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What Is Malignant Mesothelioma?

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

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Cancer starts when cells start to grow out of control. Cells in nearly any part of the body can become cancer. To learn more about how cancers start and spread, see What Is Cancer?
The mesothelium
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 against each other. 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 space in the chest containing the lungs.
The peritoneum lines the inside of the abdomen and many of the organs in the abdomen.

The pericardium covers the heart and creates the space that holds the heart in the chest.

The tunica vaginalis lines the testicles.
Mesothelial tumors can start in any of these linings. These 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 cancer cells are arranged:

About half of mesotheliomas are epithelioid. This type tends to have a better outlook (prognosis) than the other types.

About 10% of mesotheliomas are sarcomatoid (fibrous).
Mixed (biphasic) mesotheliomas have both epithelioid and sarcomatoid areas. They make up the remaining 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 does not start in 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 can 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.

Only malignant mesothelioma will be discussed further in this document.