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Mesothelioma Treatment | Articles & Abstracts
Etiolgoical Studies
Mineralogic Evaluation of Lung Tissue of Persons with Malignant Mesothelioma
Is Crocidolite Asbestos the only Type of Asbestos That Causes Mesothelioma?
BACKGROUND
The association between malignant mesothelioma and prior exposure to asbestos has been demonstrated repeatedly in epidemiologic, experimental animal, and pathologic studies. Most investigators are of the opinion that all of the commercially valuable forms of asbestos, that is, crocidolite, amosite, and chrysotile (with its contaminant tremolite) are capable of causing pleural mesothelioma in humans. However, there appears to be a fiber gradient with respect to the disease mesothelioma, with crocidolite being a more potent inducer of the disease than amosite, which is in turn more potent than chrysotile, assuming equal amounts of exposure to each fiber type.
Some investigators have claimed that crocidolite is the cause of the great majority of mesotheliomas in the world, including most cases from the United States. Since crocidolite was rarely used in shipyards and in insulation products in this country (the source of a large proportion of mesotheliomas in the U.S.), we investigated our files for the proportion of the various asbestos fiber types found in the lungs of patients with mesothelioma. The data summarized below include analyses from more than 1500 fibers from 94 patients with mesothelioma.
MESOTHELIOMA IN RELATION TO TISSUE FIBER BURDEN
Asbestos bodies may be observed in H&E stained sections of lung parenchyma in just under half of patients with mesothelioma, and are found after careful examination of iron stained sections in somewhat more than half of all cases. This finding is indicative of an elevated tissue asbestos content. When more sensitive digestion procedures are employed, about 83% of mesothelioma patients have a tissue asbestos body content which exceeds background values. In an additional 6% of mesothelioma cases, an abnormal asbestos fiber concentration can be detected by scanning electron microscopy in the presence of an asbestos body content within the normal range. Thus at least 89% of mesotheliomas appear to be asbestos related based on analysis of lung tissue burdens. This finding is similar to the rate of asbestos related cases assuming a background rate of 1 mesothelioma per million population per year and the actual observed number of about 2200 cases per year in the United States. It should be noted that asbestos bodies are not observed in histologic sections of mesothelioma (except when there is invasion of underlying lung parenchyma), and that the asbestos content of mesothelial tumor tissue is very low (if detectable at all) in comparison to the underlying lung.
MESOTHELIOMA AND FIBER TYPE
Studies of the mineral fiber content of lung in U.S. patients with mesothelioma have shown that amosite is the most common fiber type, accounting for nearly 60% of all fibers 5 or greater in length among more than 1500 fibers analyzed from 94 cases. Ten percent of the fibers analyzed were tremolite, and 3% were chrysotile. (Chrysotile tends to break down in the lungs over time so that there is a relative enrichment of the more stable tremolite contaminant). Crocidolite accounted for only 3% of fibers. Studies have shown that fibers in this size range are capable of reaching the visceral and parietal pleura.
Studies from Canada have shown an increased rate of mesothelioma in workers at the Thetford mines, analyses of whose lung tissues have demonstrated chrysotile and tremolite fibers only. These findings as well as earlier such studies demonstrate unequivocally that chrysotile ore dust can cause mesothelioma in humans. It remains unclear whether it is the chrysotile fiber itself or the contaminating tremolite (or both) which is the offending agent. Churg's studies have suggested that mesotheliomas only occur in these miners at a high fiber burden (equivalent to levels seen with asbestosis). Our own findings on end-product users suggest that this is not necessarily the case. However, mesothelioma cases in which tremolite is the only fiber type identified in increased amounts account for only a small percentage of the total number of mesothelioma cases that we have studied (about 3%).
SUMMARY
All the commercially valuable forms of asbestos cause mesotheliomas in humans. Amosite, a commercial amphibole commonly used in the past in insulation products and in shipbuilding in this country, appears to be the primary culprit. Tremolite, a common contaminant of chrysotile asbestos, appears to be next in importance. Crocidolite is the least important and was uncommonly found in U.S. shipyards and insulation products. Most exposures resulting in mesothelioma were to mixtures of amosite and chrysotile. Exposure to chrysotile dust alone can cause mesothelioma in humans, but does so uncommonly.
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