Malignant mesothelioma is a rare kind of cancer that starts in the mesothelial cells. Mesothelial cells form a lining called the mesothelium. This lining protects the outer surface of internal organs, such as the lungs, stomach, heart, and others. Tumors can start in any of these places.
The most common type of mesothelioma starts in the lining on the outer surface of the lungs. These are called pleural mesotheliomas. Pericardial mesotheliamas begin in the sac around the heart. Most of the remaining mesotheliomas are called peritoneal mesotheliomas and begin in the abdominal cavity. Rarely, mesotheliomas start in the scrotal sac surrounding the testes.