Mesothelioma and asbestos fiber type. Evidence from lung tissue analyses

Our plain-English summary of Mesothelioma and asbestos fiber type. Evidence from lung tissue analyses, by J.C. McDonald, B. Armstrong, B. Case, et al (Cancer, 1989).

The study

• Authors: J.C. McDonald, B. Armstrong, B. Case, D. Doell, W.T. McCaughey, A.D. McDonald, P. Sébastien
• Journal: Cancer
• Published: 1989;63(8):1544-7
• Cited by: 124 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 000127).

Abstract

Lung tissue samples from 78 cases from autopsy of mesothelioma in Canada, 1980 through 1984, and from matched referents were examined by optical and analytical transmission electron microscopic study. Concentrations of amosite, crocidolite, and tremolite fibers, and of typical asbestos bodies discriminated sharply between cases and referents. The distributions of chrysotile and anthophyllite/talc fibers and of all other natural and man-made inorganic fibers (greater than or equal to 8 microns) in the two series were quite similar. Relative risk was related to the concentration of long (greater than or equal to 8 microns) amphibole fibers with no additional information provided by shorter fibers. The proportion of long fibers was much higher for amphiboles than chrysotile and, except for chrysotile, systematically higher in cases than referents. Amphibole asbestos fibers could explain most mesothelioma cases in Canada and other inorganic fibers, including chrysotile, very few. Fibrous tremolite, contaminant of many industrial minerals including chrysotile, probably explained most cases in the Quebec mining region and perhaps 20% elsewhere.

In plain English

This Canadian case-referent study measured mineral fibers in lung tissue from 78 autopsied deaths reported as mesothelioma; review judged 33 definite, 25 probable, 7 possible, 4 doubtful, 9 unclassifiable. Each was matched to a referent from the same autopsy register dying of non-malignant, non-respiratory causes. The authors measured amosite, crocidolite, tremolite, talc/anthophyllite and chrysotile fibers, separating long (8 micrometres or more) from short fibers. Amosite, crocidolite and tremolite differed substantially between cases and referents; talc/anthophyllite and chrysotile much less. Adjusting for other fiber types, long amphibole fibers remained significantly associated with mesothelioma (attributable risk 68%), but long chrysotile fibers were not. Short fibers showed no significant association once long fibers were considered. Excluding nine Quebec mining-region cases, the authors attributed 14 of 69 cases (about 20%) to tremolite; formal analysis gave 22% (approximate 95% CI 7 to 33). They judged some such cases likely due to low-level tremolite contamination of industrial chrysotile. They concluded long amphibole fibers could explain a high proportion of mesotheliomas in both sexes, noting too few female cases for reliable interpretation and quite approximate attributable-risk estimates.

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: Matched case-referent (case-control) study comparing fiber content of lung tissue in autopsied mesothelioma cases and matched referents, analysed by conditional logistic regression
• Who was studied: 78 autopsied cases reported as mesothelioma (61 men, 17 women) from nine Canadian provinces, with British Columbia excluded. Each case was paired with a referent from the same autopsy register who died of causes other than malignant or respiratory disease. Referents were matched for sex, date of death (within 1 year), tissue type (wet tissue or paraffin block) and, as nearly as possible, date of birth. In all there were 52 pairs of wet tissue and 26 pairs of paraffin blocks. Deaths were mainly 1982 to 1983, with extra lung material accepted for deaths in 1979 to 1981 and the first half of 1984.
• Exposure: Lung tissue concentrations of the following, among 25 mineral fiber types identified in total: amosite, crocidolite, tremolite, talc/anthophyllite and chrysotile fibers, plus other natural and man-made mineral fibers. Fibers were measured by electron microscopy as long (8 micrometres or more) or short (under 8 micrometres). Ferruginous (asbestos) bodies were counted by light microscopy.
• Main results:
  • On pathology review of the 78 autopsy cases, 33 were considered definite mesotheliomas, 25 probable, 7 possible and 4 doubtful. Nine could not be classified, mainly because tissue samples were inadequate.
  • Both long and short fibers of amosite, crocidolite and tremolite differed substantially between cases and referents. The differences were much smaller for anthophyllite/talc and chrysotile.
  • When each fiber type was analysed separately, rising risk with rising concentration was statistically significant for amphiboles at P less than 0.001, and less so for short and long chrysotile fibers (P less than 0.05).
  • Short fibers were not significantly associated with excess risk once long fibers were taken into account. Long-fiber findings stayed significant after allowing for short fibers.
  • In the analysis allowing for confounding of one fiber type by another, long amphibole fibers remained significant. Long chrysotile fibers were no longer associated with increased risk; the authors suggested the earlier chrysotile association may have been due to confounding.
  • In the multivariate analysis, the risk increment for long amphibole fibers (all types) was 32.1 per fiber per microgram of dry lung (95% CI 8 to 120), with P less than 0.001 and an attributable risk of 68%.
  • In the multivariate analysis, the attributable risk for long tremolite fibers was 30%.
  • The differences in risk increment between the four amphibole types could be due to chance (chi-square 4.4 on 3 degrees of freedom, P greater than 0.10).
  • The much lower incremental risk for chrysotile, compared with amphiboles, could not be accounted for by chance (chi-square 8.0 on 1 degree of freedom).
  • Results were generally similar for women and men. The share of cases attributed to long amphiboles was 66% for the 17 female cases and 70% for the 61 male cases.
  • In the 66 case-referent pairs with ferruginous body counts per milligram of dried lung, the increment in relative risk was 1.46 (95% CI 0.2 to 10, P less than 0.001), with an attributable risk of 62%.
  • Of 8921 fibers measured, the proportion of long fibers was higher for amphiboles than for chrysotile. Except for chrysotile, the proportion of long fibers was higher in cases than in referents.
  • The authors estimated that perhaps 23 of the 78 cases were due to tremolite, including all 9 cases from the Quebec mining region. Excluding those 9 left 14 of 69 cases (about 20%); a more formal analysis gave 22% (approximate 95% CI 7 to 33).
  • In the incidence survey, 83 fatal cases were recorded for 1982 to 1983. Compared with 1960 through 1975, male incidence continued a steep upward trend, while female incidence stayed the same.
• The authors' conclusion: The authors concluded that, if long-term fiber retention matters most for mesothelioma, long amphibole fibers could explain a high proportion of mesotheliomas in both men and women. They judged it likely that some cases attributed to tremolite resulted from low-level tremolite contamination of industrial chrysotile. They added that chrysotile free of fibrous tremolite would probably be much safer as far as mesothelioma is concerned.
• Limitations the authors state:
  • The odds ratios by concentration group are subject to considerable sampling variability and depend on how the groups were drawn.
  • The attributable risk estimates are quite approximate: they rest on small numbers and partly on the rather arbitrary choice of the linear relative risk model.
  • There were too few female cases for reliable interpretation.
  • How the findings should be read as to cause depends on the relative importance of short-term versus long-term fiber retention in the respiratory tract.

Topics

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

Medical terms in this study, explained in our Lexicon: Mesothelioma, Autopsy, Amosite, Crocidolite, Tremolite, Asbestos Bodies, Chrysotile, Anthophyllite.

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