What is mesothelioma?

Mesothelioma starts in mesothelial cells. These cells form a membrane (lining) that covers and protects most internal organs in your body. This membrane is called the mesothelium. It is made up of 2 layers. The inner (visceral) layer wraps around organs, including the lungs, heart or stomach. The outer (parietal) layer forms a sac around the inner layer. The mesothelium makes fluid which is inl the space between the 2 layers so that the organs can move easily. The mesothelium has different names, depending on where it is in the body.

The pleura is the mesothelium that protects and cushions the lungs. The inner layer (visceral pleura) covers the lungs. The outer layer (parietal pleura) lines the inside wall of the chest. There is a small amount of fluid (pleural fluid) in the space between the 2 layers (called the pleural space). The pleural fluid helps the layers slide over each other as you breathe.

The peritoneum is the mesothelium that protects and cushions the organs in the abdomen, such as the liver, stomach and intestines. The inner layer covers the abdominal organs. The outer layer lines the wall of the abdomen. There is a small amount of fluid (peritoneal fluid) in the space between the 2 layers. The peritoneal fluid helps the organs move smoothly inside your abdomen.

Mesothelioma is a very rare type of cancer. It usually develops in the pleura or the peritoneum.

• Pleural mesothelioma starts in the pleura. (It is sometimes mistakenly called a lung cancer.) As the cancer cells grow, the membranes thicken and press on the lung. Sometimes too much fluid collects between the 2 layers of the pleura. This is called a pleural effusion. Changes to the pleura or pleural fluid can make breathing difficult.
• Peritoneal mesothelioma starts in the peritoneum. It causes the membranes to thicken. Too much fluid may collect in the abdomen and cause the abdomen to swell. This is called ascites.