Asbestos-related Pleural Plaques and Lung Cancer: Material and Methods

Asbestos-related Pleural Plaques and Lung Cancer: Material and MethodsEpidemiologic studies over the past three decades have established that workers exposed to asbestos in most circumstances are at increased risk of developing lung cancer. Recent literature reveals disagreement about the concept that the increased risk is limited to those workers who first develop asbestosis. Several animal experiments support this concept, but controversy and uncertainty about the idea continue.
Another approach in considering the validity of this concept is to examine the relationship between asbestos-related pleural plaques and the risk of lung cancer in the absence of asbestosis. Pleural plaques among workers exposed to asbestos may be used as a marker of the exposure. If the risk of lung cancer is not elevated in such persons compared with the risk in unexposed persons or the general population, then this observation would provide additional evidence for the concept that asbestosis is a necessary precursor for asbestos-related lung cancer. In this article, the term “asbestosis” is defined as diffuse parenchymal fibrosis caused by asbestos. I will review the relevant literature and weigh the evidence after assessing the methodologic aspects of each study.
The literature published in English on asbestos-related disease and on the etiology and epidemiology of lung cancer was obtained through an ongoing search of Index Medicus and MEDLARS II covering the period from 1965 to mid-1992. References cited in the published articles were also examined if relevant. An article published in a foreign language was included if there was an English abstract with sufficient information in it.
Studies accepted for analysis were those dealing with the relationship between pleural plaques unaccompanied by asbestosis (parenchymal disease) and lung cancer. The exclusion of asbestosis was determined by whatever method was used to detect the pleural disease (eg, radiographic or autopsy evidence). The types of investigations were limited to epidemiologic studies with cohort or case-control design and to prevalence studies in autopsy populations. If more than one report was published on the same study, the latest publication was used.
The report of each cohort investigation was examined for information on the location of research, type of work or exposure to asbestos, type of asbestos, number of and source of cohort members with pleural plaques and of control subjects, degree of completeness and period of follow-up, sources of possible bias, control for confounders, consideration of latency, and statistical method. When the source of cohort members was a population survey, the degree of compliance was noted if given.
The outcome measure was the relative risk for lung cancer, with the probability value or 95 percent confidence limit (CL) provided by the authors. In most studies the relative risk was measured by the standardized mortality/morbidity ratio (SMR). If the measure was morbidity, data were obtained from a cancer registry. When a probability value or CL was not provided by the investigators, I calculated these by standard methods. Comparisons between persons with plaques and control subjects were restricted in this analysis to those in which the controls were not exposed to asbestos or were members of the general population.
Reports of case-control studies were examined for information of similar nature plus information on the method for reading the chest x-ray films for pleural plaques, including the number of readers and whether readings of case and control films were concurrent and blind to case-control status. The outcome measure was the odds ratio with the probability value, usually determined from a x* test.
Similar information was obtained from the autopsy studies. The outcome measure was the “relative risk” with 95 percent CL as provided by the authors in one study or the odds ratio I calculated from the data given, with a probability value based on a x* test either given by the authors or calculated from the data.

This entry was posted in Lung Cancer and tagged asbestos, autopsy, lung cancer, relative risk, smoking.