Role of asbestos and other fibres in the development of diffuse malignant mesothelioma.

Descriptions of diffuse malignant mesothelioma date from the nineteenth century' but it was not until 1960 that Wagner et al2 described a series of cases of pleural mesothelioma from South Africa and linked them to crocidolite asbestos exposure. Malignant mesotheliomas may arise from the pleura, peritoneum, and pericardium and all may be associated with asbestos exposure. Malignant mesothelioma of the pleura should be distinguished from the so called localised pleural mesothelioma (pleural fibroma), which is rarely malignant3 and which is not associated with asbestos exposure. Malignant mesothelioma is an uncommon tumour with an annual male incidence of 7-13/1 000 000 and a female incidence of 1-2/ 1 000 000 in the United States.4 In Great Britain the death rate (equivalent to the incidence) in 1983 was 17 5/ 1 000 000 for men and 3 2/1 000 000 for women.5 The average age at presentation is about 60 years but occasionally cases have occurred in children and young adults.67 There is a long latent interval between first exposure to asbestos and the development of the tumour, usually over 20 years and averaging 30-40 years, which is unaffected by age at first exposure.8 Mesothelioma has a characteristic but not pathognomonic macroscopic appearance. It shows considerable histological diversity and its ability to mimic other tumours often makes diagnosis difficult. Good, well illustrated histopathological descriptions of these tumours are available"' but are beyond the scope of this article. There is no satisfactory treatment at present and the prognosis is very poor; most patients die within 18 months of diagnosis. Most cases of malignant mesothelioma are associated with asbestos exposure but other causes are recognised.'2 Since the paper by Wagner et al in 19602 there has been an explosion of research and scientific publications on the topic and controversy over the roles of the different fibre types and their capacity to cause disease. There are several reasons for this controversy: (1) the scarcity of populations exposed to only one fibre type; (2) the long latent periods between exposure and the development of the disease; (3) the problems of diagnosis; and (4) the lack of satisfactory data on the airborne fibre levels to which workers have been exposed. This review is an attempt to synthesise and put into perspective the current information contributed by epidemiological, pathological, and mineralogical investigations to our understanding ofthe role of the various mineral fibre types in the development of malignant mesothelioma. The evidence from in vitro and in vivo experiments will also be considered and their relevance to the human disease discussed.