Malignant mesothelioma is a rare form of cancer that is diagnosed in approximately 3,000 people each year. The disease usually affects the thin membrane that lines the chest cavity and the lungs. This membrane is called the pleura.

It also can affect the lining of the abdominal cavity, called the peritoneum, and, very rarely, it affects the lining around the heart, called the pericardium.

Most cases of malignant mesothelioma are caused by exposure to asbestos, a naturally occurring mineral that was used in a variety of industries. A small percentage of cases have been linked to exposure to a mineral silicate called zeolite or to a type of radiation contrast dye (Thorotrast) that was used before 1960 to help blood vessels show up on X-rays. In a few cases, the cause is unknown.

People exposed to asbestos for a long time or exposed to high levels have an increased risk of developing malignant mesothelioma, but even people exposed for a very short time can develop this disease. The disease develops at least 15 years (typically 20 to 40 years) after exposure to asbestos. People usually are diagnosed with this disease between ages 50 to 70. More men than women get this cancer, probably because men are more likely to have worked in the industries that use asbestos.

There are three main types of malignant mesothelioma: epithelial, sarcomatoidand mixed. The epithelial type is the most common.

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