Mesothelioma is an uncommon tumour arising from mesothelial lining of serous cavities, most often in the pleural (covering of lungs) cavity, and rarely in peritoneal (coverings of abdomen) cavity and pericardial (covering of the heart) sac. They are of two types --- benign (solitary) and malignant (diffuse). The biological behavior of the mesotheliomas can be usually predicted by their gross appearance; thoses forming solitary, discrete masses are generally benign, whereas those, which grow diffusely, are usually malignant.

BENIGN (SOLITARY) MESOTHELIOMA

Benign or solitary mesothelioma is also called pleural fibroma. Asbestos exposure plays no role on the etiology of benign mesothelioma. Grossly it consists of of a solitary, circumscribed, small, firm mass, generally less than 3cm in diameter. Cut surface shows whorls of dense fibrous tissue. Microscopically, the tumour is predominantly composed of whorls of collagen fibres and reticulin with interspersed fibroblasts. Rarely, mesothelial-lined clefts are seen in the tumour. Benign mesothelioma causes no symptoms and is detected as an incidental radiologic finding. Sometimes the tumour is associated with systemic syndrome of osteoarthropathy or hypoglycemia. Removal of the tumour is generally curative.

MALIGNANT (DIFFUSE) MESOTHELIOMA

Malignant or diffuse mesothelioma is rare. It is a highly malignant tumour associated with high mortality. The tumour is significant in view of its recognized association with occupational exposure to asbestos for a number of years, usually 20-40 years. About 90% of malignant mesotheliomas are asbestos related. Mechanism of carcinogenecity by asbestos is not quite clear but it appears that prolonged exposure of amphibole type of asbestos is capable of inducing oncogenic mutation in the mesothelium. However prolonged exposure is considered more significant rather than heavy exposure as documented by its occurrence in the family of asbestos workers. Although combination of cigarette smoking and asbestos exposure greatly increases risk to develop bronchgenic carcinoma, there is no such extra increased risk of developing mesothelioma in asbestos workers who smoke. Recently, SV40 (simian vacuolating virus) has also been implicated in the etiology of mesothelioma.

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