Mesothelioma

What Is Mesothelioma?

Malignant mesothelioma is an uncommon, but no longer rare, cancer that is difficult to diagnose and poorly responsive to therapy. Malignant mesothelioma is the most serious of all asbestos-related diseases.

A layer of specialized cells called mesothelial cells lines the chest cavity, abdominal cavity, and the cavity around the heart. These cells also cover the outer surface of most internal organs. The tissue formed by these cells is called mesothelium.

The mesothelium helps protect the organs by producing a special lubricating fluid that allows organs to move around. For example, this fluid makes it easier for the lungs to move inside the chest during breathing. The mesothelium of the chest is called the pleura and the mesothelium of the abdomen is known as the peritoneum. The mesothelium of the pericardial cavity (the "sac-like" space around the heart) is called the pericardium.

Tumors of the mesothelium can be benign (noncancerous) or malignant (cancerous). A malignant tumor of the mesothelium is called a malignant mesothelioma. Because most mesothelial tumors are cancerous, malignant mesothelioma is often simply called mesothelioma.

Mesothelioma was recognized as a tumor of the pleura, peritoneum and pericardium in the late 1700's. However it was not until much later, in 1960, that this particular type of tumor was described in more detail and even more importantly, its association with asbestos exposure was recognized. The first report linking mesothelioma to asbestos exposure was written by J.C.Wagner, and described 32 cases of workers in the "Asbestos Hills" in South Africa. Since then, the relationship between mesothelioma and asbestos exposure has been confirmed in studies around the world.

The incidence of mesothelioma in the United States remains very low, with 14 cases occurring per million people per year. Despite these numbers, the noticed threefold increase in mesothelioma in males between 1970 and 1984, is directly associated with environmental and occupational exposure to asbestos, mostly in areas of asbestos product plants and shipbuilding facilities.

Although the disease is much more commonly seen in 60-year-old men, it has been described in women and early childhood as well. The cause of the disease is not so well understood in these latter two groups, but there is some evidence of possible asbestos exposure for some of these cases as well.

Malignant mesothelioma is divided into three main types. About 50% to 70% of mesothelioma occurrences are the epithelioid type. This type has the best prognosis (outlook for survival). The other two types are the sarcomatoid type (7%-20%), and the mixed/biphasic type (20%-35%). Treatment options for all three types are the same.

About three-fourths of mesothelioma occurrences start in the chest cavity and is known as pleural mesothelioma. Another 10% to 20% begin in the abdomen and is called peritoneal mesothelioma. Pericardial mesothelioma, starting in the cavity around the heart, is very rare. The covering layer of the testicles is actually an outpouching of peritoneum into the scrotum. Mesothelioma that affects this covering of the testicles is quite rare.

Who Is At Risk for Mesothelioma?

The only known cause of mesothelioma is previous exposure to asbestos fibers. Often times workers are not told they are working around asbestos, and even single exposures to very low doses of fibers can produce serious long-term consequences.

Asbestos diseases are said to follow the trail of exposure. That means that wherever people have received asbestos exposure, regardless of their trade, age, sex or race, they are at risk of cancer and other diseases.

Almost anyone who has inhaled asbestos dust is at risk. Even a worker's families and friends can be at risk, because asbestos can often be carried on clothing.

Exposure to asbestos dust can occur at major construction job sites, in shipyards, in industry, and during construction or renovation of commercial buildings. See the list at right for certain occupations that are known to be those where risk of exposure to asbestos is great, and the numbers of people in such occupations contracting asbestos disease are high.

The period between asbestos exposure and the development of mesothelioma is generally 35-40 years. Usually, people who develop mesothelioma have been exposed to asbestos for a prolonged period of time; however, some people with only brief exposures have also developed mesothelioma. On the other hand, not all workers who are heavily exposed develop mesothelioma.

Mesothelioma Causes

Asbestos

The major known cause of mesothelioma is exposure to asbestos. A known history of asbestos exposure is present in 70-80% of all cases of mesothelioma. Asbestos likely causes cancer by physically irritating the cells. When asbestos fibers are breathed in, they reach the ends of the small airways and penetrate into the pleura (lining of the lung). These fibers may then injure mesothelial cells of the pleura and eventually cause mesothelioma.

The risk of developing mesothelioma rises with the quantity of asbestos exposure. However, genetic factors also play a role in determining who develops the condition. This is probably the reason why all people exposed to high levels of asbestos do not develop mesothelioma.

Smoking

Although smoking has not been associated with the development of mesothelioma, the combination of smoking and asbestos exposure significantly increases a person's risk of developing pleural mesothelioma.

Other Causes

Mesothelioma also has been linked to the following:

Radiation: Mesothelioma has been reported to occur in few people following exposure to therapeutic radiation using thorium dioxide (Thorotrast).

Zeolite: Zeolite is a silicate mineral (chemically related to asbestos) commonly found in the soil of the Anatoli region of Turkey. A few cases of mesothelioma reported from this region may have been caused by zeolite.

Simian virus 40 (SV40): The role of SV40 in the development of mesothelioma has also been suggested. This virus has been identified in human mesothelioma cells. In studies conducted in animals, it has been shown to induce the development of mesothelioma. However, further research is needed to confirm the link.

Mesothelioma Symptoms

Symptoms of mesothelioma may not appear until 30 to 50 years after exposure to asbestos. The onset of symptoms is gradual, and a person often experiences symptoms for four to six months before the diagnosis is made.

You should see a health-care provider if you have any of the following symptoms:

• Difficulty in breathing (dyspnea) is the most common complaint
  
• Chest discomfort
  
• Chest pain
  
• Cough
  
• Easy fatigability
  
• Fever
  
• Weight loss

The above listed symptoms may be caused by mesothelioma or by other, less serious diseases. Only a health-care provider can make a diagnosis with the help of various exams and tests.

Web Links

Mesothelioma Aid, available at http://www.mesothelioma-aid.org/
Mesothelioma-net, available at http://www.mesothelioma-net.com/
Mesothelioma Web, available at http://www.mesotheliomaweb.org/
Mesothelioma Information, available at http://www.mesoinfo.com/
Mesothelioma Information and Research Group, available at http://www.mirg.org/
Asbestos Update, available at http://www.mesothel.com/
Mesothelioma Facts and Information Newsletter, available at http://www.mesothelioma-facts.com/
Mesothelioma News, available at http://www.mesotheliomaweb.org/news.htm
OncoLink, What is Mesothelioma? available at http://www.oncolink.org/types/article.cfm?c=9&s=32&ss=249&id=8411
National Cancer Institute, Malignant Mesothelioma, available at http://cancerweb.ncl.ac.uk/cancernet/201071.html
American Cancer Society, What is Malignant Mesothelioma? Available at http://www.cancer.org/docroot/CRI/content/CRI_2_4_1X_What_is_malignant_mesothelioma_29.asp
American Lung Association, Mesothelioma, available at http://www.lungusa.org/diseases/mesothelioma.html
Cancer Information Specialists, Online Assistance, available at http://cancer.gov/common/popUps/liveHelp.aspx