Mesothelioma EPIDEMIOLOGY

While an association of malignant, mesotheliomas with radiation or zeolite exposure, severe pulmonary disease, recurrent diverticulosis, and even one case of lipoid aspiration pneumonia has been reported, asbestos remains the most frequent etiologic cause with contact documented in perhaps half of all patients with malignant pleural and peritoneal primaries. While only one of the first 13 cases of malignant testicular mesothelioma reported in the literature involved an asbestos-exposed individual, 3 of the 4 most recently described cases had been heavily exposed. To date, asbestos does not appear to be implicated in the etiology of benign mesotheliomas of any site or of malignant pericardial primaries.

The association of asbestos contact and an excess risk of bronchogenic cancer had been established in 1955 by Doll, although case reports first appeared as early as 1935. The role of asbestos in the etiology of mesothelioma was first convincingly documented by Wagner et al. in 1960. They observed that 45 of 47 South African patients with this previously rare malignancy had had a history of contact with asbestos. Of particular significance was that most of the patients were not asbestos miners but rather had lived and worked in the vicinity where asbestos was utilized.

The percentage of mesothelioma patients from whom a history of contact with asbestos can be elicited has ranged from zero to almost 80-90% in series from coastal areas with shipping industries in the United States and England, and in South African and Canadian mining areas . A proportion of the patients without a history of asbestos contact may have had a long forgotten or cryptic exposure. Asbestos bodies can be found frequently at autopsy among individuals from the general population. However, the lack of asbestos bodies on careful examination of lung tissue taken at autopsy from mesothelioma patients with no known asbestos contact supports the lack of a significant asbestos exposure. Asbestos bodies, also called ferruginous bodies, consist of hemosiderin- and glycoprotein-coated fiber cores. While other organic and inorganic fibers may produce ferruginous bodies,those found in most human lungs have asbestos cores. Uncoated fibers in the lung greatly outnumber visible ferruginous bodies. Since 1960, a number of prominent epidemiologists have quantitated, chiefly via carefully designed case-control studies, the risks of asbestos-associated cancer. The lifetime risk of mesothelioma for heavily exposed individuals is estimated to be as high as 7-10'r in various studies.

While the ratio of peritoneal to pleural mesotheliomas in some cohorts of heavily exposed asbestos workers is approximately 3:2 , an overall ratio of 2:5 is observed in most series probably because of the much larger number of transiently exposed individuals. Striking is the long latent period for the development of mesothelioma. While a few cases have been reported 20 years or less after first exposure, the peak incidence of the disease occurs after 35-45 years.