Asbestos-related Pleural Plaques and Lung Cancer: Results

The studies in this review are examined in order of decreasing design value: cohort, case-control, and autopsy reports. A proportional mortality ratio study by Navratil et al was excluded because this type of investigation does not include data on populations at risk. A cohort study by Loomis et al was also excluded because it was based on a vague definition of pleural abnormality in a survey of a sample of the US population and because there was no relationship between pleural abnormality and asbestos exposure elicited through a nonspecific occupational history. Six studies are summarized in Tables 1 through 3. They date from 1972 to 1992, beginning with two from the city of Barrow in the United Kingdom, a shipbuilding site. The earliest investigation was done by Fletcher, who generated the hypothesis that persons with pleural plaques have an increased risk of developing lung cancer.
The study by Fletcher is seriously flawed. He “felt that an epidemiological study should be carried out but permission could not be obtained for a prospective study… so retrospective examinations of existing collections of routine administrative and mass survey films were used.” This search revealed 408 men with pleural plaques. He then chose a group of 404 men with normal chest films matched by age as controls. Both groups were followed up for 10 years (1960 through 1970), and their mortality experience was compared with that of the Barrow general population using the SMR (the observed-to-expected ratio). Only age and sex were controlled in these comparisons. Unfortunately, Fletcher used “best evidence” for the causes of death in the men with plaques (ie, information from medical records as well as death certificates), but he used only death certificate diagnoses to calculate the expected numbers of deaths from rates in the Barrow population. He did not state whether “best evidence” was pursued in the control group. The SMR for the men with plaques was 2.37 (p<0.005), while that for the control group was 1.25 (p = 0.52).
A few years later a second study from Barrow was reported by Edge with similar results, but metho-dologic details were not provided. In a later article on the same investigation, Edge stated that “it did not prove possible to select a cohort. As an alternative, all men with radiologic evidence of pleural plaques known to the author… were compared with a control group from the neighboring city of Carlisle, where there is no known industrial asbestos exposure [italics added].” Controls were matched by age, sex, and date of first radiograph, and none had plaques. Follow-up covered 6 or more years.

Table 1—Design of Cohort Studies of Pleural Plaques and Lung Cancer in Persons Exposed to Asbestos

Study/yr/Location, Type of Exposure Type of Asbestos* Persons With Plaques Controls
No. Source No. Source
Fletcherie/1972/Barrow, UK Shipyard 408 Review of film collections Local population
Edgeie/1979/Barrow, UK Shipyard 429 “Known to author” UK mortality rates
Kiviluoto et alie/1979/Finland Near mines 700 X-ray survey 700 X-ray survey in area with no exposure
Hughes and Weill/1991/ New Orleans Asbestos cement plants Ch, Am, Cr 62t X-ray survey in 1969 Louisiana mortality rates
Sanden et al”/1992/Sweden Shipyard Ch, Am, Cr 837 Employee survey Local population rates
Partanen et al/l992/Finland Near mine and plant Mainly An 604 Community survey 604 Community survey

Table 2—Follow-up, Confounders, and Latency in Cohort Studies of Pleural Plaques and Lung Cancer in Persons Exposed to Asbestos

Study/yr/Location Follow-up Confounders Controlled LatencyConsidered
% Period Smoking Age and Sex Other
Fletcherie/1972/Barrow, UK 100 1960-70 No Yes No No
Edge/1979/Barrow, UK 98-99 1964-77 No Yes Date of first x-ray study No
Kiviluoto et alie/1979/Finland 1962-77 No No No No
Hughes and Weill/1991/ New Orleans 99.9 1969-83 Similar to US blue-collar workers, 1970 Yes No 20+ years
Sanden et al^/l992/Sweden 99 1978-87 Similar to general population Yes Period 20+ years
Partanen et al/1992/F inland 1972-89 No Yes Community No

Table 3—Statistical Method and Relative Risk in Cohort Studies of Pleural Plaques and Lung Cancer in Persons Exposed to Asbestos

Study/yr/Location Statistical Method 0 Relative Risk*E O/E P or 95% CL Comments
Fletcherie/1972/Barrow, UK SMR 16 6.74 2.37 0.0047 Potential selection bias; best evidence for subjects
Edge/1979/Barrow, UK SMR for 407 with plaques 16 8.5 1.88 <0.02 8 of 16 case patients died in first 2 yr
Kiviluoto et alie/1979/ Finland Cumulative incidence 13 14 0.93 >0.10 Calcified plaques studied; data from cancer registry
Hughes and Weill/1991/ New Orleans SMR 2 1.5 1.33 >0.10
Sanden et al*°/1992/Sweden SMR 8 9.9 0.81 >0.10 Data from cancer registry
Partanen et al/1992/Finland Proportional hazards model 28 25 1.07 0.62-1.83 Survey compliance 40%
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