The presence of pleural plaques present on the inner surface of the ribcage and diaphragm are sometimes the first sign of mesothelioma. 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. Pleural plaques by themselves are benign and cannot change into cancer. However, even though there isn’t a consensus about whether the existence of pleural plaques is always an indication of mesothelioma due to asbestos exposure, 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.
Pleural plaques are sometimes detectable on x-ray. 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 %.
Not all workers exposed to asbestos will develop pleural plaques, and the reason is that there may be differences in people’s immune systems. When asbestos fibers are inhaled, certain individuals will have a different immune response to those fibers, whereas for others, the existence of asbestos fibers may not develop into mesothelioma. Some reasons may be the density of the fiber, the type of asbestos fibers, the location of the fibers in the body, and other individualized circumstances that may make some people more prone to develop mesothelioma than others. These circumstances are known as the “fiber burden.”
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).
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 individuals own immune system. Therefore, absence of plaques does not rule out significant asbestos exposure.
There is evidence to conclude that individuals with pleural plaques have a higher risk of developing mesothelioma.
Patients with stable pleural plaques are at increased risk of developing diffuse pleural fibrosis; a significant fiber burden is indicated.
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.
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.