational Exposure.

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