Mesothelioma is a tumor of the mesothelium. This is the thin lining (membrane) that covers the outer surface of most of our body’s organs.
The mesothelium has different names in different parts of the body. For example:
- in the chest it’s called the pleura
- in the abdomen it’s called the peritoneum.
A cancer of the mesothelium is called a malignant mesothelioma. However, it’s usually referred to simply as mesothelioma. There are other tumors of the mesothelium, such as adenomatoid tumors, benign cystic mesotheliomas and solitary fibrous tumors of the pleura.
There are two main types of malignant mesothelioma: pleural mesothelioma and peritoneal mesothelioma. Pleural mesothelioma is much more common than peritoneal mesothelioma. Around 9 out of 10 cases (90%) of mesothelioma develop in the pleura, compared with around 1 in 10 (10%) in the peritoneum.
The pleura is the smooth outer lining (membrane) that covers each lung. It has two layers: the inner (visceral) layer, which is next to the lung, and the outer (parietal) layer, which lines the chest wall. The two layers of the pleura are usually in contact and slide over each other as we breathe. The layers produce fluid, which allows them to move smoothly over each other. When mesothelioma develops in the pleura (pleural mesothelioma), the delicate layers of the pleura thicken and may press inwards on the lung. Fluid may also collect between the two layers, which is known as a pleural effusion. Pleural mesothelioma can sometimes spread to the lymph nodes in the chest or above the collarbone, or elsewhere in the body.
The outer lining (membrane) that covers the organs in the abdomen is known as the peritoneum. The peritoneum helps protect the organs of the abdomen and keep them in place. It also has two layers: the inner (visceral) layer, which is next to the abdominal organs, and the outer (parietal) layer, which lines the abdominal wall. Mesothelioma in the peritoneum is called peritoneal mesothelioma. It causes thickening of the peritoneum and a collection of fluid in the abdomen. The collection of fluid is called ascites and causes swelling of the tummy (abdomen).
Rare types of mesothelioma
Very rarely, a mesothelioma may develop in the outer lining of the heart (pericardial mesothelioma) or in the outer lining of the testes (testicular mesothelioma). These are not discussed here.
Cell types of mesothelioma
Pleural and peritoneal mesothelioma can also be grouped according to how the cells look under a microscope.
There are three main types:
- epithelial – this is the most common
- sarcomatoid (fibrous)
- mixed (biphasic) – this has both epithelial and sarcomatoid cells.
Knowing the type of cell involved may give your doctors an idea of how well the disease will respond to treatment.
Asbestos is the most common cause of mesothelioma. Up to nine out of ten cases of mesothelioma (90%) are caused by exposure to asbestos fibers. Asbestos is a natural mineral found in many countries. It acts as an insulator (to keep heat in and cold out) and it protects against fire and corrosion. There are three main types of asbestos: blue (crocidolite), brown (amosite) and white (chrysotile). Exposure to blue and brown asbestos is commonly linked with mesothelioma. However, exposure to all types of asbestos is harmful. Mesothelioma doesn’t usually develop until many years after exposure to asbestos. It can take any time from 10–60 years, although the average is about 30–40 years after exposure.
When asbestos is disturbed or damaged, it releases tiny fibers that can be breathed into the lungs. Asbestos fibers are very fine and can make their way into the smallest airways of the lungs. Once the fibers are in the lungs, the body’s defense mechanisms try to break them down and remove them. This leads to inflammation in the lung tissue. The asbestos fibers can also travel through the lung tissue to settle in the outer lining of the lung (the pleura). Over many years they can cause mesothelioma or other lung diseases to develop.
Asbestos fibers can also be swallowed, and some of the fibers can stick in the digestive system. They can then move into the outer lining of the abdomen (the peritoneum). Here, they cause swelling and thickening of the lining and can lead to peritoneal mesothelioma.
Exposure to asbestos
People most likely to have been exposed to asbestos at work include:
- joiners and construction workers
As these jobs were mostly done by men, mesothelioma is five times more common in men than in women.
People who haven’t worked directly with asbestos can also sometimes develop mesothelioma. These can include:
- family members of people who’ve worked with asbestos and brought the dust home on their clothes
- people who lived near asbestos factories
- people who worked in buildings containing asbestos materials that were disturbed or damaged.
Occasionally, mesothelioma develops in people who have never been exposed to asbestos.
The other causes of the disease are not fully understood, but, in rare cases, mesothelioma has been linked to exposure to radiation. A mineral called erionite, which has been found in Turkey and North America, has also been linked to mesothelioma.
Mesothelioma isn’t infectious and can’t be passed on to other people. It isn’t caused by inherited faulty genes, so family members don’t have an increased risk of developing it, unless they have also been exposed to asbestos.