Malignant Mesothelioma of the Pleura

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What is Malignant Mesothelioma of the Pleura?

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Malignant mesothelioma of the pleura is a cancer which arises from the mesothelial cells found within the pleural membranes that cover the lungs. There are two pleurae: the visceral pleura, which covers the actual lung itself; and the parietal pleura, which covers the inside of the chest wall. The purpose of the pleura is to provide a smooth surface so that, as the chest expands, the lungs expand smoothly within the chest cavity. The pleural membranes normally secrete a small amount of fluid for lubrication. This allows the visceral pleura (lining the lungs) to glide over the parietal pleura (lining the inside of the chest cavity). Malignant mesothelioma develops from the mesothelial cells of the pleura.

Who gets Malignant Mesothelioma of the Pleura?

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Malignant mesothelioma is relatively uncommon compared to other tumours. Australia has the highest incidence of mesothelioma in the world, at 40 cases per million people. Other industrialised countries which have mined and used asbestos, such as the United States, England, Canada and South Africa, have similar but slightly lower rates of disease.

The incidence of mesothelioma increases with age, with most patients presenting between 50 and 80 years of age. This is because there is a latency period of as long as 50 years between exposure to asbestos and development of mesothelioma. Mesothelioma is far more common in men than in women, at a ratio of 4:1.

Predisposing Factors

The main predisposing factor is contact with asbestos. There are two major types of asbestos fibres: the long, thin 'amphiboles', and the feathery 'chrysotile' fibres.

Amphibole fibres include the known cancer-causing blue asbestos fibres, which have been definitively linked to the development of malignant mesothelioma. The long, thin shape of these fibres is thought to allow them to penetrate into the pleura, causing cycles of irritation, inflammation and thickening. Chrysotile fibres (including 'white asbestos') may also increase the risk of malignant mesothelioma, but the link is less clear.

People particularly at risk are those who have worked in the mining of asbestos, especially blue asbestos. Other at-risk jobs include ship builders, insulation workers, fitters, carpenters and electricians. Immediate family members of workers whose clothes have been contaminated with asbestos fibres are also at risk, even though the amount of exposure is very small.

Progression

Malignant mesothelioma grows in the pleural space surrounding the lungs. The tumour may eventually encase the entire lung. Local spread also occurs into nearby structures, such as the lung, chest wall, or the heart. In half of all cases, malignant mesothelioma spreads to other organs of the body, including the liver or, rarely, bone.

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