Mesothelioma, more precisely malignant mesothelioma is a rare type of cancer. Malignant (cancerous) cells develop in the mesothelium, the protective sac that covers most of the body’s internal organs. Mesothelioma generally starts in the outer membrane of the lungs (pleura), but can also occur in the lining of the abdominal cavity (peritoneum). Uncommonly, the heart or reproductive organs may be affected. Treatment depends on where the cancer is found and whether it has spread.

Mesothelioma is almost always caused by exposure to asbestos and can develop decades after the exposure.