What is malignant mesothelioma?

Malignant mesothelioma is a cancer that starts in cells in the linings of certain parts of the body, especially the chest cavity or abdominal cavity..

A layer of specialized cells called mesothelial cells lines the inside of the chest, the abdomen, and the space around your heart. These cells also cover the outer surface of most of your internal organs. The lining formed by these cells is called the mesothelium.

The mesothelium helps protect your organs by making a special lubricating fluid that allows organs to move. For example, this fluid makes it easier for your lungs to move (expand and contract) inside the chest when you breathe. The mesothelium has different names in different parts of the body:

• The pleura coats the lungs and the cavity containing the lungs in the chest.
• The peritoneum coats the abdominal cavity and many of the organs within that cavity.
• The tunica vaginalis coats the testicles.
• The pericardium coats the heart and creates the cavity that holds the heart in the chest.

Mesothelial tumors can be non-cancerous (benign) or cancerous (malignant). 

Malignant mesothelioma

A cancerous tumor of the mesothelium is called a malignant mesothelioma, although this is often shortened to just mesothelioma. Mesotheliomas can start in 4 main areas in the body.

• Pleural mesotheliomas start in the chest. About 3 out of 4 mesotheliomas are pleural mesotheliomas.
• Peritoneal mesotheliomas begin in the abdomen. They make up most of the remaining cases.
• Pericardial mesotheliomas start in the covering around the heart and are very rare.
• Mesotheliomas of the tunica vaginalis are very rare tumors that start in the covering layer of the testicles.

Malignant mesotheliomas can also be classified into 3 main types based on how the cells are arranged:

• About 50% to 60% of mesotheliomas are epithelioid. This type tends to have a better outlook (prognosis) than the other types.
• About 10% to 20% of mesotheliomas are sarcomatoid (fibrous).
• Mixed (biphasic) mesotheliomas have both epithelioid and sarcomatoid areas. They make up about 30% to 40% of mesotheliomas.

Benign tumors of the mesothelium

Benign (non-cancerous) tumors can also start in the mesothelium. These tumors are typically removed by surgery, and there is often no need for additional treatment.

Localized fibrous tumor of the pleura

This type of benign tumor can form in the pleura surrounding the lungs. It used to be called benign fibrous mesothelioma, but doctors now know that this tumor actually starts from tissue under the mesothelium and not from mesothelial cells. This disease is usually benign, but about 1 in 10 are cancerous. A similar condition that starts in the peritoneum is called solitary fibrous tumor of the peritoneum.

Adenomatoid mesothelioma

This benign tumor can develop in the mesothelium of certain reproductive organs. In men, it often starts in the epididymis (ducts that carry sperm cells out of the testicle). In women, this tumor may begin in the fallopian tubes (tubes that carry eggs from the ovaries to the uterus).

Benign cystic mesothelioma

This rare non-cancerous tumor often begins in the peritoneum.

What are the key statistics about malignant mesothelioma?

Mesothelioma is fairly rare. About 3,000 new cases of mesothelioma are diagnosed each year in the United States. 

The rate of mesotheliomas in the United States increased from the 1970s to the early 1990s, but since then it has leveled off and even gone down slightly. These changes have largely been seen in men, and are probably related to changes in workplace exposures to asbestos (see “What are the risk factors for malignant mesothelioma?”). The rate of mesothelioma is lower in women and has been fairly steady for some time. In many other countries, the rate of mesothelioma is still increasing.

Mesothelioma is more common in whites and Hispanics/Latinos than in African Americans or Asian Americans.