Fact Sheet on Pleural Plaques
What kinds of asbestos are there?
Asbestos can be divided into two types, straight (amphiboles) and curly (chrysotile only). The straight fibers (amphiboles) include amosite, crocidolite and tremolite. Chrysotile usually contains some amphibole. Chrysotile is the most widely used in industry. It is the most water-soluble and has the fastest clearance from the body. As a consequence, the amphiboles are found at autopsy when the exposure occurred decades earlier, even though the predominant exposure was to chrysotile (7).
What are pleural plaques?
Pleural plaques are well-demarcated usually bilateral areas of fibrosis present on the inner surface of the ribcage and the diaphragm. They are often partly calcified.
Can pleural plaques change into cancer?
Pleural plaques by themselves are benign and cannot change into cancer.
What is the prevalence of pleural plaques?
The reported prevalence of pleural plaques in studies depends on:
a) The case definition of pleural plaques
b) Whether the data is derived from radiologic studies or postmortem studies
c) Whether only calcified plaques are counted in the studies
d) The length of follow-up.
With these caveats in mind, about a third to one half of those occupationally exposed to asbestos will have calcified pleural plaques thirty years after first exposure. After twenty years, 5 to 15 % will have uncalcified pleural plaques (Parkes).
Are all pleural plaques detectable on x-ray?
The prevalence of plaques is lower in radiographic surveys as compared to postmortem surveys. In order for a plaque to be detectable on x-ray, it must achieve a certain density. The sensitivity of x-rays to pick up pleural plaques is reported to vary between 8 % and 40 %.
Why don’t all asbestos-exposed workers develop pleural plaques?
The answer to this is not clear. The development of pleural plaques is thought to be related to differences in each individual’s immune response to fibers.
Is there a correlation between pleural plaques and asbestos fiber burden (ie. a dose response?)
There are a number of studies which support a correlation between pleural plaques and fiber burden:
Warnock studied the asbestos content in the lungs of twenty subjects with pleural plaque at autopsy. Sixteen had a history of asbestos exposure. The subjects with pleural plaques had significantly higher levels or amosite and crocidolite fibers.
Kishimoto investigated the relationship between number of asbestos bodies and pleural plaques. There were more asbestos bodies in workers with pleural plaques than in those with “definite pleural plaques without calcification.”
Roggli studied the asbestos content of workers with lung cancer and pleural plaques without asbestos and found higher asbestos content than in those with neither plaque nor asbestosis.
Churg found that the concentration of chrysotile in workers with pleural plaques was higher than in those with similar exposure and no plaque. Interestingly, the concentration of chrysotile was the same in workers of the same cohort with lung cancer (Becklake).
Are workers with pleural plaques at higher risk of lung cancer?
Because pleural plaques can develop in individuals with low levels of exposure, “the attribution of lung cancer to asbestos exposure must be supported by an occupational history of substantial asbestos exposure” (Consensus report).
Hillerdal followed a cohort of residents in Uppsala and found that people with pleural plaques run an increased risk of developing lung cancer and should be offered regular screening.
Partanen was not able to conclude one way or the other with respect to the risk of developing lung cancer in workers with pleural plaques.
Edelman discounted most of the studies because they did not control for smoking.
So what’s the bottom line?
Pleural plaques can have some predictive value in workers who have been employed in occupations with known asbestos exposure, e.g. insulators, pipe fitters etc. The larger the plaque, the higher the likelihood of increased fiber burden, especially so if the plaque is calcified.
Similarly, an individual who has had identical exposures without pleural plaques may still have the same fiber burden. This is because the development of plaques is largely biologically determined by an individual’s own immune system. Therefore, absence of plaques does not rule out significant asbestos exposure.
Are workers with pleural plaques at higher risk of developing
mesothelioma?
There is evidence to conclude that individuals with pleural plaques have a higher risk of developing mesothelioma. Smoking is not a factor in the development of mesothelioma.
Bianchi found that the presence of plaques at autopsy was higher among subjects with pleural mesothelioma than in the controls. Hillerdal published similar conclusions based on a radiographic study in Sweden.
Can workers with pleural plaques have a respiratory impairment?
