Malignant diffuse mesothelioma is a neoplasm arising from the mesothelial or submesothelial cells of the pleura (in more than 80% of the cases), peritoneum or (in rare instances) pericardium. The disease is directly linked to occupational asbestos exposure; however there evidence shows that mesothelioma may result from both para-occupational exposure and non-occupational, environmental exposure.
An individual risk of developing mesothelioma is age-dependent (approximately ten times higher in persons over the age of 60 than in persons under the age of 40) and continues to rise decades after exposure. According to data gathered in the US Surveillance Epidemiology and End Results programme for the period 1973–1992, a constant rate of mesothelioma has been observed in females, but a regularly higher rate has been found for males.
Incidence around the globe
Mesothelioma develops in 1 to 2 persons per million of the general population per year, and the incidence among occupationally exposed persons is more than 40 times as high. It is responsible for approximately 5 thousand deaths in Western Europe and 3 thousand deaths in the US every year. Although asbestos processing is forbidden in many industrialized countries, the incidence of the disease is expected to rise further due to its long latency period.
The estimations say that approximately 10 thousand mesotheliomas occur annually throughout North America, Australia, Scandinavia and several other countries in Western Europe. The peak incidence of the disease in Australia is expected in 2020, and in the next 20 years about 70 thousand new cases are expected in the US. The predicted peak in Europe is for the period of 2015–2020, with an anticipated incidence of 250 thousand cases over the next 40 years.
Since about 100% of those who develop malignant mesothelioma die because of it, more than 100 thousand US citizens are expected to die in the next 40 years. Data modeling for France indicates that mesothelioma mortality among French men between 50 and 79 years of age will gradually increase, reaching a peak of 1300 deaths in 2040.
In some countries, the time trend of increasing incidence after 1986 is primarily restricted to those over 50 years, which suggests that control of the occupational exposure introduced in the 1970s have been effective. However, the awareness about the dangers of asbestos exposure effects was not similar in all the countries, which could explain the fact that the relative risk of developing a pleural mesothelioma among men is 1.83 for the generation born in 1953 compared to those born in 1928.
Asbestos exposure and mesothelioma
Asbestos represents a naturally occurring fibrous silicate, and the risk of developing mesothelioma depends on the exposure to different types of the asbestos mineral fiber (i.e. serpentine or amphibole fibers). The distinction between the fibers is important since the shape of the serpentine fibers makes their clearance from the respiratory tract easier. Epidemiologic data suggests that the crocidolite from the amphibole group is associated with the highest risk of mesothelioma, while the serpentine fiber chrysotile has the lowest.
Approximately 90% of male patients with pleural mesothelioma and 60% of male patients with peritoneal mesothelioma have a history of asbestos exposure. The proportion of mesotheliomas linked to asbestos exposure is lower in females and varies among different countries (from 25% in the United States up to 70% in Australia). The median latency for the disease is 44.6 years, which increases over time in a linear fashion.
The risk for developing mesothelioma shows a dose-response relation to cumulative asbestos exposure, thus the greatest risk lies with heavy exposure. One vivid example is environmental asbestos exposure in some villages in Turkey, where the rate of mesothelioma mortality can be more than 100-fold higher than in villages without that exposure.
Occupation is perhaps the strongest indicator of intensity of asbestos exposure and shows the most consistent association with latency. Proper documentation and dissemination of all asbestos exposure modalities is of utter most importance since many of them (taking into account the large-scale use of asbestos and the lack of a threshold for the dose–response curve) are frequently not expected.
Further Reading