Mesothelioma without asbestos exposure

For a time, mesothelioma was thought to be exclusively related to asbestos, but more recent reviews indicate that a significant number of cases have occurred in the absence of any known asbestos exposure. Although the association between amphibole asbestos and mesothelioma is indisputable, fewer than 10% of the people exposed to asbestos develop mesothelioma, and fiarly large proportions (up to 50% according to some authors) of the reported cases have no documented exposure to asbestos. A comprehensive survey of adult mesothelioma cases in Canada and the U.S. carefully classified patients based on their likelihood of past exposure to asbestos. The researchers found that asbestos exposure had been unlikely in between 1/4 and 1/3 of cases (McDonald & McDonald, 1980). While it is well documented that asbestos-induced mesothelioma has a latency of 20 years or more, a number of studies have highlighted pleural and pericardial mesotheliomas in children as young as 1-1/2 years old (Lemesch et al., 1976). Surveys of reported mesotheliomas in the U.S., Canada and Israel found a combined total of more than 110 cases in persons under the age of 20. In addition to these unexplained cases of mesothelioma, a number of other fibrous and non-fibrous materials have been associated with mesothelioma induction. It is now generally accepted in the scientific community that durable, long and thin fibres have fibrogenic and carcinogenic potential. A number of natural and man-made fibres with these characteristics have been established as the cause of mesothelioma in laboratory animals. These include glass fibres, aluminum oxide, attapulgite, dawsonite, silicon carbide and potassium titanate (Stanton et al., 1977). The reported outbreak of mesothelioma in rural Turkey has been associated with exposure to fibrous zeolite found in these regions. In his 1980 report, Baris had identified 185 cases of erionite/zeolite-related mesothelioma in two areas of Turkey with no local asbestos deposits or industry. Several non-fibrous agents, both organic and inorganic, have also been shown to induce malignant mesothelioma. For example, a causal link between mesothelioma and radiation has been established based on numerous case reports of mesotheliomas developing at the exact sites of radiation therapy. Other suspected causes include biogenic silica fibres, chronic irritation stemming from tuberculosis and other factors, and heavy metals such as beryllium Polio vaccines and the SV40 virusMore recently, it has been reported that a virus (SV40) contaminating some polio vaccine preparations may well be associated with mesothelioma, as some DNA sequences of the virus are sometimes found in cancerous mesothelial cells. These vaccine preparations had been produced in 1954, some eight years before SV40 was first isolated, and had been prepared by growing polio virus in cell cultures from rhesus monkey kidney cells. As a result, millions of people have been injected with SV40-contaminated polio vaccines. Recent findings by Dr. M. Carbone and colleagues of the Dept. of Pathology at the University of Chicago and by co-workers at the National Cancer Institute first indicated that the SV40 virus, which induces mesothelioma in hamsters, is also oncogenic for humans. Later on, they found SV40-like DNA sequences in human mesothelioma cases (Carbone et al., 1994). Similar evidence is now beginning to appear from France and the U.K. Recent evidence of the significance of the SV40 virus and other potential sources of mesothelioma, suggests that factors other than asbestos exposure may have played a role in recently reported mesothelioma cases in Europe in which the victims are reported to have had only casual, low level contact with asbestos-containing products.