Workers with pleural plaques most often have normal pulmonary function. Diffuse pleural fibrosis is a separate condition that often causes an impairment characterized by restriction. Pleural plaques of sufficient size can also cause some loss of ventilatory capacity, but generally not to the degree seen in diffuse pleural fibrosis (Bourbeau, Kilburn, Kouris).
Patients with stable pleural plaques are at increased risk of developing diffuse pleural fibrosis; a significant fiber burden is indicated.
What tests should be done in patients with pleural plaques?
If a patient has pleural plaques with a ventilatory impairment, further testing is necessary. If pulmonary function studies show a decrease in diffusing capacity, then it would be appropriate to proceed to high resolution CT scanning to determine if there is sub-radiographic interstitial fibrosis.
Which patients with pleural plaques should be screened for lung cancer?
Patients with calcified pleural plaques who work in an industry with known asbestos exposure should be screened for lung cancer. Some risk factors for increased risk are: a) latency from time of first exposure is over 10 years, b) cumulative exposure to tobacco, c) age, d) exposure to other lung carcinogens like PAHs, silica, radon etc, e) impaired lung function.
How should patients be screened?
The method of choice for lung cancer screening in this population is low dose spiral CT scanning.
REFERENCES
Becklake, M: Fiber Burden and Asbestos-Related Lung Disease: Determinants of Dose-Response Relationships; Am J Respir Crit Care Med Vol 150, pp 1488-1492, 1994
Bianchi, C: Pleural Plaques as Risk Indicators for Malignant Pleural Mesothelioma: A Necropsy-Based Study; Am J Ind Med 32:445-449 (1997)
Bourbeau. J: The Relationship Between Respiratory Impairment and Asbestos-Related Pleural Abnormality in an Active Work Force; Am Rev Respir Dis 1990 142: 837-842
Edelman, D: Asbestos Exposure, Pleural Plaques and the Risk of Lung Cancer; Int Arch Occup Environ Health (1988) 60: 389-393
Hillderdal, G: Pleural Plaques and Risk for Cancer in the County of Uppsala; Europ J Respir Div Suppl 107 Vol 61, 1980 pp 111-117
Hillderdal, G: Pleural Plaques and Risk for Bronchial Carcinoma and Mesothelioma; Chest 1994, 144-150
Hillderdal, G: Asbestos, Asbestosis, Pleural Plaques and Lung Cancer; Scand J Work Environ Health; 1997; 23:93-103
Kilburn, K: Abnormal Pulmonary Function Associated with Diaphragmatic Pleural Plaques due to Exposure to Asbestos; Br J Ind Med 1990; 47: 611-614
Kishimoto T: Relationship Between Number of Asbestos Bodies in Autopsy Lung and Pleural Plaques on Chest X-ray Film: Chest 1989; 95: 549-552
Kouris S: Effects of Asbestos-Related Pleural Disease on Pulmonary Function; Scand J Work Environ Health 1991; 17: 179-183
Miller A: Diffuse Thickening Superimposed on Circumscribed Pleural Thickening Related to Asbestos Expousre: Am J Ind Med 23: 859-871, 1993
Numinen M: Is There and Association Between Pleural Plaques and Lung Cancer Without Asbestosis?; Scand J Work Environ Health 1994: 20:62-4
Partanen T: Localized Pleural Plaques and Lung Cancer; Am J Ind Med, 22; 185-192 (1992)
Roggli V: Asbestos Content of Lung Tissue and Carcinoma of the Lung: A Clinicopathologic Correlation and Mineral Fiber Analysis of 234 Cases; Ann Occup Hyg, Vol 44 No 2, pp 109-117 (2000)
Asbestos, Asbestosis and Cancer: The Helsinki Criteria for Diagnosis and Atttribution; Scand J Work Environ Health 1997; 23: 311-316
International Expert Meeting on New Advances in the Radiology and Screening of Asbestos-Related Diseases; Scand J Work Environ Health 2000; 26(5) 449-454
Warnock M: Number and Types of Asbestos Fibers in Subjects with Pleural Plaques; Am J Pathol 19982, 109:37-46
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January 7, 2004
