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

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 mesothelium.

The mesothelium helps protect your organs by making 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 has different names in different parts of the body:

• In the chest it is called the pleura.
• In the abdomen it is called the peritoneum.
• In the space around the heart it is called the pericardium.

Tumors of the mesothelium 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 simply shortened to just mesothelioma. Mesotheliomas can start in 4 main areas in the body.

• Pleural mesotheliomas start in the chest cavity. They account for about 3 out of 4 mesotheliomas.

• Peritoneal mesotheliomas begin in the abdomen. They make most of the remaining cases.

• Pericardial mesotheliomas start in the cavity 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, which is actually an outpouching of peritoneum into the scrotum.

Malignant mesotheliomas can also be classified into 3 types based on how the cells are arranged when looked at under a microscope:

• epithelioid: About 50% to 60% of mesotheliomas are of this type. It tends to have a better outlook (prognosis) than the other types.

• sarcomatoid (fibrous): About 10% to 20% of mesotheliomas are of this type.

• mixed (biphasic): These mesotheliomas have both epithelioid and sarcomatoid areas. They make up about 30% to 40% of mesotheliomas.

Benign tumors of the mesothelium

It is important not to confuse malignant mesothelioma with benign tumors that also start in the mesothelium.

Adenomatoid tumor: This benign tumor can develop in the mesothelium of certain female and male reproductive organs. In men, it often starts in the epididymis (a small collection of 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 or womb).

Benign cystic mesothelioma: This is another non-cancerous tumor that may begin in mesothelium near female reproductive organs.

Solitary 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 disease starting in the peritoneum is called solitary fibrous tumor of the peritoneum.

These benign tumors are typically removed by surgery, and there is usually no need for additional treatment.

Only malignant mesothelioma will be discussed further in this document.

Last Medical Review: 03/17/2009
Last Revised: 03/17/2009