Mesothelioma is a form of cancer caused predominantly by excessive exposure to asbestos, the other causes as yet unknown . Mesothelium is a protective lining on the internal organs on which malignant cells form. In most cases, pleura, the outer lining of lungs and chest cavity are affected. Mesothelioma on the peritoneum (the lining of the abdominal cavity) or the pericardium (the sac surrounding the heart) is also common.