Radiological findings as predictors of mortality in Quebec asbestos workers

Our plain-English summary of Radiological findings as predictors of mortality in Quebec asbestos workers, by F.D. Liddell, J.C. McDonald (British Journal of Industrial Medicine, 1980).

The study

• Authors: F.D. Liddell, J.C. McDonald
• Journal: British Journal of Industrial Medicine
• Published: 1980;37(3):257-67
• Cited by: 67 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 000829).

Abstract

Two cohorts of chrysotile miners and millers in Quebec were selected to study the extent to which chest radiographs taken while still employed predict mortality. The paper presents mainly findings in much the larger cohort, which consisted of 4559 men (two-thirds past workers) whose latest radiograph had been assessed by one of six experienced readers into what became the UICC/Cincinnati (U/C) classification; by the end of 1975 there had been 1543 deaths in this cohort. The findings were generally confirmed in the other cohort, comprising 988 current male workers, who had been examined in 1967-8 by questionnaires on respiratory symptoms and smoking and by lung function tests, and for whom all six readers had assessed their 1966 radiographs into the U/C classification; 130 men had died by the end of 1975. Men with any radiographic abnormality, heavy dust exposure, or a history of cigarette smoking had relative risks (RRs) of total mortality greater than unity. Death from pneumoconiosis was associated with small parenchymal opacities, usually irregular, of profusion l/l or more, and with heavy dust exposure but not with smoking. Most who died from lung cancer had smoked cigarettes, or had been heavily exposed to dust, or both. Small parenchymal opacities were present in most but not all the excess deaths due to lung cancer. Deaths from other malignant diseases showed no consistent dust or x-ray patterns. RRs of deaths from most other causes were raised for certain radiographic features. Failures in forecasting mortality were primarily due to deaths in which asbestos-related disease was not the primary cause but may have been a contributing factor. The main findings validated the U/C classification convincingly, particulary as the films had been taken as routine and were of modest quality. Despite objective rules for the reading and the fact that all six readers were contributing to the development of the classification, there was inevitably some observer variation. The importance of radiographic technique and the need for careful control of the reading is evident. Our results provide support for the use of the chest radiograph for surveillance of asbestos workers, and for environmental monitoring. Its protective value for individual workers, however, is limited to the extent that radiological progression continues after withdrawal from exposure, and by the carcinogenic risk associated with dust already retained.

In plain English

This study followed two groups of male chrysotile miners and millers in Quebec to see how well chest X-rays taken while the men were still employed predicted later deaths. Cohort 1 had 4559 men, of whom 1543 had died by the end of 1975. Cohort 2 had 988 men examined in 1967-8, of whom 130 had died. In cohort 1, men whose X-ray was read as less than normal had a relative risk of death from all causes of 1.74 compared with men with normal films. The figure was 1.24 in cohort 2. In cohort 1 their relative risk was 11.75 for pneumoconiosis and 3.24 for lung cancer. Cigarette smoking had a relative risk of 3.35 for lung cancer, while the authors described its effect on pneumoconiosis deaths as small. Of 118 lung cancer deaths, 48 had normal films. The authors estimated that the routine X-rays missed 27% of the excess deaths. They suggested this was mainly in deaths where asbestosis contributed but was not the main cause. The authors concluded that the results support X-ray surveillance of asbestos workers. They cautioned that readers varied and that their conclusions may not apply to other fiber types or other readers.

