What is mesothelioma?

Mesothelioma is a type of cancer that starts from mesothelial cells. These cells line the outer surface of most of the body’s internal organs, creating a protective membrane called the mesothelium.

Some mesotheliomas form a mass (tumour), while others grow along the mesothelium and form a thick covering. In later stages, mesothelioma may spread (metastasise) to other parts of the body.

Pleural mesothelioma

The mesothelium that covers each lung is called the pleura. Mesothelioma that develops in the pleura is known as malignant pleural mesothelioma or, simply, pleural mesothelioma. About 90% of all mesotheliomas are in the chest.

Although pleural mesothelioma involves the lining of the lungs, it is not lung cancer and is diagnosed and treated differently.

The pleura

There are two thin layers of tissue in the pleura. The inner layer (the visceral pleura) lines the lung surface, and the outer layer (the parietal pleura) lines the chest wall and diaphragm.

Between the two layers is the pleural cavity (also called the pleural space), which normally contains a thin film of fluid. This fluid allows the two layers of pleura to slide over each other so the lungs move smoothly against the chest wall when you breathe. When mesothelioma develops in the pleura, the layers of the pleura thicken and may press on the lung, preventing it from expanding when breathing in (inhaling). Excess fluid often collects between the two layers – this is known as pleural effusion.

Peritoneal mesothelioma

The mesothelium that lines the walls and organs of the abdomen and pelvis is called the peritoneum. Mesothelioma that develops in the peritoneum is known as malignant peritoneal mesothelioma or, simply, peritoneal mesothelioma. Less than 10% of all mesotheliomas are in the abdomen.

The peritoneum

There are two layers of thin tissue in the peritoneum. The inner layer (the visceral peritoneum) lines the surface of organs such as the bowel, liver and ovaries. The outer layer (the parietal peritoneum) lines the walls of the abdomen and pelvis.

Between the two layers is the peritoneal cavity, which normally contains a thin film of fluid. This fluid allows the two layers to slide over each other as you move around. In people with peritoneal mesothelioma, excess fluid often collects between the two layers – this is known as ascites or peritoneal effusion.

Rarely, mesothelioma occurs in the pericardium, the lining of the heart. This is called pericardial mesothelioma. Even more rarely, mesothelioma can occur in the membrane around the testicles, the tunica vaginalis. This is called testicular mesothelioma.

How common is mesothelioma?

Australia has one of the highest rates of mesothelioma in the world, with 757 Australians diagnosed in 2016. Men are four times more likely than women to be diagnosed with mesothelioma, probably because many cases have been caused by exposure to asbestos at work.

Pleural mesothelioma makes up about 93% of all mesothelioma cases. Peritoneal mesothelioma is less common and makes up nearly 7% of cases.  Mesothelioma is more common in people over the age of 65, but can occur in younger people.