Malignant mesothelioma is a disease in which malignant (cancer) cells form in the thin layer of tissue that covers the lung, chest wall, or abdomen. It may also form in the heart or testicles, but this is rare.
The type of malignant mesothelioma depends on the cell in which it began. The most common type of malignant mesothelioma is epithelial mesothelioma, which forms in the cells that line organs. The other types begin in spindle-shaped cells called sarcomatoid cells or are a mixture of both cell types. Epithelial mesothelioma may grow more slowly and have a better prognosis than other types.
The major cause of malignant mesothelioma is being exposed to asbestos over a period of time. This includes people who were exposed to asbestos in the workplace and their family members.
After a person is exposed to asbestos, it usually takes at least 20 years for malignant mesothelioma to form.