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: Retrospective cohort mortality study of two cohorts, analysed by the man-years method, with two case-control comparisons within cohort 1
• Who was studied: Cohort 1: 4559 men born 1891-1920 who worked in the Quebec asbestos production industry and had a chest X-ray while employed; 1543 had died by the end of 1975. Cohort 2: 988 men still employed in November 1966 who took part in 1967-8 respiratory studies; 130 had died by the end of 1975.
• Exposure: Chrysotile mining and milling in Quebec. Accumulated dust exposure was measured in million particles per cubic foot-years and split at 300 mpcf-y. Cigarette smoking was also studied.
• Main results:
  • Death from all causes, men with less-than-normal X-rays compared with normals: RR 1.74 in cohort 1 (chi-squared 110.72) and RR 1.24 in cohort 2 (chi-squared 0.83)
  • Cohort 1, less-than-normal compared with normal X-rays: RR 11.75 for pneumoconiosis, 6.28 for respiratory tuberculosis, 3.24 for lung cancer and 2.29 for other respiratory conditions
  • Death from all causes: RR for cigarette smoking 1.67 in cohort 1 and 1.88 in cohort 2. RR for accumulated dust exposure of at least 300 mpcf-y 1.59 in cohort 1 and 1.21 in cohort 2.
  • Heavier dust exposure: RR 2.87 for lung cancer and 8.78 for pneumoconiosis. Smoking had an RR of 3.35 for lung cancer in cohort 1. In cohort 2, smoking was the only factor with a conventionally significant effect on lung cancer (RR 4.11).
  • Cohort 1 pneumoconiosis deaths: the authors described the smoking effect as small
  • Parenchymal change had been recorded on the X-rays of 23 of the 31 men whose deaths were certified as pneumoconiosis; five of the 31 had normal films
  • Lung cancer deaths: 48 of the 118 cases had normal films. The longer the gap between the last job and death, the more often the film was normal: 9 of 48 normal when the gap was under five years, 14 of 20 when it was at least 20 years.
  • In cohort 1, 49 of the 52 excess lung cancer deaths were among men with less-than-normal X-rays, 33 of them showing small opacities
  • Men whose X-rays were read as completely normal still had an SMR of 1.17, compared with 1.44 for all of cohort 1. The routine X-rays failed to detect 27% of the excess mortality.
  • The authors suggested that the X-ray failed to predict death mainly where asbestosis was a contributory rather than the primary cause
  • Six deaths across the two cohorts were from malignant mesothelial tumour, all of the pleura. Their X-ray readings varied, and one was read as normal.
• The authors' conclusion: The authors concluded that the results support using chest X-rays to monitor asbestos workers and the work environment. They added that the protective value for individual workers is limited, because X-ray changes continue to progress after exposure stops and dust already retained carries a cancer risk. They also concluded that most, but not necessarily all, lung cancers attributable to chrysotile mining and milling probably show small opacities before death.
• Limitations the authors state:
  • Many non-independent chi-squared statistics were computed, so the authors said no statement of statistical significance could be justified. The groupings were made one factor at a time and were not independent of each other.
  • Readers varied: the share of men read as less-than-normal ranged from 20.9% to 39.9% across the six readers
  • The conclusions may not apply to other asbestos fiber types, other industrial exposures, other readers, or less well-controlled circumstances
  • Many deaths came within a year or two of the X-ray, so the 27% figure was described as somewhat optimistic
  • The excess lung cancer cases could not be specifically identified
  • The films were taken as routine and were of modest quality

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile, Chest X-Ray, UICC, Relative Risk, Pneumoconiosis, Lung Cancer, Carcinogenic.

Related studies

• The 1891-1920 birth cohort of Quebec chrysotile miners and millers: development from 1904 and mortality to 1992 — F.D. Liddell, A.D. McDonald, et al. (1997)
• The 1891-1920 birth cohort of Quebec chrysotile miners and millers: mortality 1976-88 — J.C. McDonald, F.D. Liddell, et al. (1993)
• The health of chrysotile asbestos mine and mill workers of Quebec — J.C. McDonald, M.R. Becklake, et al. (1974)
• Long-term mortality experience of chrysotile miners and millers in Thetford Mines, Quebec — W.J. Nicholson, I.J. Selikoff, et al. (1979)
• Mortality in the chrysotile asbestos mines and mills of Quebec — J.C. McDonald, A.D. McDonald, et al. (1971)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part III. Radiographic findings — H.E. Amandus, R. Althouse, et al. (1987)

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