Mesothelioma is a cancer of the mesothelium. This is a thin lining (membrane) that covers the outer surface of many of our organs. The mesothelium in the chest is called the pleura. The mesothelium in the abdomen is called the peritoneum.
Pleural mesothelioma is much more common than peritoneal mesothelioma.
The lungs sit within the chest, on either side of the heart. They supply oxygen to the organs and tissues of the body.
The lungs are covered by a lining called the pleura, which has two layers. The inner layer covers the lungs. The outer layer lines the ribcage and a sheet of muscle called the diaphragm. The diaphragm separates the chest from the tummy (abdomen).
The pleura produces a small amount of fluid. This fluid acts as a lubricant, which allows the lungs to move in and out smoothly and helps you to breathe easily.
Mesothelioma in the pleura starts as tiny lumps (nodules) that are usually spread throughout the pleura. These eventually grow together, causing the layers of the pleura to become thicker. This means the lungs and chest cannot move as freely, causing you to feel breathless. Fluid may also collect between the two layers of the pleura. This is called a pleural effusion.
The organs in the tummy (abdomen) are covered by a lining (membrane) called the peritoneum. It has an inner and outer layer. The inner layer covers the organs in the abdomen. The outer layer lines the abdominal wall.
The peritoneum helps protect the organs in the abdomen and keeps them in place. This is shown as the thick line surrounding the abdominal organs in the illustration opposite. When mesothelioma develops in the peritoneum, it causes the layers of the peritoneum to thicken. Fluid may also collect between the two layers. This is known as ascites.