Mesotheliomas form a spectrum of tumors that usually arise in the pleura or in the abdominal cavity. The malignant tumors arising within the pleura are strongly associated with prior asbestos exposure in up to 90% of cases. The time period from initial exposure to development of the cancer may range from 25-45 years and the lifetime risk of developing mesothelioma following heavy exposure ranges from 7-10%. It diffusely spreads through the pleural space, directly invading other thoracic structures and associated with pleural effusion. The underlying lung is usually encased within the tumor.

The abdominal peritoneal variant is also related to asbestos exposure and 50% of these patients may have pulmonary fibrosis. Although 50% of these cases may be limited to the abdominal cavity, intestinal obstruction is common frequently leading to death.

The classic histopathology of the mesothelioma is a biphasic tumor with both an epithelioid and sarcomatoid or spindle cell component. From this description, it can be readily appreciated how the diagnosis may be very difficult to confirm. The table below lists the major histologic variants of the tumor. The task of the pathologist is to separate the mesothelioma from its many mimics including adenocarcinoma from the lung (in pleural cases) and soft tissue sarcomas such as malignant fibrous histiocytoma (in peritoneal cases). Immunoperoxidase studies and electron microscopy may aid in this distinction.

Unfortunately, the disease has a progressively downhill course. The importance in making the correct diagnosis is more of a medicolegal issue, because of the association of asbestos exposure, rather than a therapeutic one.

Asbestos is a group of fibrous silicate minerals that includes two mineralogic groups: amphiboles and serpentines. While the carcinogenic role of amphiboles (eg, crocidolite and amosite) is well established, medical "experts" that tend to strongly advocate their views currently argue in medicolegal cases multiple specific issues regarding the carcinogenicity of asbestos fibers.
 

For example, it is controversial whether chrysotile causes malignant mesothelioma (MM); what are the specific carcinogenic thresholds for amphiboles and chrysotile; what occupations are truly at risk to develop MM as a result of asbestos exposure; what is the role of chrysotile in the development of peritoneal MM; how to assign causation in individuals exposed to multiple industrial products containing variable concentrations of various asbestos fibers; and, what criteria should be used to accept causation in household exposure cases and others. The causation criteria currently acceptable in U.S. courts are surprisingly flexible and subject to variable interpretation by medical "experts." At a time where thousands of individuals are claiming causation of MM by asbestos exposure, there is a need to develop more specific causation guidelines based on scientific evidence. Evidence-based medicine has been proposed as a new approach to the study, teaching, and the practice of medicine and has been used as a process of systematically reviewing the relevant studies in the literature to assess their scientific validity and development of guidelines.

This article summarizes some of the current controversies regarding the role of asbestos exposure in the causation of MM and suggests the need for future evidence-based medicine-type studies to develop causation guidelines that could be used consistently during litigation.