Malignant mesothelioma is an uncommon, but no longer rare, cancer that is difficult to diagnose and poorly responsive to therapy. Malignant mesothelioma is the most serious of all asbestos related diseases. 

A layer of specialized cells called mesothelial cells lines the chest cavity, abdominal cavity, and the cavity around the heart. These cells also cover the outer surface of most internal organs. The tissue formed by these cells is called mesothelium. 

Mesothelioma was recognized as a tumor in the late 1700's. However it was not until much later, in 1960, that this particular type of tumor was described in more detail and even more importantly, its association with asbestos exposure was recognized. 

Although the disease is much more commonly seen in 60-year-old men, it has been described in women and early childhood as well. The cause of the disease is not so well understood in these latter two groups, but there is some evidence of possible asbestos exposure for some of these cases as well. 

About three-fourths of mesothelioma occurrences start in the chest cavity and is known as pleural mesothelioma . Another 10% to 20% begin in the abdomen and is called peritoneal mesothelioma. Pericardial mesothelioma , starting in the cavity around the heart, is very rare. The covering layer of the testicles is actually an outpouching of peritoneum into the scrotum. Mesothelioma that affects this covering of the testicles is quite rare.