What is Mesothelioma?

What is mesothelioma? You may have heard the term somewhere, seen advertisements related to the diagnosis or heard the diagnosis suggested by a physician. Until recently, it has not been a household, familiar type of cancer like breast cancer or lung cancer.

So what is mesothelioma? Mesothelioma is a rare tumor of mesothelium (see below). It is most strongly associated with a prior, significant exposure to asbestos.

Mesothelium is a tissue in the body which forms a membrane which covers and coats the inside of the body cavities, most notably the chest and abdominal cavity. The two largest cavities in the body are the thoracic (chest) cavity and the abdominal cavity.

Most of the chest cavity is lined by a mesothelium called pleura which covers the outer surface of the lungs and the surrounding body wall. The pericardium, which is the sac which encases the heart between the lungs, is also a form of mesothelium.

The entire abdominal cavity (or peritoneal cavity) is lined by another type of mesothelium called peritoneum. It covers the outer surface of many organs including part of the liver and much of the small and large intestines, as well as the body wall surrounding them.

Mesothelium also exists around the testes in males and the uterus in females.

Because mesothelioma is a tumor of mesothelium, it can theoretically occur anywhere that mesothelial tissues are normally found. The pleura in the chest surrounding the lungs is the most common location for mesothelioma. Because it occurs in the pleura surrounding the lungs and can invade the lungs it is often mistaken for a form of lung cancer. However, it does not arise from the lung tissue itself. The peritoneum in the abdomen is also a possible location for mesothelioma to develop but is less common.

Although the exact causes of mesothelioma are not fully known, it is known that this tumor is very strongly linked with significant exposure to some types of asbestos. Asbestos is a naturally occuring silicate mineral that forms long, fibrous crystals. Until the late 70's or 80's many products, especially various forms of insulation, were made with these asbestos fibers. Since that time use of asbestos-containing products has been illegal in the United States and many other countries. However, many people were exposed to asbestos and many houses and buildings still contain asbestos-containing products.

The most common mode of exposure to asbestos is by inhalation of asbestos fibers. The individuals at highest risk for these exposures are generally people who had occupational contact with asbestos over prolonged periods of time. It is thought that the insoluble and indigestible fibers cause tissue injury and inflammation and over time can lead to the development of these tumors. The cancer generally presents many years after the initial exposure to asbestos.

Mesothelioma has been reported in patients with no known exposure to asbestos, however this is rare.

The large majority of mesothelioma tumors are malignant, meaning that they are indeed cancer. However, rarely, benign forms of tumors resembling mesothelioma do occur. These are also called solitary fibrous tumors of the pleura or peritoneum.