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

The mesothelium helps protect your organs by producing a special lubricating fluid that allows organs to move around. 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 "sac-like" space around the heart (pericardial cavity) is called the pericardium.

Tumors of the mesothelium can be non-cancerous (benign) or cancerous (malignant). A malignant tumor of the mesothelium is called a malignant mesothelioma; however, malignant mesothelioma is often simply called mesothelioma.

Malignant mesotheliomas are divided into 3 main types:

• epithelioid (50% to 70% are of this type) -- this type has the best outlook for survival (prognosis)

• sarcomatoid (7% to 20% are of this type)

• mixed/biphasic (20% to 35% are of this type)

Treatment options are the same for all 3 types.

About 75% of mesotheliomas start in the chest cavity. They are known as pleural mesotheliomas. Another 10% to 20% begin in the abdomen. These are called peritoneal mesotheliomas. Pericardial mesotheliomas start in the cavity around the heart and are very rare. The covering layer of the testicles is actually an outpouching of peritoneum into the scrotum. Mesotheliomas that affect this covering of the testicles can occur but are quite rare.

It is important not to confuse malignant mesothelioma with benign tumors that also start in the mesothelium. The mesothelium of certain female and male reproductive organs may develop a type of benign tumor called an adenomatoid tumor. In men, this non-cancerous tumor 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). Another non-cancerous tumor that may begin in mesothelium near female reproductive organs is called benign cystic mesothelioma.

A type of benign tumor that used to be called benign fibrous mesothelioma can form in the pleura surrounding the lungs. Doctors now know that this tumor actually starts from tissue under the mesothelium and not from mesothelial cells. For this reason, the new name of this tumor is solitary fibrous tumor of the pleura. This disease is usually not cancerous, but cancerous forms can occur. A similar disease starting in the peritoneum is called solitary fibrous tumor of the peritoneum. The tumors described in this paragraph are usually removed by surgery, and there is no need for additional treatment. Only malignant mesothelioma will be discussed further in this document. Last Revised: 10/19/2006