Malignant mesothelioma is a rare cancer that occurs in the thin layer of tissue that covers the majority of your internal organs (mesothelium). Doctors divide mesothelioma into different types based on what part of the mesothelium is affected, including : -malignant mesothelioma, which affects the tissue that surrounds the lungs and is the most common form of mesothelioma.Peritoneal mesothelioma, which occurs in the tissue in your abdomen.Pericardial mesothelioma, which affects the tissue surrounding the heart.Mesothelioma of the tunica vaginalis, which occurs in the lining around the testicles.