Malignant Mesothelioma Following Radiation

Malignant Mesothelioma Following Radiation
The strong association between asbestos exposure and malignant mesothelioma has been widely accepted since 1960. Although asbestos is the primary etiologic agent for this tumor, a significant number of patients who develop mesothelioma have no known asbestos exposure. Radiation, nonasbestos mineral fibers, organic chemicals, chronic inflammation, and simian virus 40 (SV40) exposure have also been suggested as risk factors for mesothelioma in humans.

Because asbestos is ubiquitous, past exposures are often difficult to quantitate. Past asbestos exposure may be assessed by a standardized questionnaire that collects information on occupational, paraoccupational, environmental, and domestic contact with asbestos from insulation, mining, milling, heating trades, shipyard work, and construction. 


However, the long latency period of 20 years or longer from the onset of exposure to the development of malignant mesothelioma likely influences the accuracy of the exposure information obtained.

More objective evidence of asbestos exposure includes radiologic findings such as bibasilar fibrosis and calcified pleural plaques, the presence of asbestos fibers in sputum or bronchoalveolar lavage samples, and evidence of interstitial fibrosis or ferruginous bodies in lung tissue. 


These criteria have been used to try to exclude asbestos as the causal factor in some cases of mesothelioma.

In published case series, the proportion of mesothelioma cases that have an asbestos exposure history ranges from 16% to 77%. Of 668 patients who died of malignant mesothelioma in Canada and the United States from 1960 to 1975, only 50% of men and 5% of women had known asbestos exposure. 


Occupational asbestos exposure in women and children is rare; therefore, most asbestos exposure in these individuals is thought to come from a household member who is employed in an asbestos industry.

The occurrence of malignant mesothelioma in children may not be related to asbestos at all. In a report of 13 children diagnosed with mesothelioma in the United States, the short latency period from the time of exposure to tumor development and the absence of geographic clustering argued against an environmental cause for this malignancy in children.

Therefore, the disease appears to have a “natural” incidence of undetermined origin. Radiation is a possible etiologic agent for mesothelioma that may act independently or may have a synergistic effect with asbestos.


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Identification of Asbestos

Identification of Asbestos
Over the years much data have been accumulated about asbestos, which suggests that amphibole asbestos and its nonasbestos analogues possess very different biologic potential.


 Davis et al demonstrated that although asbestiform tremolite was extremely carcinogenic when
injected into peritoneal cavities of rats, nonasbestiform tremolite samples had little or no carcinogenic potential. Therefore, it is important to distinguish between asbestiform and nonasbestiform amphiboles and types of fibers in bulk, air, and tissue samples. 


There are some problems related to the mineralogic techniques necessary to prepare and characterize samples. The designation of the shape and size of fibrous materials can be relatively easily revealed by optical examination. Optics became the technique of choice to investigate the occurrence of inorganic fibrous airborne particulates at occupational sites, in schools, or any buildings, and even outdoors where filters could be set up to obtain a representative aliquot of the air. 


However, the light (optical) microscope does not have enough spatial resolution and so is not sufficient on its own for positive identification of minerals. It is difficult to identify some fibers such as chrysotile in the tissue samples under the optical microscope because of the small fiber sizes. Since the small fiber size of chrysotile in the tissue samples preclude the use of optical microscopes, morphologic, chemical, and structural identifications are done by combinations of methods in order to makeunambiguous mineral identifications. 


The crystal chemical range of potentially hazardous inorganic and mineral species should be accurately identified. Morphologic identifications can be performed by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Chemical information is most commonly obtained by energy dispersive spectroscopy (EDS) or wavelength dispersive spectroscopy (WDS), which is an integral part of SEM or TEM. 


A relative error percentage for EDS is about 10% and for WDS is about 1%. Therefore, EDS provides only semiquantitative information, but WDS provides more quantitative information on chemical composition of the sample. 


Crystal structures can be determined by electron diffraction (ED) on samples. Powder x-ray diffraction (XRD) is a powerful technique providing that enough material is available, but not for a mineral present at low percentage in tissue and air samples. Certain regulations may require specific species of amphiboles; thus, quantitative chemical data may be necessary. 


For example, substitution solid solution series of amphiboles, such as a tremolite and an actinolite, must be identified. The SEM studies combined with EDS may not be conclusive because of the lack of information on the mineral structure. It is also very difficult to observe chrysotile through the electron microscope because of its beam sensitivity. 


Analysts tend to measure fibers that are more stable under beam conditions. Lung burden studies indicate that chrysotile is often inhaled as a shorter fiber than amphiboles. Therefore, in a tissue with both amphibole and chrysotile, it is possible to make a misjudgment unless the fibers are identified individually. 

The levels of sensitivity using the high-resolution techniques now available mandate that we follow up the reactions delineated as interference of inorganic materials in the biologic environment. The information on the inorganic fibrous particulates can be matched with the equally high-resolution techniques applied to analyses of tissues, with data gathered at the cellular and molecular levels. 

The advances in techniques increase the possibilities that we can test hypotheses and, it is hoped, gain greater understanding from the anatomic to the genetic of the reactions that lead to induction of disease. Coordinating ultramicroscopic levels with the health and mineralogic investigations for a particular geographic area should enable us to refine the possibilities. The exchange of information among the several disciplines is needed to advance our knowledge.

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Asbestos Mineralogy and Health Effects

Asbestos Mineralogy and Health Effects

Fibers and fibrous minerals, for example, the asbestos minerals, erionite (one of the many natural and synthetic zeolite species), fiberglass, or other silica forms (diatoms) have been shown to be extremely hazardous. Their airborne character is paramount, and the specific gravity of the species, the size, and an appropriate morphology that permits suspension are of primary consideration. 

Asbestos as a ubiquitous natural resource refers to several types of fibrous minerals formed by earth processes and made up of microscopic bundles of fibers. The dangers associated with inhalation of asbestos fibers haveeen known for more than 30 years. Asbestos is known as a group A human carcinogen. The potential hazards of exposure to asbestos materials are of concern worldwide. 

There are several modes of exposure to airborne fibers including occupational exposure and the erosion of natural deposits in asbestos-bearing rocks. Asbestos may also be dispersed in water from a number of sources, including erosion of natural deposits, corrosion, and disintegration of asbestos materials. Governments and industries have introduced regulatory measures requiring safety controls throughout the product life cycle to limit asbestos exposure to the general public and workers. 

Although asbestos materials have been well documented as to their physical and chemical characteristics, they remain under investigation both by mineralogists studying geologic aspects and by pathologists/epidemiologists studying medical aspects. The term asbestos may be well known, but the precise definition, safe level of exposure, duration of exposure, and asbestos types of these fibrous materials still raise questions and often lead to differences of opinions and arguments as well as legal disputes.

Asbestos Mineralogy and Health Effects

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